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NAB SATELLITE UPLINK OPERATORS TRAINING COURSE

TEXT AND CLASSROOM NOTES

October, 2006

By: Norman Weinhouse and Sidney Skjei, P.E.

Skjei Telecom, Inc. 7777 Leesburg Pike, Suite 315N Falls Church, Virginia 22043

Phone: 703-917-9167 Email: [email protected] www.skjeitelecom.com

NAB SATELLITE UPLINK OPERATORS TRAINING

COURSE

TEXT AND CLASSROOM NOTES

October, 2006

By: Norman Weinhouse and Sidney Skjei, P.E.

Skjei Telecom, Inc. 7777 Leesburg Pike, Suite 315N Falls Church, Virginia 22043

Phone: 703-917-9167 Email: [email protected] www.skjeitelecom.com

Skjei Telecom, Inc.

7777 Leesburg Pike, Suite 315N

Falls Church, Virginia 22043

703-917-9167

All rights reserved. No part of this text may be reproduced or utilized in any form or by any means, electronic or mechanical, including photocopying, recording, or by information storage and retrieval systems, without permission in writing from Skjei Telecom, Inc.

TABLE OF REVISIONS

Revision Purpose/changes Author Date No. R0.1 Initial Release Norman May 5, Weinhouse 2006 and Sidney Skjei R9 Update Sidney Skjei Sept, 2006

CHAPTER 1: INTRODUCTION 1

BRIEF HISTORY OF COMMUNICATION SATELLITES IN U.S. 2 INTERNATIONAL SERVICE 7

CHAPTER 2: BASIC CONCEPTS 9

DECIBEL NOTATION 9 DIRECTIONAL ANTENNAS 10 ANTENNA GAIN 10 RADIATION PATTERN 11 GEOSTATIONARY ORBIT 12 SATELLITE LAUNCH SEQUENCE 13 SATELLITE VISIBILITY FROM EARTH 14 SUN OUTAGES 17 ANGULAR DISTANCE BETWEEN SATELLITES 19 FREQUENCY/POLARIZATION PLAN ―U.S. DOMESTIC 19 TRANSPONDERS AND FREQUENCIES 19 POLARIZATION 20 C BAND 22 KU BAND 23 THE COMMUNICATION SATELLITE 24 SPACECRAFT BUS 25 Stabilization and Station Keeping 25 Power 27 Propulsion 28 Telemetry and Control 30 THE COMMUNICATIONS PAYLOAD 30 Wideband Receiver 31 Channelization 33 Antenna Subsystem 36 SATELLITE CHARACTERISTICS (FOOTPRINTS) 38 SATURATION FLUX DENSITY (SFD, AND G/T) 38 EFFECTIVE ISOTROPIC RADIATED POWER (EIRP) 42 NOISE 43 THERMAL NOISE 43 ANTENNA NOISE 45 RECEIVER NOISE TEMPERATURE (CLEAR WEATHER) 46 POWER ADDITION OF NOISE 48 SATELLITE ACCESS METHODS 48 LINKS AND NETWORKING 51 ONE WAY (BROADCAST) LINKS 51 TWO WAY (BIDIRECTIONAL) LINKS 52 Point to Point Links 52 Networks 52

Copyright 2006 iii All rights reserved Mesh Networks 53 Star Networks 54 Factors in Choosing a Network Type 55 THE EARTH – SATELLITE LINK 55 POWER CONSIDERATIONS IN THE UPLINK 55 UPLINK THERMAL CARRIER TO NOISE RATIO 56 INTERFERENCE IN THE UPLINK 57 Antenna Sidelobe Discrimination 57 Uplink Carrier to Interference Ratio 58 THE SATELLITE–EARTH LINK (DOWNLINK) 63 DOWNLINK THERMAL CARRIER TO NOISE RATIO 63 DOWNLINK CARRIER –TO-INTERFERENCE RATIO 66 CARRIER-TO-INTERMODULATION RATIO 67 INTERFERENCE LOCATION SYSTEMS 69 AGGREGATION OF INTERFERENCE EFFECTS 71 PROPAGATION ANOMALIES 72 WEATHER RELATED FACTORS IN SATELLITE LINKS 72 Effects of Rain 73 Rain Attenuation 73 Noise Temperature Effects 73 Depolarization 75 Uplink and Downlink Effects and Countermeasures 75 Uplink Effects and Countermeasures...... 76 Downlink Effects and Countermeasures...... 76 Scattering 76 Effects of Snow 76 OTHER PROPAGATION ANOMALIES 77 OVERALL PREDETECTION CARRIER-TO-NOISE RATIO 77 CHARACTERISTICS OF C, KU AND KA BAND SATELLITE COMMUNICATIONS 78 C-BAND SATELLITES 78 KU-BAND SATELLITES 78 KA-BAND SATELLITES 79 COMPARISON OF C, KU AND KA BAND SYSTEMS 81 COMMONLY USED MODULATION TECHNIQUES 83 FREQUENCY MODULATION 83 Television–FM/TV 83 Frequency Division Multiplex, FDM/FM 84 Single Channel Per Carrier–SCPC/FM 84 DIGITAL MODULATION 84 SPREAD SPECTRUM 85 OVERMODULATION 85 SIGNAL-TO-NOISE RATIO-ANALOG SYSTEMS 86 FM TELEVISION 86 Video-Signal-to-Noise Ratio 86 Audio-Signal-to-Noise Ratio 89 FM Subcarriers 89 Sound in Synch Digital Audio 91 FM–SCPC 92 FDM/FM FM SUBCARRIERS 92

Copyright 2006 iv All rights reserved FDM/FM – SINGLE SIDEBAND SUBCARRIERS 93 DIGITAL TECHNOLOGIES 93 SOURCE CODING (BASEBAND PROCESSING) 94 Pulse Code Modulation (PCM) 94 Predictive Techniques 94 Forward Error Correction 95 DIGITAL MODULATION TECHNIQUES 95 Amplitude, Phase and Symbols 95 Biphase Modulation (BPSK) 97 Quaternary Phase Modulation (QPSK) 99 SIGNAL-TO-NOISE RATIO AND EB/NO 102 8PSK AND 16 QAM MODULATION 104 COFDM MODULATION 106 FORWARD ERROR CORRECTION CODING 107 Block Coding 107 LDPC 108 Convolutional Coding 110 System Impairments 113 Eye Patterns 114 COMPRESSED DIGITAL TELEVISION AND TRANSMISSION 114 INTRODUCTION- ANALOG TELEVISION 114 TYPES OF VIDEO 116 INTRODUCTION TO DIGITAL VIDEO 117 WHY DIGITAL TELEVISION? 117 WHY COMPRESSION? 118 DIGITAL TELEVISION–BASICS 118 The A-D Process–Sampling, Quantizing and PCM Coding 119 Sampling 119 Quantizing 120 Encoding 121 Serial or Parallel Transmission 123 COMPRESSION 123 Compression Techniques 126 Pre-Processing and Redundancy Removal 126 Prediction and Motion Compensation 126 Transformation–Frequency Decomposition 127 Quantization 127 Entropy Reduction 128 ALGORITHMS 128 DECOMPRESSION – DECODING 131 COMPLETE SYSTEM EXAMPLE 133 STANDARDIZATION 133 CURRENT STANDARDS FOR SATELLITE TRANSMISSION OF DIGITAL TELEVISION 134 DIGITAL TELEVISION STANDARD (DVB) 135 DVB-S and DVB-S2 136 EMERGING ENCODING METHODS: MPEG 4 AND JPEG 2000 137 MPEG 4 Part 10 and SMPTE VC-9 137 JPEG 2000 138 HIGH DEFINITION TELEVISION 140

Copyright 2006 v All rights reserved ADVANCED TELEVISION STANDARDS COMMITTEE (ATSC) 140 SATELLITE TRANSMISSION OF COMPRESSED TELEVISION 142 HIGH DEFINITION (HD) TRANSMISSION OVER SATELLITE 145 DIRECT BROADCAST SATELLITE SYSTEMS 145

CHAPTER 3: GROUND EQUIPMENT 147

UPLINK GROUND COMMUNICATIONS EQUIPMENT 149 Television Exciters/Uplink Video Equipment 149 Analog Exciter 149 Baseband Circuits 149 Modulation and Upconversion 150 Transmitter Identification 151 Digital Exciter 152 SCPC Uplinks 152 POWER AMPLIFIERS 154 MULTIPLEXERS AND SWITCHES 156 Switches 156 Multiplexers 156 Satellite Simulator–Non Radiation Tests 159 DOWNLINK EQUIPMENT 160 LOW NOISE AMPLIFIERS/CONVERTERS 160 POWER DIVIDERS 163 DOWNCONVERTER/DEMODULATOR 164 INTEGRATED DIGITAL RECEIVER-DECODER 164 ANTENNAS, DUPLEXER AND IFL 164 DUPLEXER 165 ANTENNAS–RADIATING ELEMENTS 166 Gain and Sidelobe Performance Verification 166 Antenna Geometry–Feed Systems 166 Mechanical Features 168 Dimensional Tolerances 169 Foundations, Mounts and Motor Drives 169 RECEIVE ONLY EARTH STATION 170 INTERFACILITY LINK (IFL) 170 POWER SYSTEMS 171 MAINTENANCE PROGRAMS 171 EARTH STATION LICENSING 172 FREQUENCY COORDINATION 174

CHAPTER 4: UPLINK OPERATION 175

OPERATING RESPONSIBILITIES 175 OPERATOR CONTROLS 176 TEST EQUIPMENT AND CALIBRATION 176 ACCESS PROCEDURES 177 ESTABLISH CONTACT WITH SATELLITE OPERATORS 177 LOCATE AND VERIFY IDENTITY OF PROPER SATELLITE 177

Copyright 2006 vi All rights reserved ANTENNA OPTIMIZATION AND PRE-TRANSMISSION ADJUSTMENTS 178 TRANSMISSION 178 SATELLITE NEWS VEHICLES–(SNG) 178 EVOLUTION OF SNG VEHICLES 179 PERTINENT DOT REGULATIONS 180 ANALOG OR DIGITAL 180 VOICE COMMUNICATIONS 181 SNG PRIORITIES 182 SAFETY 182 MICROWAVE RADIATION HAZARDS 182 POWER AMPLIFIER AND POWER SUPPLY 183 EQUIPMENT LAYOUT AND HOUSEKEEPING 183 INTERFERENCE MANAGEMENT 183 REVIEW OF COMMON OPERATOR ERRORS 184 REVIEW OF CRITICAL EQUIPMENT ITEMS 184

REFERENCES 185

APPENDICES 186

Copyright 2006 vii All rights reserved Table of Figures FIGURE 1-1 WORLDWIDE SATELLITE COORDINATION...... 3 FIGURE 1-2: U.S. DOMESTIC SERVICE ...... 5 FIGURE 1-3: FREQUENCY BAND NOMENCLATURE ...... 6 FIGURE 1-4: ATMOSPHERIC ATTENUATION AT DIFFERENT FREQUENCY BANDS...... 7

FIGURE 2-0 QUICK REFERENCE LIST OF DECIBELS...... 9 FIGURE 2.1 ILLUMINATION OF A PARABOLIC REFLECTOR ...... 12 FIGURE 2-2 GEOSTATIONARY SATELLITES...... 13 FIGURE 2-3: SATELLITE LAUNCH SEQUENCE...... 14 FIGURE 2-4: NORTH AMERICAN MAGNETIC DECLINATION ...... 15 FIGURE 2-5 GROUND ANTENNA ELEVATION AND AZIMUTH FOR STATIONARY SATELLITES ...... 16 FIGURE 2-6: GEOMETRY OF SUN OUTAGE...... 18 FIGURE 2-7: LINEAR POLARIZATION OF RADIATION FROM VERTICALLY AND HORIZONTALLY POLARIZED FEED HORNS...... 20 FIGURE 2-8: POLARIZATION AND ELEVATION ANGLE VERSUS LATITUDE AND LONGITUDE ...... 21 FIGURE 2-9: C BAND FREQUENCY/POLARIZATION PLAN...... 22 FIGURE 2-10 -U.S. DOMESTIC C-BAND GEOSYNCHRONOUS SATELLITES ...... 23 FIGURE 2-11: - U.S. DOMESTIC KU BAND SATELLITES–AND LOCATION ...... 24 FIGURE 2-12 GENERAL ARRANGEMENT OF THE DUAL SPIN SPACECRAFT...... 26 FIGURE 2-13 SPACECRAFT ORBITAL ASSIGNMENT “BOX” ...... 27 FIGURE 2-14: THREE AXIS OR BODY STABILIZED SPACECRAFT ...... 28 FIGURE 2-15: GEOMETRY OF ORBITAL INCLINATION...... 29 FIGURE 2-16: REPRESENTATIVE DAILY SATELLITE PATH OF AN INCLINED ORBIT SATELLITE ...... 30 FIGURE 2-17: SIMPLIFIED BLOCK DIAGRAM OF COMMUNICATIONS PAYLOAD ...... 31 FIGURE 2-18 TYPICAL WIDEBAND SATELLITE COMMUNICATIONS RECEIVER...... 32 FIGURE 2-19 INTERMODULATION EFFECTS...... 33 FIGURE 2-20: INTERMODULATION EFFECTS FROM MULTIPLE CARRIERS...... 34 FIGURE 2-21 TYPICAL INPUT/OUTPUT AMPLIFIER CHARACTERISTIC ...... 35 FIGURE 2-22 GENERATION OF A SHAPED BEAM ANTENNA PATTERN USING MULTIPLE FEED HORNS AND AN ASSOCIATED FEED NETWORK...... 37 FIGURE 2-23 SPACECRAFT ANTENNA BEAM SHAPING COVERAGE OF MEXICO...... 37 FIGURE 2-24 POWER FLUX DENSITY...... 39 FIGURE 2-25 GALAXY IV TRANSPONDER 23 G/T (DBK)...... 40 FIGURE 2-26 EIRP FOOTPRINT...... 43 FIGURE 2-27 MAJOR CONTRIBUTORS TO ANTENNA NOISE IN THE SATELLITE RECEIVER.. 46 FIGURE 2-28: BLOCK DIAGRAM SHOWING SATELLITE RECEIVER NOISE CONTRIBUTIONS...... 47 FIGURE 2-29: CHARACTERISTICS OF DIFFERENT METHODS OF SATELLITE ACCESS ...... 49 FIGURE 2-30: TYPICAL FDM TRANSPONDER ACCESS...... 49 FIGURE 2-31: SPREADING A SIGNAL TO PERMIT CDMA OPERATION ...... 50 FIGURE 2-32: SPECTRUM OF A SIGNAL BEFORE AND AFTER SPREADING FOR CDMA ...... 51 FIGURE 2-33: ONE WAY, POINT TO MULTIPOINT LINKS...... 52 FIGURE 2-34: MESH NETWORK ...... 53 FIGURE 2-35: STAR NETWORK TOPOLOGY...... 54 FIGURE 2-36 POWER LEVEL DIAGRAM- UPLINK...... 56 FIGURE 2-37: ANTENNA SIDELOBE DISCRIMINATION...... 58 FIGURE 2-38 UPLINK INTERFERENCE...... 59 FIGURE 2-41: PATH LOSS BETWEEN SYNCHRONOUS ORBIT AND SUB-SATELLITE POINT. .. 65 FIGURE 2-42 FREE SPACE LOSS VERSUS GROUND STATION ELEVATION ANGLE...... 66 FIGURE 2-43 DOWNLINK INTERFERENCE...... 67

Copyright 2006 viii All rights reserved FIGURE 2-44 AMPLIFIER INPUT OUTPUT CHARACTERISTIC SHOWING THEORETICAL THIRD ORDER DISTORTION AND 2 TONES...... 68 FIGURE 2-46: EXAMPLE OF ACCURACY FROM TWO LINES OF POSITION...... 70 FIGURE 2-47: TYPICAL TDOA MEASUREMENT SETUP...... 71 FIGURE 2-48 CAUSE-EFFECT DIAGRAM SHOWING VARIOUS INTERFERENCE SOURCES ...... 72 FIGURE 2-49 RAIN ATTENUATION VS. NOISE TEMPERATURE...... 74 FIGURE 2-50: RAIN ZONE MAPS IN THE US (CRANE MODEL)...... 75 FIGURE 2-51: SPOT BEAM CONFIGURATIONS ...... 80 FIGURE 2-52: TECHNICAL DIFFERENCES BETWEEN C, KU AND KA BAND SATCOM...... 81 FIGURE 2-53 MERITS OF C, KU - AND KA-BAND FOR SATELLITE COMMUNICATIONS...... 82 FIGURE 2-54 EFFECT OF MODULATION INDEX ON FREQUENCY MODULATION SPECTRUM 88 FIGURE 2-55: RELATIONSHIP OF C/N TO SNR IN AN FM CARRIER ...... 88 FIGURE 2-56: NTSC FM MODULATED CARRIER AND SIGNAL TO NOISE RATIO...... 89 FIGURE 2-57: PHASE RELATIONSHIPS IN SIGNALS...... 96 FIGURE 2-59: SIMPLIFIED BLOCK DIAGRAM, TIME DOMAIN OF BIPHASE MODULATOR...... 98 FIGURE 2-60 SIMPLIFIED BLOCK DIAGRAM, BIPHASE DEMODULATOR...... 99 FIGURE 2-61 SIMPLIFIED BLOCK DIAGRAM, QPSK MODULATOR SHOWING (GRAY CODED) PHASE STATES...... 100 FIGURE 2-62 MODULATOR DATA STREAMS FOR QPSK AND OKQPSK...... 101 FIGURE 2-63 RF ENVELOPE FOR QPSK AND OKQPSK SIGNALS...... 102 FIGURE 2-64 PLOT OF THEORETICAL EB/NO VS. BER...... 104 FIGURE 2-65: 8 PSK CHARACTERISTICS...... 104 FIGURE 2-66: 16-QAM CONSTELLATIONS...... 105 FIGURE 2-67: ERROR RATES OF PSK MODULATION SYSTEM...... 106 FIGURE 2-68: COHERENT ORTHOGONAL FREQUENCY DIVISION MULTIPLEX MODULATION ...... 107 FIGURE 2-69: BLOCK ENCODER...... 108 FIGURE 2-70: BLOCK DECODER...... 108 FIGURE 2-71: LDPC PERFORMANCE COMPARISON...... 109 FIGURE 2-72 COMPUTATIONAL BASIS OF LDPC...... 109 FIGURE 2-73: CONVOLUTIONAL ENCODER ...... 110 FIGURE 2-74: VITERBI DECODING OF CONVOLUTIONAL CODING ...... 110 FIGURE 2-75: MODEM PERFORMANCE WITH AND WITHOUT FEC...... 111 FIGURE 2-76: CONCATENATED CODING...... 112 FIGURE 2-77: CONCEPT OF INTERLEAVING...... 112 FIGURE 2-78: DISPERSAL OF ERRORS IN AN INTERLEAVER ...... 113 FIGURE 2-79 NTSC COUNTRIES...... 114 FIGURE 2-80: SECAM COUNTRIES ...... 115 FIGURE 2-81 NTSC SIGNAL...... 115 FIGURE 2-82: NTSC WAVEFORM...... 116 FIGURE 2-83: TYPES OF VIDEO...... 117 FIGURE 2-84 OVERVIEW OF THE A TO D CONVERSION PROCESS ...... 119 FIGURE 2-85: VIDEO SAMPLING FREQUENCIES AND BIT RATES ...... 120 FIGURE 2-86 SAMPLING POINTS (FS = 4F SC) ...... 120 FIGURE 2-86: COMPOSITE QUANTIZING LEVELS ...... 121 FIGURE 2-87: 8 BIT BINARY CODES...... 122 123 FIGURE 2-88 BINARY WORDS FOR BURST SAMPLES ...... 123 FIGURE 2-89: UNCOMPRESSED VIDEO DATA RATES ...... 124 FIGURE 2-90: MOTION COMPENSATION IN MPEG 2...... 125 FIGURE 2-91 ENCODING PROCESS SIMPLIFIED ...... 126 FIGURE 2-92: BASIC ELEMENTS IN MPEG 2 ENCODER...... 129 FIGURE 2-93 MPEG TRANSPORT PACKET STREAM...... 130 FIGURE 2-94 MPEG TRANSPORT STREAM PACKET...... 130

Copyright 2006 ix All rights reserved FIGURE 2-95 MPEG TRANSPORT STREAM PACKET MULTIPLEXING...... 131 FIGURE 2-96 PACKET DEMULTIPLEXING...... 132 FIGURE 2-97 BASIC ELEMENTS OF THE MPEG-2 DECODER...... 132 FIGURE 2-98: COMPLETE DIGITAL SYSTEM ...... 133 FIGURE 2-99 MPEG-2 LEVELS AND PROFILES ...... 135 FIGURE 2-100 MPEG 4-10 ENCODING PROCESS...... 138 FIGURE 2-101: JPEG 2000 PROCESS...... 139 FIGURE 2-102 HDTV STANDARDS AND IMPLEMENTATION ...... 140 FIGURE 2-103 : ATSC DIGITAL TELEVISION LAYERS ...... 141 FIGURE 2-104: FOUR CURRENTLY DEFINED ATSC HDTV FORMATS...... 142 FIGURE 2-105 COMPRESSED VIDEO LINK BUDGET (OUTPUTS IN ITALICS) ...... 144 FIGURE 2-106: DIFFERENCE BETWEEN KU FSS AND BSS ...... 145 FIGURE 2-107 COMPARISON OF DBS SYSTEMS...... 145 FIGURE 2-108 REPRESENTATIVE DIRECTV SPOT BEAM COVERAGE ...... 146

FIGURE 3-1 COMPOSITE SATELLITE EARTH STATION...... 147 FIGURE 3-2 LARGE KU BAND EARTH STATION...... 148 FIGURE 3-3 BASIC ELEMENTS OF AN ANALOG TV EXCITER ...... 150 FIGURE 3-4 DUAL CONVERSION PROCESS – ...... 151 FIGURE 3-5 SUBCARRIER ATIS-BLOCK DIAGRAM...... 152 FIGURE 3-6 DIGITAL EXCITER ...... 153 FIGURE 3-7 SIMPLIFIED BLOCK DIAGRAM OF DIGITAL SCPC UPLINK...... 153 FIGURE 3-8 TWO OR MORE SCPC CHANNELS FEEDING A COMMON UPCONVERTER ...... 154 FIGURE 3-9 CHARACTERISTICS OF DIFFERENT POWER AMPLIFIERS...... 155 FIGURE 3-10 SHARING AN UPLINK WITH MORE THAN ONE ANTENNA...... 157 FIGURE 3-11 SIX CHANNEL FILTER DIPLEXER MULTIPLEXER...... 158 FIGURE 3-12 SIX CHANNEL HYBRID MULTIPLEXER ...... 158 FIGURE 3-13 TWELVE CHANNEL MULTIPLEXER USES FILTERS AND HYBRID ...... 159 FIGURE 3-14 MONITOR AND NON RADIATION TEST APPARATUS...... 160 FIGURE 3-15 GENERAL CONFIGURATION OF LNA’S, LNB’S AND LNC’S...... 161 FIGURE 3-16: L BAND TO C AND KU CONVERSION CHART ...... 162 FIGURE 3-17 EXAMPLE WHERE POST AMPLIFIER IS REQUIRED TO BOOST LEVELS AND DECREASE NOISE...... 163 FIGURE 3-18: INTEGRATED RECEIVER-DECODER...... 164 FIGURE 3-19 PRIME FOCUS AND DUAL REFLECTOR GEOMETRY...... 147 FIGURE 3-20 OFFSET FED ANTENNA GEOMETRY ...... 167 FIGURE 3-21: VIDEO RECEIVE ONLY EARTH STATION...... 170

FIGURE 4.1 CAUSES OF INTERFERENCE (SOURCE: SUIRG)...... 175

CHAPTER 1: INTRODUCTION

This document is intended primarily as a textbook for the training of earth station operators who have responsibility for accessing satellites (uplinks). The primary thrust is directed toward operations with U.S. Domestic satellites. However, the operator must be constantly aware of the fact that he (or she) is a part of a worldwide telecommunications infrastructure. Improper operation of an uplink earth station can adversely affect: 1) the network of which the earth station is a part, 2) other networks in the same satellite, 3) other U.S. domestic satellites, 4) other foreign or regional satellites and 5) international telecommunications traffic.

There can be severe economic consequences due to improper earth station operations. The importance to the network in which the earth station is a part will vary with the circumstances. Each operator should be aware of this and act in accordance with the interests of his (or her) employer or client. Furthermore, interference to other systems can result in criminal prosecution with both fines and/or jail sentences depending on the circumstances. Repeated cases of unintentional or negligent interference can result in fines and/or loss of the FCC license for the earth station.

This text includes a short history of communication satellites in the U.S. and a discussion of the regulatory aspects of U.S. domestic and other satellite systems in this section. Section 2 deals with BASIC CONCEPTS including: a) satellite specifics (orbit/orbit control, communication subsystem, frequency/polarization plans and important parameters), b) directional antennas, c) noise and d) link budgets for various commonly used modulation techniques. Section 3 deals with GROUND EQUIPMENT, and section 4 covers UPLINK OPERATIONS.

BRIEF HISTORY OF COMMUNICATION SATELLITES IN U.S. The U.S. department of Defense and NASA initiated a number of projects in the late 1950s and early 1960s directed toward satellite communications. The first operational commercial communications satellite was Early Bird launched in 1965 for Intelsat followed by Intelsat II in 1966. In 1970, the U.S. government announced an “open skies” policy whereby an entity with the legal, technical and financial capabilities could launch and operate satellites serving the U.S. domestically. The first U.S. domestic satellite to be launched was Westar I (1974). The orbital arc is administered on a global basis by the International Telecommunications Union (ITU), which is an agency of the United Nations (UN). The FCC administers and regulates the Geostationary Orbit for commercial use in the U.S. Domestic Satellite Service. The FCC has authorized satellites to operate in the Geostationary Orbital Arc between 62˚ west longitude to 146˚ west longitude for U.S. Domestic Service.

UNITED NATIONS INTERNATIONAL TELECOMMUNICATIONS UNION

REGION 1 REGION 1 REGION 1 REGION 3 EASTERN WESTERN AFRICA ASIA/AUSTRALIA EUROPE EUROPE

REGION 2 THE AMERICAS

NORTH/CENTRAL SOUTH

AMERICA AMERICA

UNITED CANADA UNITED ORBITAL ARC

STATES MEXICO STATES DEGREES,

143 121 105 62 WEST

Figure 1-1 Worldwide Satellite Coordination

As shown in Figure 1-1, Coordination of all satellites is done by a branch of the International Telecommunications Union (ITU), which in turn is a part of the United Nations (UN). The U.S. Department of State is the official U.S. member of ITU, and the FCC provides support. An application for a space station must be submitted to the FCC in accordance with the FCC rules. The FCC then forwards the pertinent information to the ITU, which then coordinates with other entities for any potential conflict or interference potential. Three classes of satellite services have been established in the U.S. They are: a) Fixed Satellite Service (FSS), b) Broadcast Satellite Service (BSS) and c) Mobile Satellite Service (MSS). All three classes are administered by the International Bureau of the FCC, as shown in Figure 1-2. In South and Central America rules are established by CITEL, the Inter-American Telecommunications Commission.

This course of study will emphasize the FSS. The rules dealing with the FSS are given in the "Code of Federal Regulations 47 Part 25". An uplink operator should be familiar with these rules and maintain the latest published version at the earth station. For uplink stations that operate in the Broadcast Satellite Service (BSS), the rules are given in Code of Federal Regulations 47, Part 100.

FCC

INTERNATIONAL BUREAU

BROADCAST FIXED MOBILE SATELLITE SATELLITE SATELLITE SERVICE SERVICE SERVICE

-4 degree spacing -2 degree spacing -9 degree spacing - circular polarization -linear - circular --L, S, C and Ku Band polarization, polarization -C, Ku, Ka Band --Ku, Ka Band -assignments by channel or frequency -assignments by -assignments by orbital arc location channel or frequency

Figure 1-2: U.S. Domestic Service

There are three frequency bands currently used in the FSS in the United States. They are C Band (5925 to 6425 MHz up, 3700 to 4200 MHz down–commonly called 6/4 GHz), Ku band (14000 to 14500 MHz up, 11700 to 12200 MHz down–commonly called 14/12 GHz and Ka Band, (28,350 to 28600 and 29,250 to 30,000 uplink and 18300 to 18,800 and 19,700 to 20.200 downlink) commonly called 20/30 GHz. Full details are given later in this text. A chart of all frequency bands is given in Figure 1-3.

During classroom sessions, the various characteristics, advantages and disadvantages of the various frequency bands will be discussed. Differentiators include beam sizes available, sharing of the band with other services, and antenna sizes required, as well as the relative amount of atmospheric attenuation that affects the signal as it passes through the earth’s atmosphere. As shown in Figure 1-4, this attenuation differs significantly for the three FSS frequency bands of interest.

FREQUENCY (MHz) DESIGNATION Ref. Data for Radio US Navy RSGB Engineers I 100 - 150 G 150 - 225 P 225 - 390 225 - 390 L 390 - 1,550 390 - 1,550 1,000 - 2,000 S 1,550 - 5,200 1,550 - 3,900 2,000 - 4,000 C 3,900 - 6,200 3,900 - 6,200 4,000 - 8,000 X 5,200 - 10,900 6,200 - 10,900 8,000 - 12,000

15,350 - 15,250 - 12,000 - Ku 10,900 - 17,250 10,900 - 17,250 18,000 - 18,000 K 36,000 36,000 26,500 33,000 - 33,000 - 26,500 - Ka 36,000 36,000 40,000 Q 36,000 - 46,000 36,000 - 46,000 33,000 - 50,000 U 40,000 - 60,000 V 46,000 - 56,000 46,000 - 56,000 W 56,000 - 100,000 56,000 - 100,000

Figure 1-3: Frequency Band Nomenclature

Copyright 2006 6 All Rights Reserved Different authors use different nomenclature for frequency bands. The chart above shows three- a standard engineering text, the US Navy and the Radio Society of Great Britain.

International Service There are satellites other than U.S. domestic ones within the field of view of earth stations located in the U.S. These include satellites serving other countries in North and South America, as well as international satellites located above the Atlantic and Pacific Oceans. Of particular concern are satellites serving the northern hemisphere. They are mentioned here because faulty operation of earth stations in the U.S. domestic service can cause interference to these satellites.

The U.S. has formal arrangements with Canada and Mexico regarding assignments of satellites. South American satellites will be interspersed with North American satellites. Future discussion in this text dealing with the geostationary orbital arc and directional antennas will indicate why reasonably large antennas are required in uplinking. The South American Satellites are, or will be, located as little as one degree or less from U.S. Domestic satellites. 1

0.5 O2

0.2

0.1 H O 0.05 2

0.02 ATTENUATION (dB/km) 0.01

0.004 3106020 30 FREQUENCY (GHz) C-BAND Ku-BAND Ka-BAND

Figure 1-4: Atmospheric Attenuation at Different Frequency Bands

CHAPTER 2: BASIC CONCEPTS

In order to properly operate a satellite uplink earth station, an understanding of the overall infrastructure in which the station is a part is necessary. There are no mysteries here. A satellite is sometimes referred to as a “bent pipe” in space, and the popular press almost always refers to a satellite transmission as a signal “bounced” off of a satellite 22,300 miles away. In fact, most, but not all present day satellites can be called “Microwave Heterodyne Repeaters” and a satellite link can usually be characterized as a two-hop microwave system. Of course, the paths are rather long as compared to terrestrial links and the repeater “tower” is rather tall.

Decibel Notation

Decibel notation is used extensively in this course and in satellite communications, normally when dealing with power and bandwidth. Decibels were invented by engineers as a tool to easily multiply large and small numbers without the need for calculators, computers and slide rules. A tutorial on decibels is given in Appendix B of this text. However, decibels are easy to use when certain principals are understood and a few reference numbers are able to be referred to. Figure 2-0 provides a quick reference list of decibels and their corresponding linear (“normal”) power or bandwidth values.

Linear- (Multiply) dB- (Add) 1 0 dB 1.26 1 dB 2 3.0 dB 3 4.8 dB 4 6.0 dB 5 7.0 dB 7 8.5 dB 10 10 dB 20 13 dB 30 14.8 dB 40 16 dB 50 17 dB 70 18.5 dB 100 20 dB 200 23 dB 1000 30 dB 10,000 40 dB Figure 2-0 Quick Reference list of Decibels

Directional Antennas

Before proceeding with specifics of satellites and satellite links, an initial discussion of directional antennas is warranted, since orderly use of satellites is dependent on antenna characteristics. Antennas will discussed further in other sections of the text, but at this point a few basics should be understood. Antenna Gain The question most frequently asked about antennas is, “How can a passive device, like an antenna, have gain?” The answer is that antenna gain is a measure of how well the antenna concentrates its radiated power in a given direction. Gain is the ratio of the power radiated in a given direction to the power radiated in the same direction by a standard antenna (usually an isotropic radiator). An isotropic radiator is one where the radiated power is the same in all directions (point source).

The gain of an antenna can be related to the effective area (Ar) of its aperture by the formula:

4πA r G = λ 2 Where: λ is wavelength, and

Ar = η Α Where: η is the efficiency, and

A is the actual area of the aperture.

In most satellite earth station applications, a paraboloidal surface is used as the main reflecting surface. Typical values of efficiency in the direction of maximum radiation are 50 to 70 percent, depending on design.

It is instantly obvious that the larger the antenna the higher the (on-axis) gain, assuming that the efficiency is the same. Antenna gain specifications from manufacturers imply maximum (on-axis) radiation relative to an isotropic radiator.

3x108 Solution: λ = = 0.05meters 6.0x109

π D2 Α = πr 2 = 4

2 4π 2 x 0.65 x (10) G = =2.5661.105 4 x().05 2

Gain (in decibels) = 54.1 dB

Radiation Pattern

In a practical antenna, not all of the available power is radiated in (or received) from just one direction. Energy is lost in: 1) feed losses, 2) spillover from feed to reflector(s), 3) forming the main beam, and 4) sidelobes. A typical radiation pattern is shown in Figure 2-1. The shape of the main beam and sidelobe levels is a function of the intrinsic design and mechanical imperfections in the reflecting surface(s).

Figure 2-1 shows what is commonly called the “co-pol” or co-polarization pattern. The cross polarization pattern is also important and will be discussed later.

Further discussion on antennas is contained in Chapter 3. In that section, practical consideration of antenna performance and maintenance are considered.

Figure 2.1 Illumination of a Parabolic Reflector Geostationary Orbit

The earth rotates about its N-S axis at the rate of one revolution per day (24 hours). The moon, which is an earth satellite, rotates about that axis at a rate of about one revolution per month. Low altitude earth satellites such as an orbiting space shuttle operating at an altitude of 150 miles, has a rotational rate of about 90 minutes. An object placed on a line 22,300 statute miles above the earth’s equator will have a rotational rate that is exactly the same as the earth’s rotational rate. This is the well known “Geostationary Orbit”, and has the unique property of being fixed in space relative to all points on the earth. Figure 2-2 shows this unique orbit.

Figure 2-2 Geostationary Satellites

The advantage of this orbit is obvious. Expensive earth station antenna tracking is not required as long as the satellite is kept within the beam of the earth station antenna. Domestic satellites are required to maintain their assigned orbital position within 0.05˚. This puts a practical limit at C Band of about 35 feet diameter (for 0.5 dB loss) on earth station dish size for no tracking function.

Satellite Launch Sequence As shown in Figure 2-3 below, a satellite launched into geosynchronous orbit is first launched into a circular orbit (a) which circles the earth. It then is placed into an elliptical transfer orbit (b) by firing a rocket or expending fuel at the transfer orbit’s perigee. After several rotations of the earth in the transfer orbit, an apogee kick motor or other propellant is fired at the apogee, and the satellite is placed into geosynchronous orbit.

Figure 2-3: Satellite Launch Sequence

Satellite Visibility from Earth

As indicated above, the FCC has authorized U.S. Domestic satellites in the orbital arc from 62˚ to 146˚ west longitude. Assuming there are no local obstacles (buildings, mountains, etc.) line of sight can be maintained from all points in the contiguous 48 states, continental U.S. (CONUS), with greater than 5˚ elevation angle of the earth station antenna. Less than 5˚ is generally undesirable at C Band and less than 10˚ elevation angle is not desirable at Ku Band.

Formulas for calculating the pointing angles of earth station antennas in the northern hemisphere to satellites in the geostationary arc are as follows:

Copyright 2006 14 All rights reserved Skjei Telecom, Inc. ⎛ tan Θ ⎞ True Azimuth, AZ = 180˚ + arc tan ⎜ ⎟ ⎝ sinα ⎠ Where: α is the earth station latitude, and Θ is the relative longitude of the earth station with respect to the satellite longitude. (Satellite longitude minus the earth station longitude).

True Elevation (with respect to earth), ⎛ cosΘcosα − R / D ⎞ EL = - arc tan ⎜ ⎟ ⎝ sin Θ/sin AZ ⎠ where: R is radius of earth (3,957 miles), and D is radius of the satellite orbit (26,244 miles). It should be noted that when attempting to point the antenna, if a compass is used for azimuth, magnetic declination must be taken into account. Figure 2-4 refers.

Figure 2-4: North American Magnetic Declination

Figure 2-5 Ground Antenna Elevation and Azimuth for Stationary Satellites

Example: Earth Station location: Los Angeles

34° 03΄ 30” N. Latitude 118° 07΄ 40” W. Longitude Satellite: SPACENET II @ 69° W. Longitude

Solution: Station Latitude = a = 34.06°

Station Longitude = 118.13° Θ= 69 – 118.13 = -49.13°

tan Θ = - 1.155 sin a = 0.560 cos Θ = 0.654 sin Θ = -0.756 cos a = 0.828

R/D = 0.15077

−1.155 AZ = 180 + arc tan = 180 − 64.13 0.560

AZ = 115.86°

sin AZ = 0.8998

0.654 x 0.828 − .15077 EL = −arctan − 0.756 / 0.8998

EL = - arc tan (-.465)

EL = 24.94°

Sun Outages

As indicated above, geostationary satellites are in an orbital arc above the equator, which means they are in the equatorial plane. During the spring (vernal) and fall

As seen from the ground, the sun seems to pass behind a satellite once a day. During the time when both the satellite and sun are in the earth station antenna field of view, the RF energy from the sun can overpower the signal from the satellite. It is this loss or degradation of signal that is referred to as Sun Outage.

Figure 2-6: Geometry of Sun Outage.

The severity and duration of a sun outage depends on many factors, and will not be dealt with here. However, the date and time of maximum effect is predictable. For practical antennas and for practical satellite signal strengths, an outage will usually occur for 3 or 4 days in each Equinox for a period of 1 to 5 minutes. Satellite operators can assist any user with specific information.

There are commercially available programs that can also provide predictions.

A simplified formula for the outage angle shown in Figure 2-6 is:

Outage angle = 3 dB Beamwidth + apparent radius of sun = 11/F/D + 2.5°

Where: F is downlink frequency in GHz D is diameter of antenna in meters.

For a 5 meter antenna at 4 GHz, the outage angle is approximately 3°.

It should be noted that in the FSS service, orbital slots whether assigned or not call for a uniform 2° spacing between Ku band satellites (12/14 GHz), and C Band satellites (4/6 GHz). This 2° spacing policy has been in effect since 1983.

From an uplink standpoint, it is obvious that to preclude interference to adjacent satellites, a large antenna (narrow beam) and low sidelobes are required.

In future sections of this text, interference considerations are quantified. However, paramount to an uplink operator should be the understanding that his (or her) antenna should be pointed accurately and should meet the FCC standard for sidelobes.

Frequency/Polarization Plan ―U.S. Domestic Details of the satellite communications subsystem (payload) are given in section 2.4 below. However, before describing the spacecraft subsystems, it is worthwhile to discuss the channels of communications of which the satellites are a part. Transponders and Frequencies In the context of this section, a channel of communication and the term, “transponder” are used interchangeably. In spacecraft terms, as we shall see later, a transponder is a channel of communication. A transponder is characterized as having: 1) a center frequency, 2) a usable bandwidth, 3) certain uplink sensitivity and saturation characteristics, 4) certain power output characteristics and 5) coverage (footprint) for characteristics 3 and 4. We will also see in later sections, that a transponder can support more than one channel of communication because of its relatively wide bandwidth. In some cases, multiple channels are modulated on a single carrier. In other cases, a single channel is modulated on a carrier, and a multiplicity of carriers is transmitted through a transponder.

The FCC has set aside 500 MHz of bandwidth for uplinking and downlinking to both the C band and Ku band for the U.S. Domestic FSS Service. The rules do not dictate how this bandwidth is to be utilized. The uplink frequency range at C Band is 5,925 to 6,425 MHz, and the C Band downlink is 3,700 to 4,200 MHz. Elsewhere in the world, “extended C Band is starting to be used, but it has not been assigned in the US at this time. At Ku Band, the uplink frequency range is 14,000 to 14,500 MHz, and the downlink is 11,700 to 12, 200 MHz. Similarly, elsewhere in the world, extended Ku band has been assigned but it has not been assigned by the FCC for US use. The FCC has also set aside assignments for Ka band as discussed in section 1 and these satellites are just now starting to be placed into service.

Copyright 2006 19 All rights reserved Skjei Telecom, Inc. Polarization Electromagnetic waves and antennas are always polarized in some manner. The polarization may be linear or (approximately) circular. Linear polarizations and circular polarizations are aligned in space as shown in Figure 2-7. Most domestic FSS satellites are linearly polarized. A linearly polarized antenna receives maximum power from an incident linearly polarized wave if the tilt angles of the wave and the antenna polarizations are aligned similarly in space). The wave is then said to be co-polarized. As the tilt angle of the wave or antenna rotates from co-polarization, the received power decreases. When the tilt angles are 90˚ apart as shown in Figure 2-7, the antenna is cross polarized to the wave and receives no power from it. The antenna and the wave then have orthogonal polarizations. A given satellite can employ two orthogonal polarizations that exist simultaneously and carry different information without interference. This principle, frequency reuse, is used to increase the “information capacity” of satellites and of the geosynchronous orbit. Early satellites (Westar 1, 2, 3, and SBS 1, 2, 3, 4) utilized a single polarization for transmission. Modern satellites utilize orthogonal linear polarization, and therefore are capable of more channels of communication through frequency reuse. One of the most common errors made in uplink transmission is to transmit on the wrong polarization. Even more common is to have a slightly misadjusted antenna polarization or a defective antenna with poor polarization isolation. Erroneous polarization or poor polarization isolation in the uplink antenna can cause harmful interference to adjacent channels on opposite polarization, or to adjacent satellites on the same channel but opposite polarization.

Figure 2-7: Linear Polarization of Radiation from Vertically and Horizontally Polarized Feed Horns

Copyright 2006 20 All rights reserved Skjei Telecom, Inc. The terms vertical and horizontal polarizations apply to linear orthogonal polarizations of the satellite antenna at the sub-satellite longitude. The polarization angle of an antenna transmitting to or receiving from a satellite will depend on the earth station location. Figure 2-8 shows polarization angle and elevation angle for latitude and longitude of the earth station. This figure should be used as an approximation only. Final adjustment should be made by coordination with the satellite operator.

Figure 2-8: Polarization and Elevation angle Versus Latitude and Longitude

Copyright 2006 21 All rights reserved Skjei Telecom, Inc. For completeness, some satellites not in the domestic FSS arc also used a type of polarization known as “circular” polarization. Intelsat uses this at C band for historical reasons and MSS and BSS (Broadcast Satellites) use this at Ku band to simplify antenna installation for DBS dishes. In circular polarization, the antenna polarity does not need to be specifically aligned relative to the satellite.

C Band A de-facto standard frequency plan has evolved at C band. The FCC places U.S. domestic satellites so that adjacent satellites are of opposite polarization. Figure 2-9 below is a frequency polarization plan that reflects the current situation at C band. CENTER POLARIZATION POLARIZATION FREQUENCY PLAN A PLAN B UPLINK DOWNLINK NO. UP DN NO. UP DN 5945 3720 1 H V 1 V H 5965 3740 2 V H 2 H V 5985 3760 3 H V 3 V H 6005 3780 4 V H 4 H V 6025 3800 5 H V 5 V H 6045 3820 6 V H 6 H V 6065 3840 7 H V 7 V H 6085 3860 8 V H 8 H V 6105 3880 9 H V 9 V H 6125 3900 10 V H 10 H V 6145 3920 11 H V 11 V H 6165 3940 12 V H 12 H V 6185 3960 13 H V 13 V H 6205 3980 14 V H 14 H V 6225 4000 15 H V 15 V H 6245 4020 16 V H 16 H V 6265 4040 17 H V 17 V H 6285 4060 18 V H 18 H V 6305 4080 19 H V 19 V H 6325 4100 20 V H 20 H V 6345 4120 21 H V 21 V H 6365 4140 22 V H 22 H V 6385 4160 23 H V 23 V H 6405 4180 24 V H 24 H V Figure 2-9: C Band Frequency/Polarization Plan

Figure 2-10 below shows current active C band satellites, their orbital location and the transponder/polarization plan.

Copyright 2006 22 All rights reserved Skjei Telecom, Inc. Satellite West Longitude Polarization Plan AMC 8 139 A AMC 7 137 B AMC 10 135 A GALAXY 1R 133 B AMC 11 131 A INTELSAT AMERICA 7 129 B GALAXY 13/HORIZONS 1 127 A GALAXY 12 125 B GALAXY 10R 123 A INTELSAT AMERICA 13 121 B ANIK E2 119 A SATMEX 5 117 B SOLIDARIDAD 2 113 ANIK F1 107 B AMC-2 105 A AMC 1 103 B AMC 4 101 A GALAXY 4R 99 B INTELSAT AMERICA 5 97 A GALAXY 3C 95 B INTELSAT AMERICA 6 93 A GALAXY 11 91 B AMC 3 87 B AMC 9 85 A AMC 6 72 A Figure 2-10 -U.S. Domestic C-Band Geosynchronous Satellites Ku Band At Ku Band there is no current standard (de-facto or mandated) for the operational satellites frequency and polarization plans. A trend appears to be forming similar to the De-Facto C Band plan whereby most modern satellites have 24, 36 MHz transponders with 40 MHz spacing of center frequency on each polarization. The net effect of this

Copyright 2006 23 All rights reserved Skjei Telecom, Inc. non-standardization is that unless careful coordination is done, polarization isolation cannot be counted on in any link involving interference to or from other satellites in the orbital arc. The co-polarized sidelobe response of the ground antenna is the only tool available to the transmitting or receiving earth station to avoid interference. Satellite West Longitude INTELSAT AMERICA 7 129 GALAXY 13/HORIZONS I 127 GALAXY 10R 123 INTELSAT AMERICA 13 121 ANIK E2 118 SATMEX 5 117 SOLIDARIDAD 2 113 ANIK F2 111 ANIK F1 107 AMC 2 105 AMC 1 103 AMC 4 101 GALAXY 4R 99 INTELSAT AMERICA 5 97 GALAXY 3C 95 INTELSAT AMERICA 6 93 GALAXY 11 91 AMC 3 87 AMC 9 85 AMC 5 79 SBS 6 74 AMC 6 72 ESTRELA DO SUL 63 Figure 2-11: - U.S. Domestic Ku Band Satellites–and Location

The Communication Satellite There are two main hardware sections that comprise a communications satellite. They are the communications payload containing the actual radio communications equipment

Copyright 2006 24 All rights reserved Skjei Telecom, Inc. for receiving and transmitting signals, and the spacecraft bus which provides the supporting vehicle to house and operate the payload. Each major section consists of subsystems, which contribute to the efficient functioning of the satellite. Only the fundamentals are given here and mainly as they apply to the necessary skill and knowledge of an uplink operator. For a more complete description, the interested student is urged to read references 1 and 2. Reference 1 is an excellent treatment without mathematical encumbrance. Reference 2 delves more deeply into the mathematics and physics involved in orbital dynamics, and is directed to the engineering professional in the field.

Spacecraft Bus A satellite has a directive antenna on board as part of the communications payload. The satellite antenna pointing and/or attitude control systems affect earth stations accessing it. It is worthwhile therefore, to know and understand some of the imperfections in that orbit.

The treatment here is general in nature; therefore, the operator is urged to obtain specific information on the satellite he (or she) is working with. This information can generally be obtained from the satellite operator/owner.

Stabilization and Station Keeping There are two types of stabilization in present day satellites. They are: 1) spin stabilization and 2) body (three axis) stabilization. Modern satellites are body stabilized to support a more powerful satellite.

Figure 2-12 depicts the elements of a simple spinner. The simple spinner produces a very stable and reliable design. However, it has limited communications capability. The spinner is unconditionally stable, meaning that the spacecraft will stay erect and even correct itself if disturbed by an external force. In the design and construction of the simple spinner, the body and major components are arranged to provide maximum rotational inertia about the spin axis. This produces a drum shape more akin to that of a tuna can than to that of a pencil.

Geosynchronous satellites are assigned specific longitudinal positions above the equator. To reduce adjacent satellite interference, satellite stations or "boxes" are defined at these positions in the east-west and north-south directions, as shown in Figure 2-13.

A satellite’s box size is assigned based upon its operational frequency band and is defined as assigned longitude +0.05° and 0° latitude + 0.05° (box size of 0.10° in the E- W/N-S directions); approximately 45 miles on a side.

Figure 2-12 General Arrangement of the Dual Spin Spacecraft.

It is normal for satellites to move within their box due to gravitational and other effects associated with the Earth, sun, and moon. Normally, the satellite operator will allow the spacecraft to drift from one “end” of the box to the far end before utilizing the spacecraft’s onboard stationkeeping (normally hydrazine) fuel to position it at the opposite end of the box. For this reason, it is important that ground station antennas be aligned when the satellite is passing through box center for peak performance, particularly for large, non-tracking antennas. This information can usually be obtained from the spacecraft operator, normally on his web site.

It should be noted that the entire communications payload is despun. This allows great flexibility in the antenna beam forming through a multiplicity of antenna feeds. It also allows a larger payload. Dynamic stability is much more complex in the dual spinner and the interested student can gain insight in references 1 and 2. Antenna pointing in spinning satellites is usually provided by use of a ground beacon and tracking system. Earth and sun sensors can augment the ground beacon.

.10 Box (Nominal Assigned Orbital Position)

Figure 2-13 Spacecraft Orbital Assignment “Box”

Body stabilized satellites utilize high speed gyroscopes or momentum wheels to provide stiffness in three directions and act as an inertial reference. Figure 2-14 shows the general arrangement. Antennas are usually mounted on the earth facing side, and antenna pointing is augmented by sun and earth sensors.

Power Power to operate a communications satellite is derived from solar cells and a storage battery, which is necessary during periods when the satellite is in eclipse (the period in which the solar cells are not able to provide power because the satellite is in the earth’s shadow). Modern satellites have sufficient battery capacity to withstand these eclipses and provide full time power to all on-board electronics. The battery is an important factor in the life of a satellite and careful conditioning must be exercised by ground control to ensure that the batteries

Figure 2-14: Three Axis or Body Stabilized Spacecraft

Propulsion The process by which a satellite is launched and placed in orbit is beyond the scope of this training course. However, the uplink operator should be aware that a satellite has a propulsion system. An Apogee Kick Motor (AKM) is on board and is used as a retro- rocket to slow the satellite at its proper apogee (22,300 miles above earth) as it crosses the equatorial plane (see Figure 2-15). That is the only function of the AKM.

A system of small thrusters with a supply of hydrazine fuel is also on board. These thrusters can be used to make corrections to a slight error of the main booster system or AKM firing. Should such a need exist for these thrusters, valuable fuel would be consumed and detract from the available fuel for its main function of station keeping and attitude control.

Copyright 2006 28 All rights reserved Skjei Telecom, Inc. There are several factors in space to force a satellite out of a geostationary orbit. The most important ones are: 1) gravity from objects other than earth in our solar system, 2) solar winds, and 3) thermal gradient in the satellite. Imperfections in the stability system necessitate occasional correction in attitude control.

As indicated earlier the FCC mandates a maximum excursion of a satellite to 0.05˚ in both north-south and east-west station keeping (the “box”). To maintain the satellite in the box, fuel is used. A great deal more fuel is used to maintain the “vertical” (north- south) position in the box than is used to maintain the “horizontal” position (east-west),

Sometimes, when a satellite runs low on fuel, the satellite operator will cease north- south stationkeeping. The satellite will gradually trace what appears from earth to be a “figure 8” trajectory within the assigned box. This figure 8 will increase by 0.9 degree per year. Satellites in such an orbit are called “inclined orbit” satellites. Figure 2-15 depicts the geometry of orbital inclination and Figure 2-16 shows the daily track of such a satellite as seen from one earth location.

Figure 2-15: Geometry of Orbital inclination

Figure 2-16: Representative daily satellite path of an inclined orbit satellite

Telemetry and Control A telemetry system of sensors and a transmitter monitors the health and conditions of various elements in the spacecraft. A command receiver and the actuators to control various elements in the spacecraft are also included. The telemetry transmitters and command receiver are connected to an omnidirectional antenna so that communication with the TT and C ground station can be maintained during launch and in emergencies. It is worthwhile to note that several commands can affect an uplink station. In most satellites, ground controlled attenuators can affect the amount of power required from the uplink station. In extreme circumstances, if an uplink station is causing harmful interference, transponders can be turned off thereby cutting the desired channel of communication. The Communications Payload The term bent pipe, used in the introduction of this chapter, was a greatly exaggerated simplification of a communication satellite. It is analogous to a microwave repeater of the heterodyne type, in which, the microwave carrier is merely displaced in frequency and retransmitted without demodulation or further processing. Most current satellites are of this type, although some specialized satellites have been implemented in Europe where on-board processing can increase the communications capacity, by utilizing the maximum available power.

Copyright 2006 30 All rights reserved Skjei Telecom, Inc. Figure 2-17 is a greatly simplified block diagram of a satellite repeater. It should be noted that there are three main subsystems. They are: 1) wideband receiver that is common to all channels, 2) antenna subsystem also common to all channels, and 3) a means for channelization of the signals. For frequency reuse, by polarization isolation, items 1 and 3 are replicated. The antenna is common to both polarizations. The term transponder is a contraction of the words transmitter and responder. The owner or lessee of a transponder of a 24 channel satellite is therefore an owner or lessee of 1/24 of the common equipment, and one of the active transmitters.

Figure 2-17: Simplified Block Diagram of Communications Payload

Wideband Receiver The concept of using a single wideband receiver to accommodate the full frequency range of input signals of a communications system is unique to satellite design. Figure 2-18 depicts the elements contained in this subsystem. Usually, this subsystem is redundant on a 1:1 basis, which means that a frequency reuse system contains four such subsystems.

Figure 2-18 Typical Wideband Satellite Communications Receiver.

A highly stable oscillator consisting of a quartz crystal oscillator and multiplier produce a (low side) local oscillator where:

FLO = FUP – FDOWN

This maintains an upright downlink signal. The stability is such that the most critical of narrowband transmissions and transmissions with critical phase noise requirements are not materially affected. Sufficient gain elements are included to drive the channelized transmitters to saturation.

The uplink operator should be aware of the fact that the design is such that tolerable intermodulation is maintained with the nominal levels to obtain saturation of the channelized transmitters. It is therefore important that the uplink station operate at nominal power dictated by the satellite operator. Too much power could cause harmful interference to other users on the same polarization through intermodulation distortion in this wideband portion of the satellite. Figure 2-19 shows how two signals create an

Copyright 2006 32 All rights reserved Skjei Telecom, Inc. interference signal due to intermodulation, and how the intermodulation components are affected if one of the signal’s level increases. Figure 2-20 shows the effects from more than two signals.

(a)

f =2f -f 3 2 1 f4=2f1-f2

f4 f1 f2 f3 f3 and f4 are intermodulation products produced by third order distortion due to equal level input signals f1 and f2

(b) X dB increase

X dB increase X

2 X dB increase f4 f1 f2 f3

If one of the input signals of condition (a) above is increased by X dB, the intermodulation products are affected as shown.

Figure 2-19 Intermodulation Effects.

Channelization The concept of channelization was introduced earlier in this chapter (2.3). Reference to figure 2-17, will show in a general way how channelization is accomplished in a satellite. The series of circulators and filters to the left of the amplifier constitutes what is known as an input multiplexer. The function of the input multiplexer is to: 1) efficiently transfer all signals to the separate amplifiers (circulator function), and 2) pass the desired signal and reject the unwanted signals to the separate amplifiers (filter function). The uplink operator at this point must realize the importance of the fact that the output of his (or her) station should be contained in the channel which has been assigned to that station. The uplink signal should be at the proper frequency, and be relatively free from spurious outputs or overmodulation (splatter), which can get into other satellite channels.

Figure 2-20: Intermodulation Effects from Multiple Carriers

• The amplifier in the block diagram constitutes the active transmitter in the repeater chain. It is usually a traveling wave tube amplifier (TWTA), although in some satellites, solid-state power amplifiers (SSPA) are used. The need for reliability in this transmitter is obvious. Redundancy is included in modern satellites. Normal protection is 5:4 or 3:2 meaning that one spare is available for 4 or 2 operating amplifiers, respectively. Ground controlled attenuators are included in the channels. • Transponder Amplifiers typically consist of two amplifier stages and a common electric power conditioner (EPC): 1 The first stage is the Driver Amplifier (DA)

Copyright 2006 34 All rights reserved Skjei Telecom, Inc. • Typically, the DA is a high gain, low power, broadband, solid state amplifier • The DA provides the commandable gain control for the transponder • Some DA units also have an automatic level control circuit that maintains the output signal level constant as the input signal level varies over a large range 2. The second stage is the Power Amplifier (PA) • Typically, the PA is a high gain, high power, broadband amplifier • The PA provides the RF power required for the downlink EIRP • Some PA units also have a linearizer that functions to optimize the phase & amplitude and which permit the transponder to operate at reduced backoff for the same level of intermodulation products. The amplifiers have an input/output characteristic generally common to all amplifiers. A typical input/output curve is shown in figure 2-21. Of special interest to uplink operators is the fact that SSPA’s will have a slightly different saturation characteristic. The TWTA type has a soft saturation whereas the SSPA has a harder saturation. The net effect is that the SSPA will have a slightly greater linear range than the TWTA, and the output will droop less with input drive beyond saturation. From the uplink operation standpoint, he (or she) should realize:

Figure 2-21 Typical Input/Output Amplifier Characteristic.

Copyright 2006 35 All rights reserved Skjei Telecom, Inc. 1. For a single carrier in the amplifier, increasing the uplink power will not increase the output power beyond saturation. Beyond saturation, the output power will probably decrease.

2. For multiple carriers in the amplifier, the satellite operator will operate the amplifier with less than maximum total output power (back off) in order to control intermodulation in the amplifier. The actual back off power is at the discretion of the satellite operator and normally varies from 2.5 to 4 dB. The satellite operator in this case will normally assign the amount of power taken from the satellite by each carrier. This is controlled by the user’s uplink power.

In any case, the uplink station should operate with the assigned power output and not exceed assigned levels, because to do so would drive the amplifier into saturation, and intermodulation interference would be generated which would degrade or disrupt service to all users in the transponders.

Again referring to figure 2-17, the filters on the output of the amplifier are used to efficiently transfer energy from the amplifiers to the antenna. This complement of parts is known as the output multiplexer.

Antenna Subsystem From synchronous altitude, the earth subtends a solid angle of about 19˚. To transmit and receive signals to and from earth would require an antenna with this beamwidth. This could be accomplished with a simple flared waveguide horn. Indeed this kind of simple horn antenna is used in international satellites for global coverage. For domestic (U.S. or foreign) systems, such an antenna would not only be wasteful of power, it would seriously hamper the number of satellites which can be used for domestic service, or place an unreasonable burden on earth stations accessing those satellites. In the case of the Continental United States (CONUS), a beam with about 3˚ (North-South) and 8˚ (East-West) is required.

Figure 2-22 shows in a general way how shaped beams can be formed using a single parabolic reflector and a multiplicity of feed horns. Figures 2-23 shows how a domestic land mass (Mexico) can be efficiently covered by use of beam shaping and demonstrating that a Shaped Beam is More Efficient than an Elliptical Beam. .

Figure 2-22 Generation of a Shaped Beam Antenna Pattern Using Multiple Feed Horns and an Associated Feed Network.

Figure 2-23 Spacecraft Antenna Beam Shaping Coverage of Mexico

Copyright 2006 37 All rights reserved Skjei Telecom, Inc. In the case of U.S. domestic satellites, some satellites have antennas that have coverage that is more favorable to densely populated areas. Other satellites have east and west spot beams. Still other proposed satellites have time zone beams. Almost all have very narrow spot beams covering Hawaii or Puerto Rico. Some proposed systems utilizing time zone beams, can use the spatial or geographic isolation in the beams to re-use the same frequencies in addition to re-use though polarization isolation.

It should be apparent that as the beamwidth of a satellite antenna system is narrowed by beam shaping or by use of large antennas, there is a concomitant requirement for stability in antenna pointing and/or attitude control. A shaped beam antenna will have a steep drop off at edges of coverage as compared to a simple beam. The operator of a fixed earth station should be in contact with the satellite operator to know what the variation in coverage might be. For satellites with ground beacon pointing, the variations will be minimal. For satellites with earth and sun sensors, the variations could be substantial at the edge of coverage.

Satellite Characteristics (Footprints) So far in our study of the basic concepts, we have generalized about directional antennas, and satellite specific items such as orbit and orbital control, and the various spacecraft subsystems as to how they relate to the uplink operator. In this section, we will be more specific about the important satellite characteristics that are sometimes called footprints. These characteristics are important in determining the ultimate performance of a satellite link. For the uplink, the important characteristics are the Gain to Temperature Ratio (G/T) and the Saturation Flux Density (SFD). The two are related, and a single footprint can characterize both. On the downlink, the single important characteristic is Effective Isotropic Radiated Power (EIRP). Saturation Flux Density (SFD, and G/T) In any radio link, the amount of transmitted power required for a certain desired performance, is dependent on the losses in the link and the sensitivity of the receiver (absent external interference). In a satellite uplink, the amount of power required is dependent on the location of the station relative to the satellite because: 1) the slant range to the satellite is different (although slight) and, 2) there is a directive antenna on the satellite, whose gain will vary depending on the direction of the earth station with respect to the direction of maximum gain.

The concept of flux density can be explained by reference to figure 2-24. From the satellite, the earth station antenna, for all practical purposes, looks like a point source with an effective isotropic radiated power in the direction of the satellite. This power must traverse a distance d. From the inverse square law, the power at the satellite is reduced by, 1/ (4πd 2 ). The power flux density at the satellite is therefore:

Distance d

EIRP

Point Source p (Power flux density) Radiator EIRP p = ()from inverse square law 4 π d 2

Figure 2-24 Power Flux Density.

⎛ dBW ⎞ ⎛ dBW ⎞ p⎜ ⎟ =[]EIRP ()dBW −10log4π d 2 ⎜ ⎟ ⎝ m2 ⎠ ⎝ m2 ⎠

where: d is synchronous altitude in meters = 3.59 x 107 meters

dBW p = []EIRP()dBW −162.09 m2

The term 1/ (4πd 2 ) represents isotropic loss in the link and is independent of frequency. It is a convenient number, as we shall see in the link analysis that follows.

Fortunately, satellite operators simplify the problem by publishing footprints of the SFD and/or the G/T. Figure 2-25 is a footprint of G/T for transponder 23, a 36 MHz C band transponder on Galaxy 4 (no longer in operation).

NOTE: A user of a satellite transponder (owner or lessee) or an entity in serious negotiation for purchase or lease is entitled to obtain “Transponder Specific Footprints.”

Figure 2-25 Galaxy IV Transponder 23 G/T (dBK).

As examples of how a G/T footprint can be used by an uplink operation consider:

1. An earth station in Seattle wishes to access Galaxy IV–transponder 23 to transmit a wideband television transmission (saturated carrier). It has an antenna with a 10 meter diameter whose gain is 53 dB, and there is a 3 dB loss between the HPA and the antenna. What HPA power output is required? The spacecraft has a 6 dB attenuator in the transponder used.

Solution:

From figure 2-25;

(a) The G/T in Seattle is:

+2 dB/K

(b) Therefore,

SFD = - (+2 + 89) + 6 (Pad) = - 85 dBW/m2

SFD = (EIRP – 162.1) dBW m2 and

EIRP = (PA + GA) dBW

PA = (PT – 3) dBW

(d) Therefore PT = (SFD + 162.1 – GA + 3) dBW

= - 85 + 162.1 – 53 + 3 = 27.1 dBW = 513 watts

2. An earth station in Honolulu is accessing Galaxy 4 transponder 23, 36 MHz transponder with a radio (audio) signal utilizing 5% of the available power in the satellite. In this (partial transponder) service, Galaxy 4 operates this transponder with a total output power back off of 5 dB which corresponds to an input power back off of 9 dB. The earth station antenna gain is 53 dB and the transmission line loss is 3 dB. What transmitter power is required? The spacecraft has a 3 dB attenuator installed in this channel.

Solution:

From Figure 2-25

(a) The G/T in Honolulu is – 5 dB/K (b) Therefore,

SFD = - (-5 + 89) + 3 = - 81 dBW/m2

FD = SFD – BOI – PD

(d) Since the carrier takes only 5% of the total available power,

PD = 10 log .05 = -13dB

(e) Flux density at the satellite is:

FD = –81 – 13 – 9 = –103 dBW/m2

PT = –103 + 162.1 – 53 + 3 = 9.1 dBW = 8.1 Watts

It should be noted that the G/T footprint of Figure 2-25 represents a specific transponder on Galaxy 4. Most operators have in their data bank actual footprints for a particular transponder. In some cases, data resides in computer memory and specific SFD or G/T information can be obtained based on the geographic coordinates of the uplink station.

Another factor that should be noted is that in the calculation of isotropic loss,

⎛ 1 ⎞ ⎜10 log ⎟ ⎝ 4πd 2 ⎠

The distance used was the synchronous altitude of a satellite, not the slant range from earth station to satellite.

Effective Isotropic Radiated Power (EIRP) By definition, EIRP is the product of power into an antenna and the gain of the antenna referenced to an isotropic radiator. Up to now in this course of study, we have used the term EIRP, but we have only considered the maximum or on-axis gain of directional antennas. We have also discussed the nature of directional antennas where a beam is formed along with unavoidable sidelobes. In the section on satellite antennas, we discussed beam shaping techniques. In an ideal situation, the spacecraft antenna designed would like to shape the beam for uniform gain over the coverage area with low loss. We live in an imperfect world and coverage is not uniform. EIRP should properly be expressed as:

EIRP = PAGA ()Θ

Where Θ is the angle off boresight and GA (Θ) is antenna gain at angle Θ.

This concept is important in the future discussion on earth station antennas and their potential for interference to other systems.

Satellite operators publish EIRP footprints. Figure 2-26 is such a footprint for transponder 23 on Galaxy 4. EIRP footprints apply to the saturated EIRP from the satellite. If an uplink station does not drive the transponder to saturation, or if it overdrives the transponder, the EIRP will be reduced in accordance with Figure 2-21.

Satellite EIRP applies to the downlink signal, and therefore represents an important parameter in the design of a receiving earth station.

Figure 2-26 EIRP Footprint. Noise Noise in the context of this course of training is defined as any undesired signal in a communication circuit. The discussion deals with two basic categories of undesired signals. They are: 1) thermal noise, and 2) interference noise. In our treatment of thermal noise, we will include a category called antenna noise because in satellite technology it is intrinsic and has the same characteristics as thermal noise. Interference will be treated in more detail in later sections dealing with the overall satellite link. Thermal Noise Thermal noise is a result of random electron motion. It is characterized by a uniform energy distribution over a given frequency bandwidth, and a normal or Gaussian distribution of levels. In our treatment of thermal noise, we will use the temperature scale normally used in scientific work dealing with the MKS system of measures. This is known as the absolute or Kelvin scale. There is a one-for-one correspondence with the Celsius scale. The relation is:

Kelvins = ˚ C + 273.18

At absolute zero degrees (0˚ K or – 273.18˚ C), there is no molecular motion. No molecular motion means no thermal noise.

Copyright 2006 43 All rights reserved Skjei Telecom, Inc. Thermal noise power is proportional to bandwidth and absolute temperature. The connecting relationship is Boltzmann’s constant and is mathematically expressed as:

N (noise power) = kTB

where: k is Boltzmann’s constant 1.38 x 10 –23 Joules/K, T is temperature in Kelvins, and B is effective noise bandwidth in Hz.

Noise factor is a measure of the noise produced by a practical network compared to an ideal network (i.e. one that is noiseless). Another way of defining the term is the amount of excess noise on the output of a device over the amount of noise that would be present if the device were ideal.

Expressing this in a formula

N Noise Factor = NF = practical Nideal

An ideal (noiseless) receiver would have on its output a noise power of:

N ideal = GkT0B

Where T0 is ambient temperature, G is gain of the receiver, k is Boltzmann’s constant, and B is the effective noise bandwidth in Hz.

A practical receiver would have an equivalent input noise temperature Te and the output noise due to its internal noise would be:

N int = GkTeB

The output of the practical receiver therefore is:

N pract = N ideal + N int = Gk (T0 + Te) B

Gk(T T )B Noise Factor = 0 + e GkT0B

T T = 0 + e T0

T = 1+ e T0 or alternatively, the equivalent noise temperature:

Te = T0 (NF – 1)

Noise figure is the noise factor expressed in decibels:

F dB = 10 log 10 NF

Example: A receiver has a noise figure of 1.5 dB. What is its equivalent noise temperature?

1.5 NF = alog = 10.15 = 1.4125 10 Te = 290 ()1.4125 − 1 = 120° K

Antenna Noise One of the least understood aspects of satellite technology by practitioners is the concept of antenna noise. The question arising most frequently is, “How can a passive device create noise?” The answer lies in the fact that the environment in which the antenna is placed is not free from noise and therefore in addition to picking up the desired signal, the antenna also picks up this noise. In most terrestrial microwave applications this noise is small compared to the receiver noise and for all practical purposes, it can be ignored. However, in a satellite application it represents a substantial part of the total system noise.

As indicated above, the environment is polluted by a variety of radio frequency energy. Most of these pollutants meet the test of the definition for thermal noise given above. That is, it has a uniform frequency spectrum and has a Gaussian level distribution. Some noise is man-made as a result of electric motors, neon signs, power lines, ignition systems and a plethora of industrial, scientific and medical instrumentation. Fortunately, virtually all of this type of pollution is at low frequency and is not a factor in satellite communication. Broadcast applications and HF communications (2 to 30 MHz) are plagued with man-made noise and it is even extended into the VHF and UHF frequencies in some urban areas. In satellite microwave systems however, the noise in the environment comes from natural sources.

Under normal conditions, there are three major contributors to antenna noise in a satellite receiving system. They are: 1) galactic noise, 2) reflections from a hot earth and 3) moisture absorption in the atmosphere.

In an earlier section, we discussed the abnormal situation called sun outage, where the radiation from the sun can completely overwhelm the satellite signal.

Galactic noise arises from a universe that has an almost infinite number of stars in various stages. At 4 GHz the range of galactic noise is 8˚ to 12˚ K for practical antennas.

Copyright 2006 45 All rights reserved Skjei Telecom, Inc. The earth temperature varies by a small amount around 17˚ C (290˚ K). This energy arrives at the antenna output terminals through the sidelobes of the antenna and is reduced by the sidelobes. The amount depends on sidelobe levels and the elevation angle. It may be of some interest to note that the antenna on board a domestic satellite has a minimum noise temperature of 290˚ K since it points at the earth.

Moisture in the atmosphere contributes a significant amount of noise, particularly at certain frequency bands, as discussed previously. The amount of noise contributed also depends on elevation angle in that the length of the path through the moisture laden atmosphere decreases as elevation angle increases. Figure 2-27 graphically shows he major contributors to antenna noise in the satellite receiver. For high elevation angles (more than 20 degrees), noise is fairly constant between 20 and 30 degrees Kelvin.

Figure 2-27 Major Contributors To Antenna Noise In The Satellite Receiver. Receiver Noise Temperature (Clear Weather) The block diagram of Figure 2-28 shows a receiver consisting of: 1) antenna and feed, 2) losses (L1) between antenna and LNA, 3) LNA 4) losses (L2) between LNA and receiver/demodulator and 5) receiver/demodulator. Loss factor L1 consists of resistive feed and waveguide loss and mismatch loss that arises from imperfect impedance match between antenna and LNA.

Figure 2-28: Block Diagram Showing Satellite Receiver Noise Contributions.

Loss factor L2 consists of cable and/or power divider losses between LNA and the receiver/demodulator.

Referenced to the input of the LNA, the effective noise temperature of the system is calculated as follows, using the equation in figure 2-28. Where To is ambient temperature in Kelvins (290 K) L1 is loss (power ratio) Ta is antenna noise temperature Tp is LNA noise temperature L2 is loss (power ratio) Gp is gain of LNA Tr is receiver noise temperature

Practical values for these factors are:

Ta = 30 K (elevation angle greater than 20) L1 = 0.2 dB = 1.047 L2 = 10 dB = 10 Tp = 120 K Gp = 50 dB = 100,000 Tr = (noise figure of 15 dB = 290 (31.6 – 1) = 8880 39 ()1.047 −1 290 (10 −1) 290 (10)(8880) T = + +120 + + eff 1.047 1.047 100,000 100,000

Teff= 28.6 + 13 + 120 + 0.026 + 0.888

= 162.5˚ k

If the losses L1 and L2 are 0 dB (1.0), the expression reduces to:

TR Teff = Ta + Tp + GR

= 30 + 120 + 0.09 = 150.09˚ K

Power Addition of Noise

Since noise is random in nature, addition of two or more noise signals is done on a power (incoherent) basis. Thus,

Nt = N1 + N2 - - - - +Nn

Example: N1 = -30 dBm N2 = -40 dBm

−3 −4 NG = 10log(10 +10 )

= 10log.0011

= -29.586 dBm

Satellite Access Methods The term “satellite access” is used to describe the method employed to permit multiple users to utilize a satellite transponder. In general, three different methods of satellite access are in use today: Frequency Division Multiple Access, or FDMA, Time Division Multiple Access, or TDMA and Code Division Multiple Access, or CDMA. Figure 2-29 below describes the characteristics of these methods.

FDMA (SCPC) TDM/TDMA CDMA

Advantages Simple, low cost Flexibility of BW Simple equipment reallocation. Efficiency

Disadvantages Not flexible on short Cost, requires control Requires power term basis. Inefficient station control use of transponder power Usage today Very Common Common for VSATs Not common but increasing

Other Can allow greater Licensing issues, bandwidth per plus and minus transponder

Figure 2-29: Characteristics of Different Methods of Satellite Access FDMA, sometimes called SCPC or Single Channel per Carrier, involves assignment of each satellite access, or user, to a unique and different frequency range. No two users occupy the same bandwidth at any time. This requires that the satellite transponder be backed off to prevent intermodulation products and thus results in inefficient use of the satellite power. Figure 2-30 shows an example of such access.

Figure 2-30: Typical FDM Transponder Access

Copyright 2006 49 All rights reserved Skjei Telecom, Inc. This inefficiency can be avoided if TDMA or TDM access is used. If only one carrier accesses the transponder, it can be operated at Saturation. In this type of access, users are separated not in frequency but in time, with each user instantaneously occupying the full signal bandwidth, but only for a short time, typically called a time slot (if TDM) or a burst (if TDMA). This method is sometimes also called MCPC or Multiple Channel per Carrier. It should be noted, however, that while TDMA, TDM and MCPC type carriers can enjoy the efficiency of full transponder power, they can also be operated in transponder backed off mode if the full transponder bandwidth is not used.

A third but uncommonly used technique, code division multiple access (CDMA) or spread spectrum, shares the transponders by allowing coded signals to overlap in time and frequency, essentially to be layered in the transponder. CDMA involves users sharing the same bandwidth at the same time but using different codes such that each users signal can be accurately recovered if the correct code is known. Figure 2-31 shows how the signal is spread using CDMA and Figure 2-32 shows the signal before and after spreading.

Figure 2-31: Spreading a Signal to Permit CDMA Operation

Figure 2-32: Spectrum of a Signal Before and after Spreading for CDMA

Links and Networking Satellite links can be divided into one way and bi-directional (two way) links. These will be discussed briefly below. One Way (Broadcast) Links One of, if the most important, aspect of satellite communications is its point to multipoint capability, as shown in Figure 2-33 below. The capability of “connectivity” is exploited by these links, wherein all receivers within the coverage area are able to simultaneously receive a given transmission. These links are typically characterized by larger transmitting antennas and small, low cost receive antennas. The disadvantage of these links is that if a given receiver fails to receive the signal, it is unable to request a retransmission unless a separate path is available.

Figure 2-33: One Way, Point to Multipoint Links

Two Way (Bidirectional) Links Two way satellite links can be characterized as either point-to-point or networked links.

Point to Point Links Point to point satellite links are normally only employed where a mobile platform is involved or where terrestrial transmission methods are not present or are prohibitively expensive, for example a link from Hawaii to Antarctica. The exception to this would be a disaster recovery backup link put in place to provide communications in the event of terrestrial link failure.

Networks Satellite networks are very popular for a variety of purposes: internet communications, mobile platform communications, rural communications, etc. Two basic types of networks exist, although many variants of these two types are in existence: mesh networks and star networks.

Copyright 2006 52 All rights reserved Skjei Telecom, Inc. Mesh Networks The primary characteristics of mesh networks is that any site can communicate with any other site using only one satellite “hop” or connection. This is shown in Figure 2-34 below. This type network minimizes satellite delay time (latency) and also utilizes less bandwidth than other types of network, because each connection requires only one uplink and one downlink per simplex channel. Also, mesh networks can be the simplest networks, and the most reliable, because they do not rely on a central control station or hub. As shown in the figure, each station transmits, or is cable of transmitting, a link to the other stations (in this case 3 other stations) and is similarly capable of receiving from 3 other stations. In the past, this type network was prohibitively expensive, requiring expensive terminals and placing severe demands on satellite power and bandwidth. This is no longer the case, and mesh networks are today both cost effective from a terminal and a satellite bandwidth standpoint. They can be used for either circuit switched or packet switched data. They can employ Demand Assigned Multiple Access, or DAMA controllers to further minimize the satellite bandwidth required. Good applications for mesh networks include all types of voice connectivity in which both parties utilize satellite terminals, disaster recovery (terrestrial network replication), videoconferencing, satellite newsgathering, and any other application in which transmission delay or latency is important.

Figure 2-34: Mesh Network

Figure 2-35: Star Network Topology

In a star network, remote terminals can only talk to the central hub, which controls the network and allocates satellite bandwidth to the remotes based on their requests or according to a fixed schedule. This type of network has the advantage that the remote terminal cost is minimized as is satellite bandwidth for networks with varying bandwidth demands per terminal. The hub cost, however, is typically high because the hub is a larger antenna, has sophisticated control and monitoring equipment and requires a high degree of redundancy because a hub outage results in a total network outage. Typical applications for star networks include interactive data networks ( e.g. point of sale credit card approval) utilizing store and forward data transmission, internet access, sub-meter VSAT terminals such as mobile terminals, and voice networks in which one party is connected terrestrially to the hub.

Copyright 2006 54 All rights reserved Skjei Telecom, Inc. Factors in Choosing a Network Type Many factors are involved in selecting the type of network for a particular application. Some of the most important are: • Permanently mounted or transportable/deployable terminals • Network topology: star versus mesh • Reliability (susceptibility to single point failure) • Fixed or demand assignment of satellite resources • Type of communications: voice, data, video • Bandwidth required and flexibility in expansion of bandwidth • Capital and operating cost • User needs • Security requirements

The Earth – Satellite Link Power Considerations in the Uplink In any radio link regardless of the form of modulation used, the performance depends on the ratio of the carrier power-to-noise power (C/N) or the ratio of the carrier power to the noise power density (C/No). The latter expression is most often used with digital links for reasons that are provided later in the text. At this point, however, this discussion will center on the derivation of the C/N ratio, however, as it is more readily understood.

Consider the power level diagram of Figure 2.36 below. An uplink transmitter has power output PT. Some loss of power is taken on the way to an antenna (LT). The antenna provides gain (GA) relative to an isotropic antenna, and a loss of power is taken in the path to the satellite (LS). An antenna on board the satellite provides gain (Gas) and power is delivered to the satellite receiver (PR).

Figure 2-36 Power Level Diagram- Uplink Uplink Thermal Carrier to Noise Ratio

The expression for received satellite power (PR) represents the carrier power at the satellite. Subtracting the thermal noise (N) in decibels yields an expression for the carrier to noise ratio (C/N)UP:

()C / N UP = PR − N = PT − LT + GAT − LS + GAS − N But, N = (K + T + B) dB

And, PT – L + GAT = EIRPUP

Therefore:

(C/N)UP = EIRPUP – LS + GAS – k – T – B

A more general expression can be derived by performing some manipulations based on the following:

(a) (G -T) in dB = G/T (b) Flux Density at the satellite, FD = EIRPUP – Li where Li is isotropic loss.

Where λ is wavelength

(d) The flux density at the satellite is related to saturation flux density by: Flux Density (FD) = Saturation Flux Density – Input Backoff – Power Division Loss

FD = SFD – BOI – PD

Combining (b), (c) and (d),

EIRPUP – LS = SFD – BOI- PD + Li – (Li-Gi) =SFD – BOI – PD + Gi

Substituting produces the general expression for (C/N)UP:

(C/N)UP = [SFD – BOI – PD + Gi + (G/T)SAT – k – B] dB.

For a transponder operating at saturation, BOI and PD are 0 dB, and the expression reduces to:

(C/N)UP (Saturated) = [SFD + Gi + (G/T)SAT – k – B] dB.

Interference in the Uplink In this treatment of interference in the uplink, consideration is given to how an earth station can cause interference into other systems, and how interference from other systems can affect operations of a system to which an earth station is a part.

Antenna Sidelobe Discrimination In a previous section, we discussed EIRP as it applies to the downlink signal, and referred to the fact that EIRP at angles off-boresight of uplink antennas is important. Consider figure 2-37. It is obvious that if a satellite is located at Θ1, only slight discrimination is given to this satellite. If the satellite is located at Θ2, it is afforded discrimination from interference by the sidelobe characteristics of the antenna, which is specifed by the FCC.

Figure 2-37: Antenna Sidelobe Discrimination

Uplink Carrier to Interference Ratio Now, consider figure 2-38. It shows several earth stations accessing several satellites that are spaced Θ degrees apart. The power flux density at satellite A consists of power flux density from earth station A (desired signal –C), and the sum of all flux density from the other earth stations accessing other satellites (interference –I).

Figure 2-38 Uplink Interference.

Expressed as an equation:

N ()C / I UP = FDA − ∑⊕ FDi i−1 where ⊕ denotes power summation.

But flux density is proportional to EIRP. Therefore:

N ()C / I UP = EIRPA − ∑⊕ []EIRPi − (Gi − Goi ) i−1 where Gi is the (on-axis gain of an interfering earth station antenna, and Goi is the gain of that earth station antenna in the direction of the satellite which is being interfered with.

The interference power can enter one of several transponders. See figure 2-39 In this example the interference signal is at the same frequency and occupies the same bandwidth as transponder number 5 of the interfered with satellite. Depending on its polarization, it can affect transponders 5 and 6 as well as transponder 5. It should also be obvious that even if the signal labeled interfering earth station signal is the desired signal to transponder 5, if its polarization is not co-polarized with transponder 5, interference into transponder 4 and 6 will occur. The equation for C/I given above must therefore be modified to account for polarization effects and frequency effects. These

Copyright 2006 59 All rights reserved Skjei Telecom, Inc. effects are actually improvement factors. Each interference component is therefore reduced by the fact that there may be a cross polarization component (polarization improvement) or that the entire spectral power of the interference may not enter the interfered with channel (frequency improvement).

The complete expression for (C/I)UP is:

N ()C / I UP = EIRPA − ∑⊕ []EIRPi − (Gi − Goi )+ Fi + Pi i=1

Where, Fi is frequency improvement factor Pi is polarization improvement factor. To illustrate how satellite communications can be affected by uplink interference let us take a few examples:

Adjacent Satellite Interference

Galaxy 3 at 95 W 2 4 6 8 10 OOOA

1 3 5 7 9 11 OOM A O

Intelsat Americas 5 at 97 W 1 357 9 11 A O A O 2 4 6 8 10 S M AAA

Galaxy 4R at 99W 2 4 6 8 10 M S AA 1 3 5 7 9 11 SSSA

AMC 4 at 101 1 3 5 7 9 11 S OOA 2 4 6 8 10 AAS S

AMC-1 at 103 2 4 6 8 10 AA 1 3 579 11 AAAM

Legend: A= Analog Video O = Occasional Use M= Digital Full Transponder August 9, 2004 S= Digital SCPC

Figure 2-39 Example: Frequency and Polarization Effects in Uplink Interference.

Example A. Assume that the desired channel of communication is an FM/TV signal which saturates the transponder it is contained in. Assume further, that it is a C Band transmission, and that all of the C band satellites in the orbital arc are identical (homogeneous space segment) and are uniformly 2º apart. Also assume that all other transponders on the same, and adjacent frequencies are also carrying saturating FM/TV signals. Furthermore, assume that adjacent satellites are cross polarized on the uplink. Under these conditions the (on axis) EIRP from every uplink station would be the same, and the interference power from each station would depend entirely on its antenna discrimination or the difference between maximum gain and off axis gain.

Assume all ground antennas are 10 meters in diameter (Gain = 53 dB).

1. Interference from earth stations accessing satellites 2º from the desired satellite:

(a) Co-channel, cross polarized (Fi = 0dB)

()C / I U1 = 53 − [19 − 25log2]− 3(2 earth stations) =53 – 11.5 – 3 = 38.5dB.

(b) Adjacent channel, co-polarized (Fi = -6.5dB)*

()C / N U 2 = 53 − [29 − 25log2]+ 6.5 − 6(4 int erferers) = 53 – 21.5 + 6.5-6 = 32dB

Note: Fi = 6.5 dB denotes a yellow mask spectrum for the TV signal, and is used in international satellite coordination.

This is considered a rather conservative criteria representing worst case. A more commonly used “mask” is used by the FCC in domestic satellite analyses where Fi = 15 dB for FM/TV into FM/TV on adjacent channels that are co-polarized.

2. Interference from earth stations accessing satellites 4º apart.

(a) Co-channel (co-polarized) – (Fi = 0)

()C / I U 3 = 53 − [29 − 25log4]− 3 = 53 – 13.9 – 3 = 36 dB

(b) Adjacent channel, cross polarized) – (Fi = -6.5)

()C / I U 4 = 53 − [19 − 25log4]+ 6.5 − 6 = 53 – 3.9 + 6.5-6

Power addition yields:

()C / I UP = (C / I )U1 ⊕ (C / I )Us ⊕ (C / I )U 3 ⊕ (C / I )U 4 ⊕ = 38.5 ⊕ 32 ⊕ 36 ⊕ 49.6 = 29.8 dB This example is highly idealized, and does not represent the real world, because 1) not all adjacent satellites are cross polarized on the uplink, 2) not all satellites have the same saturation flux density (SFD), 3) not all earth stations are identical, etc. It is presented as an example only.

Example B: Suppose just one of the adjacent (2º) satellites is not cross polarized with the satellite being interfered with. Further assume that the earth station accessing that adjacent satellite is a 5 meter transportable.

The single entry interference from the transportable would be C/I = 25.5 dB into the co- channel. This level of single entry interference is considered unacceptable for broadcast television transmission. If the transportable in this example were not pointing properly or if its sidelobes were not in accordance with the FCC standard, the interference from this source would be worse. The Satellite–Earth Link (Downlink) Analysis of the downlink performance is treated in a similar manner to the previous analysis for the uplink. Thermal noise and interference noise are treated separately.

Downlink Thermal Carrier to Noise Ratio The link power levels are shown in Figure 2-40. Expressed as an equation:

Figure 2-40 Power Levels in Downlink.

Carrier Power (PR) = EIRP SAT – LP + GR (dB)

To determine C/N, add –N to both sides of the equation.

()C / N D = EIRPSAT − LP + GAR − (k + T + B)LdB

⎛ G AR ⎞ ()C / N D = EIRP SAT − L P + ⎜ ⎟ − k − B L dB ⎝ T ⎠

Where, LP is clear weather space loss = 36.6 + 20logf + 20 log r f in MHz r in statute miles.

EIRP SAT is the Effective Isotropic Radiated Power from the satellite and was discussed in section 2.5.2 above. (GAR/T) is the earth station gain to temperature ratio and was discussed in section 2.6.3 above.

Figures 2-41 and 2-42 are given here to assist in the determination of LP.

Figure 2-41: Path Loss Between Synchronous Orbit and Sub-satellite Point.

Figure 2-42 Free Space Loss versus Ground Station Elevation Angle. Downlink Carrier –to-Interference Ratio Consider the situation of figure 2-43. An earth station is receiving a desired signal from just one satellite, but is subject to interference from a multiplicity of satellites in the orbital arc. If all of the satellites are serving the same geographic area, the only defense against interference from these other satellites is the earth station’s antenna discrimination, and whatever frequency and polarization improvements that may exist.

Expressed as an equation, the downlink C/I is:

N ()C / I D = EIRPSAT + GES − ∑⊕ EIRPi + GES ()Θi + Fi + Pi i=1 where, GES is the maximum (on axis) gain of the receiving antenna.

Copyright 2006 66 All rights reserved Skjei Telecom, Inc. GES ()Θi is the gain of the receiving antenna in the direction of an interfering satellite (co-polarization). EIRPi is the EIRP of an interfering satellite Fi and Pi are frequency and polarization improvement factors.

Figure 2-43 Downlink Interference.

Carrier-to-Intermodulation Ratio IM in the satellite was discussed in section 2.4.2.1 above in a qualitative manner. In this section we will quantify this parameter in a general way, and show how a signal from an earth station can cause harmful interference into other users of the same satellite (or transponder) if it is higher in level than its normal or assigned level.

The output voltage of an amplifier can be expressed as a power series 2 3 4 5 eo = k1ei + k2ei + k3ei + k4ei + k5ei L

If the input signal consists of two sinusoids: eo = Acosω1t + Bcosω2t , expansion of the series is quite complex and not pertinent here. What is pertinent is that if this expanded series is passed through an appropriate bandpass filter, containing the fundamental 3 5 7 frequencies, only the components due to odd orders (ei ,ei ,ei ,ei ,etc) will pass. These components are:

Fundamentals: ω 1 ,ω 2 rd 3 order:(2ω 1 – ω 2), (2ω 2 – ω 1) th 5 order: (3ω 1 – 2ω 2), (3ω 2 – 2ω 1) th 7 order: (4ω 1 – 3ω 2), (4ω 2 – 3ω 1)

At saturation with two equal amplitude signals, assuming a perfect limiter, the amplitudes of the distortion components relative to the fundamentals is:

3rd order – 9.54 dB 5th order – 13.98 dB 7th order – 16.9 dB

When the input levels are backed off from saturation: 1) the third order component amplitudes drop off at a 2:1 ratio (dB), 2) fifth order at 4:1 slope, and 3) seventh order at a 6:1 slope. The important distortion component is therefore third order when attempting to minimize distortion. Figure 2-44 shows an amplifier input output characteristic similar to Figure 2-21, but showing third order distortion.

Figure 2-44 Amplifier Input Output Characteristic Showing Theoretical Third Order Distortion and 2 Tones.

If (as in the case in satellite transmission) three or more carriers pass through an amplifier the situation becomes far more complex giving rise to components which fall

Copyright 2006 68 All rights reserved Skjei Telecom, Inc. on the fundamental frequencies (triple beat). If the carriers are modulated, analysis is enormously complicated, and usually takes a great deal of computer power with very sophisticated programs. In this case, distortion is treated as a power sum. If for example a satellite has 12 equal amplitude (noncoherent) carriers on one polarization, the power sharing load is 10.8 dB. In order to maintain a 30 dB carrier to intermodulation ratio, the final wideband amplifier would have to be operated at an input backoff of at least 16 dB relative to saturation.

Now supposed two careless uplink operators increase their power by a factor of two (3 dB). There would be at least two other wideband systems in the same satellite with a C/IM of 24 dB which is unacceptable. Consider further that one of the transponders is carrying narrowband traffic (radio stations or data) where several carriers share the transponder and each carrier is operating far below saturation. The increase in intermodulation noise could produce harmful interference.

Interference Location Systems A number of resources exist to isolate interference problems when they occur. These are normally coordinated by the satellite operator- who is in the best position to know the activity of other carriers on the same and adjacent satellite. If an obvious interfering carrier is seen or known to exist, and if the satellite operator is not able to resolve an interference situation using obvious methods (noting changes in other carriers, reading ATIS codes, etc) they may utilize an interference location system which uses Time Difference of Arrival (TDOA) and or interferometry methods to locate the interfering uplink. Such systems or services are provided by companies such as Transmitter Location Systems (TLS) or Qinetiq. An example of a single line of position obtained from such a system is shown in Figure 2-45. Normally, two such lines of position can be obtained and their intersection will locate the carrier within a certain accuracy, as shown in Figure 2-46. The method of accomplishing this is shown in Figure 2-47.

Figure 2-45 Example of TDOA Single Line of Position

Figure 2-46: Example of Accuracy from Two Lines of Position

Figure 2-47: Typical TDOA Measurement Setup

Aggregation of Interference Effects Figure 2-48 presents a “cause and effect diagram” used for troubleshooting link anomalies. Each branch of the “fishbone” presents a separate source of degradation to the link. As can be seen, in troubleshooting a satellite link, a wide varied of interference effects must be considered, spanning several satellites.

Same Satellite Transponder Operating Equipment Interference Conditions Design Carrier Uplink Recovery crosspol Intermodulation AFC loop Loop adjustment Interfernce from another Interference Input level xpor uplink Clock Recovery Insufficient range Loop Transponder Cross Pol Error Reporting Backoff Clock interface/ timing Inaccuracies Inerference Downlink problems crosspol Loss of adjustment lock or Low Ant. Gain due to excess poor installation HPA Uplink BER Terrestrial or airborne/ Intermods Interference military interference Transponder Downlink Electrical power IF or IFL Pad Interference flucturations Pickup mismatch Connector/.IFL Downlink Problems Mispointed Uplink Antenna Uplink Pattern Pointing Antenna Mismatch Downlink Site Specific Interference from Antenna Problems Carrier Uplink High Spectral Sidelobes, Density Bad Crosspol Uplink Interference Downlink Sidelobe Antenna Pattern Uplink Crosspol Antenna Adjustment Crosspol EIRP Adjustment footprint mismatch Adjacent Satellite Interference

Revision 2, 16 September, 2004

Figure 2-48 Cause-Effect Diagram Showing Various Interference Sources

Propagation Anomalies Weather Related Factors in Satellite Links In general, satellite links need not provide for the same level of protection from fading as terrestrial links, since only a small portion of the path passes through the atmosphere. However, the space link is not entirely free from atmospheric effects and the attendant degradations to radio wave propagation, as was noted in Chapter 1 where a comparison of the various frequency band effects were presented.

Satellite link propagation has been the subject of intense study and a firm database has been established from experiment and theoretical studies. There are many degrading factors such as clouds, fog, turbulence, adiabatic effects, and precipitation. The effects of

Copyright 2006 72 All rights reserved Skjei Telecom, Inc. these factors are attenuation, depolarization and sky noise increase. Secondary effects are scintillation (multipath), antenna gain degradation, and bandwidth coherence reduction due to variations of these effects with frequency.

At C band, it has been demonstrated that in the contiguous 48 states, a 2 dB margin is all that is required for all atmospheric effects for more than 99.99 percent propagation reliability. Above 10 GHz, however, the effects change dramatically, especially degradations due to precipitation.

Effects of Rain

Rain Attenuation Several models exist to predict attenuation of microwave links due to rain. These models are based on rain rate data taken over a number of years over the entire earth. Details are beyond the scope of this training seminar. There are two models that are currently used by practitioners of satellite engineering. They are the Crane models (Reference 4) and the ITU model (Reference 6).

Noise Temperature Effects Not only does rain affect the path attenuation but it also affects the system noise temperature. The increase in system noise temperature is dependent on the clear weather noise temperature and the amount of rain attenuation. Figure 2-49 is a plot of the noise temperature degradation versus rain attenuation for some common values of clear weather noise temperature The expression for this increase in noise is:

⎡ ⎛ 1 ⎞⎤ ⎜ ⎟ ⎢TR + To ⎜1− ⎟⎥ ⎝ LR ⎠ ΔT = 10 log⎢ ⎥dB E ⎢ T ⎥ ⎢ R ⎥ ⎣⎢ ⎦⎥

where, TR is clear weather system noise temperature.

T0 is ambient temperature of rain = 260º K

LR is rain attenuation as a numeric.

Figure 2-49 Rain Attenuation vs. Noise Temperature

For example, if the rain model predicts rain attenuation of 5 dB, and the clear weather system noise temperature is 200º K, the degradation of system noise is:

⎡ ⎛ 3.24 −1⎞⎤ ⎢200 + 260⎜ ⎟⎥ ⎝ 3.24 ⎠ ΔT = 10log⎢ ⎥ E ⎢ 200 ⎥ ⎢ ⎥ ⎣ ⎦ = 2.8 dB.

The total fade in the link is LR + ΔTE . In this example with the conditions imposed, a margin of (5 + 2.8) = 7.3 dB must exist in the link to a C/N threshold.

At Ku Band and Ka band, rain attenuation becomes more of a factor due to increased attenuation at those frequencies. Figure 2-50 provides a map of the different Crane model rain zones in the US. Rain zone E, the Gulf Coast Region, provides the most rain attenuation, followed by zones D3, D2 and D1 in decreasing severity.

B D1

C D3 E

F E

Figure 2-50: Rain Zone Maps in the US (Crane Model)

Depolarization In frequency reuse systems, satellite channels are staggered in frequency and adjacent channels are crosspolarized. Rain induced depolarization in the path will cause received signal attenuation by changing the signal’s polarization. Once thought to be a major problem, this effect has turned out to be less severe than originally believed, but it can still cause a problem when severe rain attenuation exists.

Uplink and Downlink Effects and Countermeasures Both uplink and downlinks can be affected by rain, although normally this occurs at different time because the downlink is located in a different geographical area from the uplink. Both uplink and downlink effects vary inversely with the antenna elevation angle: they are more severe at low elevation angle because the path through the rain cell is generally longer. Several countermeasures exist which can reduce both uplink and downlink effects:

Copyright 2006 75 All rights reserved Skjei Telecom, Inc. 1. Use of adaptive coding and modulation such as described in the DVB-S2 standard. 2. Use of “store and forward” information distribution instead of “streaming” signal distribution, where possible. During rain events, the signal could be delayed and retransmitted at a later date, or only those portions affected by rain could be retransmitted. 3. In conjunction with item 2 above, use of Erasure Codes, similar to “striping” on a RAID disk, can be effective, as demonstrated by the Kencast FAZZT system.

Uplink Effects and Countermeasures Uplink effects are generally more important in broadcasting than are downlink effects because the entire network (all downlinks) are affected if the uplink fades, whereas only one or perhaps two downlinks are affected during downlink fading. Some satellites have an AGC or automatic gain control in their transponders so that in a wideband (full transponder) operation, the transponder is always at saturated EIRP. Fortunately, several countermeasures exist to reduce the severity of this fading, including: 1. Uplink power control 2. Use of a diversity uplink site 3. Use of a hard limiter or AGC circuit in the satellite, where possible 4. Cross banding (e.g. use of C Band uplink and Ku band downlink) where available

Downlink Effects and Countermeasures Downlink effects can also be mitigated by use of a diversity site and by use of a larger antenna in some cases

Scattering If an earth station antenna beam intersects the beam of a terrestrial microwave system in a rain cell, there is a possibility of reflection from the signal of either beam being sent in the direction of the receivers in the other system. At Ku band, the problem does not exist because there is no shared service. At C band, coordination requires a listing of beam intersections. C band uplink operators should be sensitive to the possibility of interference from their transmissions to terrestrial facilities in a local heavy rainstorm.

Effects of Snow From a propagation standpoint, (path attenuation) snow has very little effect, although as a signal passes through a dense cloud producing snow, ice particles can cause modest fading. The greatest effect of snow is in its effect on the ground antenna performance. This is especially true when the inevitable thaw occurs and the ground antenna is filled

Copyright 2006 76 All rights reserved Skjei Telecom, Inc. with high water content of slush. Serious beam deflection (pointing error), gain reduction and sidelobe degradation can occur. For uplink stations operating in snow areas, snow avoidance and/or removal equipment is necessary. Both reflector and feed must be kept free of snow and ice.

Other Propagation Anomalies Signals propagating between the earth and geostationary satellites are subject to certain propagation anomalies even in clear weather. These effects are generally of little significance in current FSS commercial service and are generally not included in link analysis. They are, however, worth mention in the overall context of satellite communications. The severity of these effects is dependent on the length of the path through the atmosphere, troposphere and ionosphere (elevation angle of the earth station antenna). These effects are: A. Refraction– Bending of the radio wave beam due to varying density of atmosphere and troposphere. B. Diffraction– Bending of the radio wave beam around a local physical object such as a building or mountain. Can be avoided by appropriate line of sight clearance. C. Multipath– In a satellite transmission, multipath scintillation can occur by dual refraction in the atmosphere and the ionosphere, or in low look angle applications. Diversity sites can be used as a countermeasure in some cases. D. Faraday Rotation– In addition to refraction in the ionosphere, the radio wave (normally C band) can experience a polarization rotation due to the highly charged ions in this region; this typically affects larger antennas.. E. Ionospheric Scintillation: The C or X Band signal can be affected by variations in the ion level in the ionosphere, causing varying signal levels. This is normally only a problem at the magnetic equator..

Overall Predetection Carrier-To-Noise Ratio The overall satellite system (C/N)s is given by the power sum of the previously derived expressions for C/N, C/I and C/IM.

()()()C / N s = []C / N UP ⊕ C / N DOWN ⊕ (C / I )UP ⊕ (C / I )DOWN ⊕ C / IM dB

The method of calculation of (C/N)s is to convert dB values of (C/N)up, (C/N)down, (C/I)up, (C/I)down and (C/IM) to numerical values and use the formula:

1 ()C / N = s 1 1 1 1 1 + + + + ()C / N U ()C / N D ()C / I U ()C / I D ()C / IM

This gives the system (C/N) as a numeric. To convert to dB value:

(C/N)s = 10 log (C/N)s (numeric)

(C/N)U = 30 dB = 1000 (numeric) (C/N)D = 15 dB = 31.6 (numeric) (C/I)U = 26 dB = 398 (numeric) (C/I)D = 26 dB = 398 (numeric) (C/IM) = 30 dB = 1000 (numeric)

1 1 ()C / N = = x .001+ .0316 + .0025 + .0025 + .001 .0386

= 25.9 (numeric) = 10 log 25.9 = 14.1 dB

The previously derived clear weather expressions for (C/N) should be modified to include the effects of anomalous propagation. As previously stated, at C band all of these effects can be accommodated by including a 2 dB margin in the link. At Ku band, the effects of precipitation should be included as a separate factor depending on the percentage of reliability required.

Absent from the entire discussion above on C/N is the effect of terrestrial interference on the overall system performance. From an uplink operator standpoint, interference from terrestrial sources into the satellite system is of no consequence. However, it should be kept in mind that at C band, interference to satellite receivers from 4 GHz terrestrial sources can be a factor. There is also the possibility (although very slight), that a 6 GHz terrestrial transmitter might cause interference directly into a satellite. FCC rules, and properly conducted frequency coordination procedures, generally precludes interference between satellite and terrestrial systems. Characteristics of C, Ku and Ka Band Satellite Communications Now that some of the basic concepts have been discussed, it is possible to develop a more in-depth discussion of the relative characteristics of satellite communications systems employing the three primary frequency bands, C, Ku and Ka. C-Band Satellites C-band was initially favored for communications satellites because of the favorable propagation characteristics at these frequencies. Ku-Band Satellites The higher propagation loss characteristics at these frequencies require higher spacecraft equivalent isotropic radiated power (EIRP) to achieve the same transmission performance as C-band frequencies and this is obtained from a variety of methods, including the use of greater spacecraft antenna gains, readily achievable at the higher frequencies. Since the Ku-band frequencies are not shared with terrestrial systems (as is the case of C-band) the power flux density (PFD) limitation is much less stringent and

Copyright 2006 78 All rights reserved Skjei Telecom, Inc. there is no requirement for coordination with terrestrial microwave systems; consequently, Ku Band satellites employ higher power satellite amplifiers than do C Band satellites, as much as an order of magnitude higher. The high powers permit the use of very small earth station antennas at or near the user's premises. This results in important economic advantage for many services and makes the use of this frequency band very attractive. Even so, a good part of the higher satellite power achievable is necessary to offset the additional attenuation that is experienced at these frequencies during heavy rain conditions.

There is no mandated frequency plan for transponders in this frequency band, although typical transponder bandwidths today are 36 MHz. Since the bandwidth is the same as at C band, it is possible to have a similar 24 transponder, 36 MHz frequency plan with 40 MHz channel spacing when frequency reuse is utilized.

Ka-Band Satellites The commercial application of Ka-band satellites is only in its infancy. The NASA ACTS experimental satellite has allowed successful demonstration of the use of the 17.5 GHz to 22.5 GHz downlink and 29.5 GHZ to 34.5 GHz bands for various services and use of this band for DBS video and internet service has begun. It is inevitable that these bands will be increasingly used for video applications, with considerable interest in narrow casting and point to point applications. Due to the higher frequency range, Ka band satellites generally include many small spot beams in lieu of one large CONUS or geographically large beam. This is shown in Figure 2-51 below.

Frequency Group 1 70 beams

Frequency Group 2

Frequency Group 3

Figure 2-51: Spot Beam Configurations

This significantly improves frequency re-use and overall satellite capacity, but at the expense of connectivity (ability of an uplink to simultaneously broadcast to many geographically dispersed downlinks).

Ka-band signals suffer from greater attenuation due to the presence of rain and atmospheric oxygen, than do C-band and Ku-band services. This was discussed previously in Chapter 1. This attenuation problem has historically made Ka-band rather unattractive for satellite communications, but scarcity of spectrum in other bands, desirability of small spot beams for some applications, as well as advances in satellite communications technology have mitigated this somewhat. A comparison of the technical characteristics of Ka Band and Ku band is shown in Figure 2-52.

C Band Ku Band Ka Band

Downlink 3.7-4.2 11.7-12.2 18.3 to 18.8, 19.7 to 20.2 Frequency (GHz) Primary One uplink transmits to all of One uplink transmits to Connectivity United States a “spot” beam

Total Satellite 1 GHz 5-10 GHz Bandwidth

Rain Margin Less than 2 dB Greater than C Greater than Ku Band Needed (this Band will vary) Two way 3.8 meter 1.2 meter 0.67 meter Antenna Size

Figure 2-52: Technical Differences between C, Ku and Ka Band SATCOM

Today, the primary application areas of Ka Band are Direct-to-home video and Internet access. At the time of writing, DirecTV is using Ka band on the SPACEWAY 1 satellite for providing DBS video service. Although this satellite was originally designed for on- board switching, it is being used in a “bent pipe” configuration for local-into-local video, including HDTV Another current use of Ka band, home and small office internet service, is being implemented by the Wild Blue satellite system. At the time of writing, little or no Ka band trunking or backhaul is taking place, but this type of point to point application would seem a natural application of Ka Band systems, particularly in dryer regions of the country.

Comparison of C, Ku and Ka Band Systems Two of the more important differences between C, Ku and Ka -band are the following: • C-band FSS share frequencies with terrestrial microwave systems. This places constraints on the location of C-band earth stations and it limits the permissible downlink power density. Prior to licensing a C Band transmit antenna, a frequency coordination must be performed (such frequency coordination is not required at Ku Band) • Ku-band signals and to a greater degree, Ka Band systems, are subject to significant attenuation in heavy rainfall The advantages and disadvantages of C-band, Ku-band and Ka Band, which result from these and other differences, are summarized in Figure 2-53 below:

Copyright 2006 81 All rights reserved Skjei Telecom, Inc. _ C-band Advantages Usually most reliable due to less susceptibility to rain outages. C band space segment is normally less expensive than Ku Band space segment

C-band Disadvantages Frequency band is congested because it is shared with terrestrial microwave, making frequency coordination a requirement. Requires relatively large antennas because of low satellite EIRP levels and the necessity of narrow half-power beamwidth to allow two degree spaced satellites. Avoiding terrestrial interference can make site selection a difficult process. The use of artificial shielding to block interference can increase total system cost. Faraday rotation of polarization can affect system performance.

Ku-band Advantages Frequency band is only used for satellite communication. Smaller antennas may be used because of higher gain and higher satellite EIRP. Easier site selection because of smaller antenna and reduced terrestrial interference. Suitable for direct-to-home application. Flexibility in channelization plan. Not affected by Faraday rotation.

Ku-band Disadvantages Affected by rain attenuation and depolarization. Waveguide and coaxial transmission line losses are quite high. Interference can occur from radar detectors located in passing automobiles. Site surveys should assess.

Ka Band Advantages Space segment cost-per-bit-transmitted is lowest because satellites have much greater capacity due to increased frequency re-use via spot beams. Smaller antennas (than Ku band) can be used in many areas where atmospheric attenuation is low. Higher frequency is more amenable to spot beam use and facilititates small dish uplinking, point to point applications Smaller downlink beams can be tailored to coverage area

Ka Band Disadvantages Increased atmospheric attenuation requires greater link margin, can reduce reliability All-spot beam operation results in significant disadvantage for broadcast (point to multipoint) service Figure 2-53 Merits of C, Ku - and Ka-band for Satellite Communications

Copyright 2006 82 All rights reserved Skjei Telecom, Inc. Commonly Used Modulation Techniques Up to now, our discussion of satellite communications has dealt with power considerations of the transmitted carrier and noise plus interference. Bandwidth has only been discussed from a negative standpoint as containing thermal (intrinsic) noise and as a receiver of interference. Carrier power by itself doesn’t’ carry information content. Only when the carrier is modulated does this carrier power become a useful signal. In this section, a general description of the commonly used modulation techniques is discussed.

Frequency Modulation Frequency modulation is commonly used in satellite communications in both full and partial transponder operations. FM is not spectrally efficient, but it utilizes the available bandwidth to overcome the considerable path loss and limited available power from the satellite.

Television–FM/TV Television transmission is the major user of satellite facilities. A plethora of broadcast TV, cable TV and private networks use full baseband bandwidth (4.2 MHz) transmission, to exploit the natural point to multipoint nature of satellite operation.

There are two basic baseband formats currently in use. They are: 1) NTSC and 2) MAC. There are a few variations on these basic formats depending on the scrambling technology used (if any).

The NTSC format when transmitted in the clear is usually the same as the terrestrial broadcast standard signal for video which television sets are designed to receive. Program audio is usually transmitted on a subcarrier above the video. This format probably needs no further description. In scrambled transmission, the baseband video and audio are fed to a scrambler and a variety of baseband signals emerge. The audio is usually digitally modulated as a burst transmission in the horizontal blanking interval. The video usually remains in an analog form of varying complexity but still retaining the basic NTSC form of simultaneous transmission of luminance and chrominance information.

MAC transmission stands for Multiplexed Analog Component, and is unique to satellite transmission utilizing FM. There are many forms of MAC worldwide. In the U.S., BMAC is currently utilized in several private networks. In MAC, the luminance and chrominance components are separated, and time multiplexed on each horizontal scanning line, along with program audio. The components are time compressed with chrominance occupying about 1/3 and luminance about 2/3 of the active line time. In B-MAC, 4 high quality audio signals are transmitted in a burst mode during the horizontal blanking interval. The time compression of the luminance component creates

Copyright 2006 83 All rights reserved Skjei Telecom, Inc. a higher baseband frequency so that the luminance noise is slightly higher than in NTSC transmission. However, cross color artifacts are eliminated, and if the received signal is fed to an RGB monitor, a higher quality color signal is perceived by the viewer.

Frequency Division Multiplex, FDM/FM The baseband of a satellite carrier, can be frequency multiplexed with a multiplicity of subcarriers and the aggregate can be frequency modulated on a carrier. Modulation of the subcarriers can be single sideband AM (SSB-AM), FM, or even digital.

With SSB subcarriers, a very large number of voice channels (3kHz) can be accommodated. With FM subcarriers, a reasonably large number of high quality audio signals can be accommodated with inexpensive receiving terminals. Today, however, digital modulation techniques have replaced analog FM or SSB techniques within the domestic United States.

Single Channel Per Carrier–SCPC/FM There are still a few radio networks currently using satellite facilities for distribution utilizing this form of modulation, but they are mostly or all international and not within the US. It is very similar in nature to terrestrial broadcast service. In fact, some networks use the same modulation parameters as terrestrial broadcast. Most networks operating this way use sophisticated companding techniques to enhance performance and to conserve bandwidth and/or power.

Digital Modulation Digital modulation is commonly used in satellite transmission, especially when the information to be transmitted is digital in nature (data). A wide variety of services, data rates and levels of modulation are used. In general, digital modulation is more robust than analog and is less sensitive to noise and interference under strong signal conditions. For systems operating close to threshold, errors in transmission can be reduced by a variety of coding schemes (at the expense of higher data rates and concomitant bandwidth).

Radio networks are in place that use both multiple channels (Time Division) and single channels on a satellite carrier. Compressed video networks for business teleconference and education/training is increasing in use as the cost of CODECs (coder-decoders) and earth station hardware comes down with new technology and volume production. Private business networks using VSAT (Very Small Aperture Terminals) networks are becoming a significant user of satellite facilities. These VSAT networks are usually characterized by a Hub earth station with a large antenna controlling a large number of earth stations with a small antenna. The signals inbound to the hub are single channel transmissions from a VSAT, and the outbound carrier is a time division multiplexed carrier to the community of VSATs.

Copyright 2006 84 All rights reserved Skjei Telecom, Inc. Digital technology has reached the point where entertainment television can be economically transmitted by satellite. Compression techniques have advanced such that “NTSC Broadcast Quality” can be achieved with data rates of about 6 Mb/s. Half-inch VTR quality can be achieved with data rates of about 2 to 3 Mb/s and High Definition Television (HDTV) can be achieved with data rates in the range of 15 to 20 Mb/s. HDTV is loosely defined as having the same subjective quality as 35 mm film, or about twice the resolution of NTSC video. Current practice with available satellites can provide at lease four broadcast quality NTSC television signals with Compact Disc quality audio over a single satellite transponder to relatively small receiving earth stations.

Another common use of compression technology using satellites for distribution is networks providing multichannel audio services. Usually these services Time Division Multiplex many channels of audio into a single data stream of up to 20 Mb/s, and transmit the data stream in a single transponder. Current state-of-the-art can allow a stereo pair of CD quality audio in 192 kb/s data rate. Five channels (“Surround Sound” can be reduced to a bit rate of about 300 kbps.

Spread Spectrum This special form of signal processing or coding is currently in use in some VSAT networks. In this particular commercial application, the digital data stream is made considerably tolerant to interference by mixing it with a sequence of essentially random bits at a much higher rate than the data stream. This amounts to expanding the baseband bandwidth by a considerable amount. The added random bits cause the modulated carrier to be spread over a wide band corresponding to the “chip” rate. At the receiving end, a synchronous code generator subtracts the high rate signal.

Overmodulation Regardless of the form of modulation used, an uplink operator should be constantly aware of the possibility for overmodulation. The station license contains an emission designator for each carrier transmitted by the station. Contained in that designator is the necessary bandwidth occupied by that transmission. Emission Designation is defined in FCC rules #2.201 and 2.202. Under the latest rules (December 1984), necessary bandwidth is given by three numerals and a letter where the letter occupies the decimal point. A bandwidth of 36 MHz is given as 36 MO. Under the old rules, this would be given at 36,000. Exceeding the allowed bandwidth constitutes an illegal use of the earth station, and places the license in jeopardy. In addition, harmful interference to other systems is highly probable with overmodulation.

Extreme care should be exercised when adjusting depth of modulation. A good idea is to have the satellite operator monitor the adjustments to assure that the local monitor is accurate. Furthermore, the operator should be familiar with failure mechanisms, which

Signal-To-Noise Ratio-Analog Systems In an analog transmission, the ultimate performance in any communication channel is the ratio of signal to noise delivered to the user at the receiving end. In a digital channel, the measure of performance is the bit error rate which is dependent on the bit energy to noise power density. Under strong signal conditions, S/N and Eb/No have a 1:1 relationship with the carrier to noise ratio (C/N) which was covered above.

FM Television Video-Signal-to-Noise Ratio

In any FM transmission, the RMS Signal to RMS Noise ratio out of a receiver when operating above threshold is:

2 ()S / N o = 3m (C / N )i p ΔF Where: m is modulation index = f m

ΔF is one side peak deviation of the carrier,

f m is the highest modulation frequency,

(C/N)i is the input carrier to noise ratio, and p is improvement factor due to de-emphasis, and/or other factors.

Since noise is proportional to bandwidth:

BWPD ()C / N i = ()C / N PD 2BWBB

Where: BWPD is the predetection bandwidth, BWBB is the baseband bandwidth = Fm, and (C/N)PD is the carrier-to-noise ratio in the predetection bandwidth.

For television transmission, the video signal-to-noise ratio is defined as the ratio of the peak luminance signal to the RMS (weighted) noise.

Where peak luminance is blanking level to white level (100 IRE units).

The video signal-to-noise ratio of an NTSC (4.2 MHz bandwidth) transmission is therefore:

2 2 3 ⎛ Peak Lum.⎞ ⎛ ΔFV ⎞ ⎛ BWPD ⎞ ()S / N V = ⎜ ⎟ ⎜ ⎟ ⎜ ⎟()()C / N PD pW 2 ⎝ RMS ⎠ ⎝ 4.2X106 ⎠ ⎝ 4.2X106 ⎠ where the voltage ratio: ⎛ PeakLum.⎞ ⎛ Lum ⎞⎛ P − P ⎞ ⎛100 ⎞ ⎜ ⎟ = ⎜ ⎟⎜ ⎟ = ⎜ ⎟(2 2)= 2 if synch is transmitted. ⎝ RMS ⎠ ⎝ P − P ⎠⎝ RMS ⎠ ⎝140 ⎠

Note: Ratio of peak-to-peak voltage to RMS voltage is 2 2(2.82) for a sine wave signal, and the ratio of luminance (100 IRE) to synch tip to white = 100/140. and, pW is the combined pre-emphasis and weighting improvement factor using the appropriate filter, and CCIR 405-2 pre-emphasis.

pw = 12 .8dB for FM TV

(note: When measuring (S/N)v, the weighting filter used in the measurement should be known. Some modern instruments will use the “Universal” 5 MHz filter. In that case the value for pW = 13.8dB.)

Therefore:

2 3 ⎛ ΔFu ⎞ BWPD ()()S / N V dB = 10log + 20log 2 +10log⎜ ⎟ +10log 2 ⎝ fm ⎠ fm

+ (C/N)PD + 12.8 ….dB

= 1.8+6 +12.8 +20 log ΔF + 10 Log BWPD

6 –30 log(4.2x10 ) + (C/N)PD …. dB

= [(C/N)PD + 20 log ΔF + 10 logBWPD – 178.1]dB.

A typical C band transmission might have one side deviation of 10.75 MHz, and a predetection bandwidth of 34 MHz. Then:

()()S / N = C / M + 20log10.75x106 +10log34x106 −178.1 v PD

= (C/M)PD + 37.8…. dB

The effect of varying the modulation index is shown in Figure 2-54 below.

Figure 2-55 shows the C/N versus Signal to Noise ratio for FM modulation\

Figure 2-54 Effect of modulation index on Frequency Modulation spectrum

Figure 2-55: Relationship of C/N to SNR in an FM carrier

Figure 2-56: NTSC FM modulated carrier and signal to noise ratio

Audio-Signal-to-Noise Ratio

FM Subcarriers In satellite transmission of television, the TV associated sound is sometimes transmitted by an FM subcarrier placed above the video in the main carrier baseband. The following is a derivation of S/N out of the subcarrier demodulator: The basic formula for unweighted S/N of an FM receiver operating above threshold is: 2 ()S / N o = 3m (C / N )iP ΔF where: m is the modulation index = SC f a

ΔF SC is the one side peak deviation of the subcarrier by the audio

f a is highest audio frequency, and p is deemphasis improvement

For 15 kHz audio channel, f a = 15 kHz and deemphasis is usually 75 microseconds (p = 13 dB)

But:

BWSC ()C / N i = ()C / N BWSC 2BWBB

Therefore: 3 ⎛ ΔF ⎞ ⎜ SC ⎟ ()S / N a = ⎜ ⎟()()C / N BWSC p 2 ⎝ f a ⎠

In a double FM transmission such as is the case here, the (C/N)BWSC can be expressed as:

2 ⎛ ⎞ 1 BWPD ()C / N BWSC = ⎜ ⎟()C / N PD 2 M ⎜ ⎟ ⎝ BWSC ⎠ where: BWPD is the predetection bandwidth of the wideband receiver (assuming there are no other bandwidth restrictions in the path),

(C/N)PD is the predetection carrier-to-noise ratio, and m1 is the modulation index of the main carrier due to subcarrier and is:

ΔFc m1 = f sc where: ΔFc is the one side peak deviation of the main carrier by the subcarrier, and

fsc is the center frequency of the subcarrier.

Combining all terms and substitution gives:

2 2 ⎛ ΔF ⎞ ⎛ ΔF ⎞ ⎛ BW ⎞ ⎜ SC ⎟ ⎜ C ⎟ ⎜ PD ⎟ ()S / N a = 3/ 4⎜ ⎟ ⎜ ⎟ ⎜ ⎟()()C / N PD p ⎝ f a ⎠ ⎝ f sc ⎠ ⎝ f a ⎠

Typical values for C band satellite transmission are:

ΔFSC = 237k Hz fa = 15k Hz ΔF = 2M Hz c f sc = 6.8M Hz

BWPD = 34M Hz p = 13dB.

Substituting gives typical value of:

()()S / N a = C / N PD + 58.7LdB. Some satellite TV transmissions utilize a multiplicity of FM subcarriers for a variety of reasons (multilingual, radio networks, data transmission, etc.). In this case, “spectrally efficient” subcarriers are generally used. These subcarrier receivers utilize threshold extension, and powerful signal processing (companding). The net result is high S/N with lower deviation (less power taken from the satellite) and narrower occupied bandwidth.

The formula for S/N is the same as given above for “standard subcarriers”, but the parameters are different. Typical parameters are:

ΔFSC = 75k Hz

f a = 15k Hz ΔF c = 0.18 f SC

BWPD = 34M Hz p = 30dB

Substituting gives typical values of: (S / N )a = [(C / N )PD + 61.3]LdB.

NOTE: Uplink operators should be aware that if the TV transmission utilizes the Videocypher II scrambling system that a limit of about 2 standard subcarriers or about 6 spectrally efficient subcarriers can be accommodated. Having more FM subcarriers can seriously affect the performance of the Videocypher II scrambling system.

Sound in Synch Digital Audio As mentioned in Section 2.12.1.1 above, two commonly used satellite FM/TV transmissions utilize a digital burst mode during the horizontal blanking interval for program sound. The “Videocypher II” scrambling system delivers either a Left and Right stereo signal or two monaural channels. These signals are encrypted for security. The “BMAC” system provides 4 monaural channels for 2 stereo pairs. Videocypher II employs a Pulse Code Modulation (PCM) method as source coding. BMAC employs Adaptive Delta Modulation.

Essentially error free detection is obtained with sound-in-synch of these two implementations with a predetection carrier-to-noise ratio (C/N)PD of 10 dB or less. The dynamic range (peak signal level to noticeable noise ratio) is about 70 dB which represents an extremely high quality signal. This dynamic range is maintained so long as the (C/N)PD remains above the threshold. As in virtually all digital transmission systems, the performance drops drastically below the threshold C/N.

FM–SCPC

As indicated above in 2.12.1.3, some radio networks use this form of modulation for distribution. Derivation of the S/N for this service follows the above derivation for subcarrier before consideration of double FM.

That is:

2 ⎛ ΔF ⎞ ⎛ BW ⎞ ⎜ ⎟ ⎜ PD ⎟ ()S / N a = 3/ 2⎜ ⎟ ⎜ ⎟()()C / N PD p ⎝ f a ⎠ ⎝ f a ⎠

In this case p is generally a combined improvement factor due to deemphasis and companding which provides a large subjective improvement factor. Typical values are:

ΔF = 75k Hz fa = 15k Hz

BWPD = 250k Hz p = 30dB

Substituting gives typical value of:

()()S / N a = C / N PD + 58LdB

FDM/FM FM Subcarriers As indicated above, an aggregation of FM subcarriers on a single baseband signal is currently being used by various providers of one-way audio and data services. This mode of operation has the distinct advantage of utilizing inexpensive receivers (similar to satellite television receivers).

The formula for the signal-to-noise ratio for this operation is the same as was derived above for the TV associated audio. That is: 2 2 ⎛ ΔF ⎞ ⎛ ΔF ⎞ ⎛ BW ⎞ ⎜ SC ⎟ ⎜ C ⎟ ⎜ PD ⎟ ()S / N a = 3/ 4⎜ ⎟ ⎜ ⎟ ⎜ ⎟()()C / N PD p ⎝ fa ⎠ ⎝ f SC ⎠ ⎝ fa ⎠

In this case spectrum efficient subcarriers are used, and the deviation of the main carrier

( ΔFC ) is adjusted for constant mod index for different fSC. In addition, companding is included to give a subjective improvement. Typical values are:

f a = 15k Hz ΔF C = 0.18 f sc

BWPD = 15M Hz p = 30dB

Substituting gives typical values of:

()()S / N a = []C / N PD + 57.9 LdB.

FDM/FM – Single Sideband Subcarriers FDM/FM using SSB subcarriers is a method of multiplexing many voice grade (3.1 kHz) audio channels on to one FM carrier. This is accomplished by converting each channel to an assigned frequency as a SBSC signal in the baseband frequency range. The total baseband then modulates an FM carrier. Demodulation and de-multiplexing are accomplished in the reverse order at the receive station.

FDM/FM performance is measured in terms of picowatts of noise per FDM channel. The noise-per-channel is related to the total S/N ratio in the total baseband with a test tone signal. This, in turn, may be related back to C/N ratio in the receive system IF.

The relationship between C/N in the IF and S/N in the baseband is:

⎡ΔFTT ⎤ ⎡ BIF ⎤ S / N = C / N + 20log⎢ ⎥ +10log⎢ ⎥ + P +W ⎣ fch ⎦ ⎣ Bch⎦

Where: ΔFTT = rms test tone deviation fch = highest voice channel frequency Bch = voice channel bandwidth P = top channel emphasis improvement factor W = psophometric weighting improvement factor – 2.5 dB.

Once the test tone S/N ratio has been determined, the noise per channel in picowatts may be determined from:

⎡90 − ()S / N ⎤ noise = log −1 pWpO ⎣⎢ 10 ⎦⎥

Digital Technologies In this section we discuss certain digital technologies currently popular in satellite transmission and are expected to be more popular in the future. Baseband processing

Copyright 2006 93 All rights reserved Skjei Telecom, Inc. which includes source coding and forward error correction coding (FEC) are covered. In addition, certain currently used modulation techniques in satellite transmission are described.

Discussion of these items is limited to generic methods rather than specific applications. Furthermore, the underlying theory is considered to be beyond the scope of this book. Instead, these items are treated from a user standpoint rather than a designer standpoint.

Advances in digital transmission technology have been closely tied to advances in semiconductor technology. There is a never ending quest for coding and modulation schemes to reduce the required bandwidth for faithful reproduction after transmission.

Source Coding (Baseband Processing) Aural and Visual information is analog in nature and must be coded in some manner to be transmitted digitally. The “best” method of coding depends on the transmission medium, and the state-of-the-art for practical implementation. An in-depth discussion is beyond the scope of this text. But a basic knowledge of the various methods currently in use can be helpful to a satellite uplink operator.

The basic “Sampling Theorem” states that only two samples of the highest frequency in a complex waveform contains all the information of the original message. For example, a voice signal limited to 3.4 kHz is reproduced with a sampling rate of 8 kHz. Compact Disc quality audio (20 kHz) can be sampled at a 44 kHz sample rate, and NTSC video (band limited to 4.2 MHz) can be sampled at about 10M samples per second. These samples then must be coded in a manner appropriate for transmission.

Pulse Code Modulation (PCM) PCM is a popular technique used to process digital signals for transmission. In PCM, several pulses are used as a code group to describe the quantized amplitude of a single sample. A code group of n on-off pulses (binary code) can represent 2n discrete levels including zero. A highly refined PCM system is used in telephony. Each voice channel is sampled at 8k samples/s. Each sample is quantized to 28 or 256 levels, or 8 bits of information per quantized sample. To reduce quantization noise, a non-uniform spacing of levels is used to provide smaller steps for weaker signals and larger steps for larger signals. This process is called companding. PCM is used in many other applications such as high quality digital audio and video systems, with higher quantization levels.

Predictive Techniques Delta Modulation is a form of predictive coding to reduce the transmission rate. Predictive coding is used to exploit the correlation between neighboring samples to

Copyright 2006 94 All rights reserved Skjei Telecom, Inc. reduce statistical redundancy. In PCM, differential techniques (DPCM) and adaptive quantizers are used (ADPCM) to greatly reduce the transmission rate and improve the S/N. Adaptive techniques are also used in Delta Modulation (ADM). These predictive techniques have become commonplace in telephony and high fidelity audio systems because of standardization and the availability of very large scale integrated circuit (VLSI) semiconductor chips.

A new class of predictive techniques has recently emerged which makes transmission of video practical and desirable. Digital Signal Processing (DSP) chips have given rise to powerful COMPRESSION techniques to greatly reduce the number of bits per sample. The equipment to perform this compression must have intelligence in the form of a computer and an algorithm to control it. An algorithm is a method of calculation. In this case, the algorithm contains a predictor or a multiplicity of predictors.

Forward Error Correction To control errors due to noise in a communication link there are techniques for preparing the data stream prior to modulation and transmission. The data can be coded with parity bits to expand the number of bits (requiring a concomitant increase in bandwidth) and a decoder at the receiver end used to correct errors. This is a concept difficult to explain without resorting to the mathematics of statistical communication and information theory. The concept is one in which a substantial gain in threshold performance is obtained with only a modest increase in the channel bandwidth. For example, a rate ¾ FEC can improve the threshold of a receiver by 3 to 5 dB, at the expense of about 1 dB in bandwidth. This gain will be demonstrated in a later section of this text when discussing the performance of a link

The hardware to perform this error correction function is usually imbedded in the modulator and demodulator. Again, the advantages far outweigh the cost, which is minimized by the advent of low cost VLSI technology. Most forward error correction codecs utilize an array of shift registers and switches (commutators) to provide either “block” or ”convolutional” codes or a combination of the two. Digital Modulation Techniques Since satellite power is relatively low, and the link is rather long, it is generally prudent to utilize a “Power Efficient” modulation technique. Virtually all digital satellite transmissions utilize either a biphase (2 phase states), quadrature (4 states) or 8 phase modulation known as BPSK, QPSK or 8 PSK, respectively. In some cases higher states ( 16, 32…states) are used where the data rate is high and the available bandwidth is low. In other words, for most applications power efficiency is more desirable than spectral efficiency.

Amplitude, Phase and Symbols The concepts of a signal amplitude and phase, and subdivision of a signal into “symbols” are basic to a discussion of digital modulation.

The concept of amplitude is relatively simple. The amplitude of a digital signal is just the peak or average value of the sinusoidal wave itself. Often it can be normalized to “1” for simplicity.

The phase of the digital signal refers to the state of the signal at a point in time. A sinusoidal signal carrying digital modulation goes from 0o to 360o in one cycle or one complete unique period of the sinusoid.

Figure 2-57 below shows three different sets of signals with different phase relationships in each pair, illustrating the concept of phase and phase relationships.

In Phase 180o Out of Phase 40o Out of Phase Figure 2-57: Phase Relationships in Signals

Figure 2.58 below shows the effect of modulating a baseband signal by changing its phase, and also shows an example of a “symbol”

Basically, a symbol is one complete cycle of the baseband sinusoidal wave in which the amplitude and phase is stable at some value for the duration of the symbol. Note that the concept of a symbol is primarily valid for the baseband modulation only, not for the Radio Frequency or RF wave. The RF wave will normally have many cycles of for each cycle of baseband modulation. For example, a baseband digital signal of 3 Mbps might be modulated on an RF signal of 6,000 MHz ( 6 GHz). In this case there are at least 2000 sinusoidal cycles of RF signal for each symbol “bit” of modulation ( 6,000/3=2,000)

Biphase Modulation (BPSK) BPSK is considered the simplest form of phase shift keying. Figure 2-59 is a block diagram of a typical Biphase modulator showing the signal-time domain relationships at various points in the signal path. Figure 2-60 is a simplified block diagram of a biphase demodulator. Today, BPSK is not as common as it once was because QPSK is more bandwidth efficient. It can still be found in some older VSAT systems, however, and can be useful for certain licensing purposes.

Figure 2-59: Simplified Block Diagram, Time Domain of Biphase Modulator.

Figure 2-60 Simplified Block Diagram, Biphase Demodulator.

Quaternary Phase Modulation (QPSK) QPSK transmission is the most popular satellite modulation technique in current practice. Spectral efficiency of almost twice that of BPSK can be realized with about the same power efficiency of BPSK. The demodulation methods described here are also used in QPSK. In addition to the extensions of the biphase techniques, a method called “offset keyed” quadriphase (OKQPSK) modulation is also used.

Figure 2-61 is a simplified block diagram of how a QPSK signal is generated, along with a (gray coded) vector presentation of the phase states. The serial to parallel converter may also contain differential encoding. The dashed line block showing an offset delay of Tb (one bit duration) is a means to obtain offset keying. The input data stream is split into two data streams by the serial to parallel converter. These two streams are called the I and Q channels. The I and Q channels are fed to balanced mixers along with an RF oscillator whose phase is 90˚ apart at each mixer. The QPSK can be regarded as two BPSK systems operating in quadrature.

The four phase states are generated by a unique mapping scheme of consecutive “dibits” (pairs) into symbols. The phase states are maintained during the signaling interval Ts which has a two bit duration. The four possible dibits are usually mapped in accordance with the “Gray code” as shown in Figure 2-61. The Gray code assures that a single symbol error corresponds to a single bit error.

Copyright 2006 99 All rights reserved Skjei Telecom, Inc. In some cases where several lower data rate channels are time division multiplexed (TDM) into higher rate data streams, the serial to parallel converter is bypassed and the I and Q channels are separately fed and separately used at the receiving end.

Figure 2-62 shows a timing diagram for QPSK and OKQPSK, and figure 2-63 shows the RF envelope for QPSK and OKQPSK. It should be noted that for QPSK, 180° transitions have a momentary change to zero in the envelope amplitude. For OKQPSK these 180° transitions are eliminated. Certain advantages insure for OKQPSK in satellite transmission because of this elimination of the 180° envelope amplitude change.

Figure 2-61 Simplified Block Diagram, QPSK Modulator Showing (gray coded) Phase States.

Figure 2-62 Modulator Data Streams for QPSK and OKQPSK.

Figure 2-63 RF Envelope for QPSK and OKQPSK Signals.

Signal-to-Noise Ratio and Eb/No The usual method of determining performance in a digital system is Bit Error Rate (BER). For example, a BER of 10-5 would mean that for 105 bits sent, there would be one error. If the transmission rate was 100 kb/s (105 b/s), then one error would be made every second. As indicated above (2.14.1.4) various FEC coding schemes can be used to decrease the errors in a transmission. Each service will have a “threshold” BER that is considered to be tolerable. For example, voice service can tolerate BER as high as 10-3, computer traffic is generally acceptable at 10-6, and certain highly compressed video and audio services might require BER of 10-10 or lower.

Copyright 2006 102 All rights reserved Skjei Telecom, Inc. The BER analysis of a particular mode of transmission must proceed from a noise model since incoming data bits will have a signal-to-noise ratio which depends on the link parameters. The signal-to-noise ratio in a digital transmission is given by the term Eb/No, where Eb is the bit energy and No is noise power density per hertz.

The relationship between Eb/No and the system carrier-to-noise ratio for BPSK and QPSK modulation is:

Eb = []()C / N s −10log R +10log BW db N o where: R is the information bit rate, and BW is the predetection bandwidth in Hz.

Consider the curve of figure 2-64. This curve represents the theoretical performance of BPSK and QPSK transmission. Let us assume that a link exists where the information data rate is 56 kb/s and the noise bandwidth is 84 kHz. Assume further that the system carrier-to-noise ratio is 10 dB.

E b = 10 −10log56x103 +10log84x103 N o = []10 − 47 .5 + 49 .2 db = 11.7 db

From figure 2-64, this corresponds to a BER of 2 x 10-8.

Suppose further in this example that the required BER is 10-6. What margin exists in this link to the 10-6 threshold?

The curve of Figure 2-64 indicates that BER = 10-6 is obtained with Eb/No = 10.6 db.

E Required ()C / N s = b +10log R −10log BW N o = []10.6 + 47.5 − 49.2 db = 8.9 dB

Since the link provides (C/N)s of 10 db, the margin to BER = 10-6 is 1.1 dB.

Figure 2-64 Plot of Theoretical Eb/No vs. BER.

8PSK and 16 QAM Modulation Figures 2-65 and 2-66 provide information on 8 PSK and 16 QAM modulation.

Figure 2-65: 8 PSK Characteristics

Copyright 2006 104 All rights reserved Skjei Telecom, Inc. As can be seen from Figure 2-65, one transmitted symbol or point in the constellation now transmits 3 bits of information, as opposed to 2 bits of information for QPSK or one bit for BPSK. Thus, transitioning from QPSK to 8 PSK theoretically improves overall capacity by 50%. In practice it is less than this value but still significantly higher. .

Figure 2-66: 16-QAM Constellations

Figure 2-66 shows three ways in which the next higher order of modulation, 16 Quaternary Amplitude Modulation, or 16- QAM, can be implemented. Different combinations of amplitudes and phases exist, but in each case, one symbol represents 4 bits (as compared to 8 PSK which would be 3 bits). Thus, the capacity of the link has improved by a factor of 4/3 or 33%.

Figure 2-67 shows the theoretical performance of several levels of PSK systems. No forward error correction is applied. Curves provide BER versus Eb/No. Curves for QPSK, 8 PSK, 16 QAM and 32 QAM are shown. These curves assume an infinite bandwidth channel where there are no non-linearities to produce distortions. They are based on thermal (Gaussian white) noise with no interference and no forward error correction.

Figure 2-67: Error Rates of PSK Modulation System.

COFDM Modulation Figure 2-68 below shows the waveform used for COFDM modulation. In this modulation method, the actual symbol time is increased and therefore its resistance to multipath interference is greatly increased. COFDM is seeing extensive use for Electronic Newsgathering (ENG) applications due to its ability to be reliably transmitted in a non-line-of-sight situation.

Figure 2-68: Coherent Orthogonal Frequency Division Multiplex Modulation

Forward Error Correction Coding Until now, we have been dealing with signals that do not employ error correction codes. In practice, however, almost all digital satellite signals today employ error correction codes. Such codes generally fall into two types: Block codes such as Reed Solomon Codes, BCH codes, Convolutional codes

While a complete description of these codes is beyond the scope of this course, it still will be valuable to discuss some coding basics.

Block Coding With block coding techniques, each group of K consecutive information bits is encoded into a group of N bits for transmission over the channel. Normally, the K information bits are located at the beginning of the N bit block code and the last N-K bits correspond to parity bits formed by performing a specified operation on the K information bits. The coding rate for this operation is K/N. The encoder structure is shown in the below figure 2-69. The information bits are stored in the k storage devices and then the shift register is shifted N times. For the encoder shown, K=6.

The block decoder is shown below and is essentially a digital processor

Figure 2-70: Block Decoder

LDPC One block code which has been shown to be even more powerful than Turbo coding is Low Density Parity Check. This is a block code based on an idea which is not new, but was not capable of being implemented until today’s powerful computing technology existed. Figure 2-71 shows a comparison of LDPC to the Shannon (theoretical maximum) limit and to various turbo codes.

Figure 2-71: LDPC Performance Comparison Figure 2-72 shows an example of how the LDPC works. Essentially, it uses a matrix of information bits and parity check bits and solves the matrix mathematically. For the example shown, k=message bits = 9, n-k parity bits = 7 Code Rate = k/n = 9/16

n9 = n0 + n1 + n2 n10 = n3 + n4 + n5 n11 = n6 + n7 + n8 n12 = n0 + n3 + n6 n13 = n1 + n4 + n7 n14 = n2 + n5 + n8 n15 = n12 + n13 + n14

Figure 2-72 Computational Basis of LDPC

Copyright 2006 109 All rights reserved Skjei Telecom, Inc. Convolutional Coding The next figure is a block diagram of a convolutional encoder for a rate ½ coder that will encode K information bits into N total bits. Bits are shifted in from the left and the A/B switch selects one upper path output and one lower path output for each shifted bit. The process is continuous. K is called the “constraint length” of the code.

Figure 2-73: Convolutional Encoder

Coding in this manner, a “code tree” is developed as shown below. The encoded bits depend not only on the input bit received ( 1 or 0) but also on the “state” of the encoder when it receives them.

There are several ways to “decode” or trace back the input bits- Viterbi decoding method is one method and sequential decoding is another method. These methods attempt to determine the input (information) bits by various computation and “error measurements” – essentially they are trying different paths through the “code tree” and measuring which path has the most probability of being correct.

Figure 2-74: Viterbi Decoding of Convolutional Coding

Figure 2-75: Modem Performance with and without FEC

In “concatenated coding” two different codes are used, separated by an interleaver which permits the full power of both codes to be applied. The block diagram for such a scheme is shown in figure 2-76

Input Data Outer Outer Inner Inner Modulator Encoder Interleaver Encoder Interleaver

Channel

Output Data Outer Outer Inner Inner Demodulator Decoder Deinterleaver Decoder Deinterleaver

Figure 2-76: Concatenated Coding

Figures 2-77 and 2-78 show how the interleaver is used to distribute errors left over from one decoding and disperse them so the second decoding can correct them.

Figure 2-77: Concept of Interleaving

Figure 2-78: Dispersal of errors in an interleaver

System Impairments The above discussion and example are theoretical and might not exist in practice. Nonlinearities in the link timing imperfections and phase noise can degrade the performance. The aggregate effect is called implementation loss and can vary considerably depending on the service and condition of the equipment in the link. The uplink operator must be aware and maintain the equipment to its specified levels.

Nonlinear phase versus frequency can cause “Intersymbol Interference” (ISI), and in a QPSK system, amplitude nonlinearity can cause I to Q component interference. The degree of implementation loss is difficult to predict.

A practical approach to implementation loss in link analysis is to start with MODEM vendor provided curves of BER versus Eb/No curves for the particular service. These curves are generally taken on a back-to-back basis at IF frequency. An example is given in figure 2-75, showing performance of the modem with certain FEC coding and without coding. Each service then will have a loss factor in implementation. This factor is applied to Eb/No as an increase. For example, if the implementation loss in a system using the Model of figure 2-75 is say 1.0 dB, and no FEC is applied, the threshold Eb/No for 10-6 BER is 12.0 dB (11+1). If ¾ Rate coding is used, the threshold Eb/No for 10-6 BER is 6.0 dB (5+1).

An excellent means for monitoring the relative performance of a digital link is by means of an “EYE PATTERN” on a suitable oscilloscope.. If the I and Q channels of a QPSK signal are applied to the horizontal and vertical inputs and the symbol clock is applied to the external trigger, a lissajous pattern is obtained. The horizontal time base should be adjusted for the symbol duration, and the scope should have high persistence.

A more modernized display which is similar to an eye pattern shows only the demodulated baseband bit stream. This type display is included in many pieces of digital test equipment and shows multiple demodulated bits.

The system input data stream should be constantly monitored, and any changes over time should be documented and corrective action taken. Likewise, the output data stream should be constantly monitored. Each service is likely to have unique eye patterns depending on the baseband processing (filtering and limiting). The important aspect of monitoring is to denote changes from normal. Analysis of changes and corrective action should be made by experts familiar with the system design.

Compressed Digital Television and Transmission Introduction- Analog Television Many types of video currently exist today- both analog and digital. Before describing digital video, it is useful to spend a few minutes describing analog television systems. Today the three types of analog television systems are NTSC, PAL and SECAM. Figure 2-79 and Figure 2-80 show the countries in which NTSC and SECAM are used today. The remainder of the world uses some form of PAL.

• USA • Guatemala • Canada • Honduras • Chile • Jamaica • Columbia • South Korea • Costa Rica • Mexico • Dominican Republic • Myanmar • Ecuador • Nicaragua • El Salvador • Panama • Venezuela • Peru • Taiwan • Philippines Figure 2-79 NTSC Countries

• Greece • Some Luxembourg, • Some Germany • Bulgaria, • Some Saudi Arabia • Czechoslovakia • Some Afghanistan • Hungary • Egypt, Iran • Poland, • Iraq • CIS • Lebanon • Congo • Libya • Korea • Morocco • Mongolia • Syria • France • Niger • Some Vietnam Figure 2-80: SECAM Countries Figure 2-72 below shows the NTSC signal levels, and Figure 2-73 shows the NTSC waveform including timing and framing pulses..

Figure 2-81 NTSC Signal

Figure 2-82: NTSC Waveform

Types of Video Generally speaking, all video is either in “component” form or in “composite” form. These terms refer, very simply, to whether the video signal is transmitted as one signal or is broken down into sub-signals which need to be added together to produce the video signal. There are many ways of breaking the signal down into component forms, such as RGB, YUV, Y Cr Cb, and others. Some of the advantages of component and composite are: · Effects and Graphics Compositing: Graphic or live images that are painted, moved, keyed or otherwise manipulated . Component digital systems allow free exchange of images between paint, effects and editing devices without loss of quality or regard to NTSC signal anomalies. · Standards Conversion: The recording on a D1 or Digital Betacam tape is digital component which is the native video signal format of standards conversion equipment. It presents the least adulterated signal to the converter for the best conversion quality. · Color Correction: The native video signal in color correctors is component video. · Projection: Video projectors that produce large screen images are best served by component a video source..

Copyright 2006 116 All rights reserved Skjei Telecom, Inc. · Film recording: Video programs targeted for film recording are best done on component recorders. Film recorders are presented with an RGB (Red Green and Blue) image that would be degraded if they had been composite at some point along the way Computer Images: Video intended for display on computer equipment is best presented in component form since the native internal format of the computer is RGB. : Introduction to Digital Video This section provides additional instruction and details dealing specifically with COMPRESSED digital television technology as currently implemented in satellite transmission. It is presented in a generic manner rather than being directed to a specific method of implementation.

The basic element of a digital video picture is the “pixel” or individual colored light element. The Figure below provides the number of pixels of resolution in the horizontal and vertical dimension for various types of video Resolution (Pixels) Type of Video Application 1920 x 1280 HDTV Best quality video 720 x 486 ITU-R- 601- Serial Digital Network Distribution 640 x 480 VGA Computer Display 720 x 576 DVD Consumer Disk 720 x 486 DV Camcorder 352 x 240 FCIF, MPEG1 VCR Quality 176 x 120 QCIF Videoconferencing Figure 2-83: Types of Video

Emphasis is placed on “entertainment quality” rather than “videoconference quality”. In general, videoconference quality (in today’s state-of-the-art) is transmitted at bit rates up to about 1.5 Mb/s. Entertainment quality has bit rates from 1.0 Mb/s to 20 Mb/s. These terms of “entertainment quality” and Videoconference quality” are purely subjective. Entertainment quality can be broken down to three levels: 1) ½” VCR home quality (1.0 to 2.0 Mb/s), 2) NTSC broadcast quality (2.0 to 8.0 Mb/s) and 3) High Definition-HDTV quality (12 to 20 Mb/s). These bit rates refer to the information rate. .

Why Digital Television? The digital format for television has an enormous advantage over the analog format because it is inherently more robust. There is noise immunity up to a certain threshold. It is also relatively immune to nonlinear transfer functions of transmission or recording circuits. This immunity or robustness is because each bit is either on or off (1 or 0). The NTSC color system benefits greatly because of its sensitivity to differential gain and differential phase (luminance to chrominance intermodulation) and other nonlinear distortions such as; chrominance to luminance intermodulation, luminance non-

It is for these reasons, and more that the broadcast and recording industries have embraced the digital format for many years in the studio, and in a limited way in transmission where the bandwidth is adequate to support transmission economically. The digital format is not totally immune to transmission anomalies or distortion. Timing errors due to unstable oscillators (phase noise), group delay and certain linear distortions can give rise to implementation loss.

Why Compression? The answer is: bandwidth, bandwidth, bandwidth and economics.

The basic sampling (Nyquist) theorem states that only two samples of the highest frequency in a complex waveform contain all of the information of the original message. The television industry has standardized sampling frequencies and data rate for various input signals. Table 2-4 shows the important parameters for Composite (NTSC) and Component (luminance and color difference) signals. If RGB inputs were to be digitized and coded into 8 bit words and assuming 8.4 MHz sampling frequency, the bit rate would be 8.4 x 106 x 8 x 3 or about 201 Mb/sec.

These uncompressed data rates and the concomitant bandwidth requirements are just impractical for satellite transmission. Adding parity bits for error correction requires additional bandwidth. High order modulation schemes to fit the bit stream into the available bandwidth would require rather large antennas for reception, and more complex modulation and demodulation equipment. In short, there would be no economic incentive to go digital for virtually all applications. However, modern compression technology allows from 4 to 10 entertainment quality television signals to be transmitted in a single satellite transponder with reasonable cost in the earth station segment. It is extremely cost effective in a “broadcast” mode where the receiver population is large. The receiving (decompression) equipment is reasonably inexpensive due to the current availability of single chip (VLSI) decoders. Digital Television–Basics Before delving into compression technology, it is deemed prudent to describe in some detail how a television image is transformed from an analog (complex voltage waveform) to a serial digital stream suitable for transmission. In this discussion, we will deal only with a composite video signal (NTSC video to SMPTE D-2).

Sampling Figure 2-84 provides an overview of the Analog to Digital Conversion process, and we will discuss it in detail below when reviewing Figure 2-85.

Figure 2-84 Overview of the A to D Conversion Process

Let us review in somewhat more detail the sampling and PCM coding discussed in previous paragraphs and how they are applied to NTSC television. The D-2 format suggests that for composite signals the sampling frequency be either 3 or 4 times the subcarrier frequency (10.6 or 14.4 MHz). This frequency is in turn an integral multiple of the line rate (15.75 kHz). These rates represent oversampling but allow anti-aliasing filters with gradual cutoff (similar to roofing filters used in analog video) and to simplify processing and decoding since the same time period is sampled in each active line. Figure 2-86 shows sampling of the color burst (3.58 MHz) at four samples per hertz. The samples are taken at specific burst phase. This is I-Q axis sampling.

525 LINE 59.95 FIELDS PARAMETER NTSC, SMPTE SMPTE D-1 3 FSC D-2, SMPTE RF 125 4 FSC CCIR 601 Baseband bandwidth (MHz) 4.2 4.2 -- Luminance Channel (MHz) -- -- 5.5 Color difference (MHz) -- -- 2.2 Subcarrier Frequency (MHz) 3.58 3.58 -- Sampling Frequency (MHz) 10.6 14.4 -- Luminance -- -- 13.5 Color Difference -- -- 6.75 Bit Rate (Mb/s) 8 bit word 85.9 114.5 -- Luminance -- -- 108 Chrominance -- -- 54 Figure 2-85: Video Sampling Frequencies and Bit Rates

Figure 2-86 Sampling Points (Fs = 4f sc)

Quantizing Once an analog signal is sampled, its amplitude is quantized. If an 8 bit word is assigned to each sample, there are 28 or 256 discrete levels including 0. For SMPTE D-2 only 196 levels are used for video. The balance is used for audio, data and control. Figure 2-87 shows the composite signal and the quantizing level utilization. Levels 0 and 255 are used for synchronization.

Figure 2-86: Composite Quantizing Levels If actual values fall between the discrete levels, the net result is noise and some distortion. For 8 bit quantization, the signal-to-noise ratio is about 48 dB (unweighted).

Encoding

There are many methods of encoding the quantized levels. Only one is discussed here, i.e., pure binary. Each character in a binary word is a bit and is either a 1 or 0. Figure 2- 87 shows the relationship between decimal number and the binary code.

Decimal Binary 0 00000000 1 00000001 2 00000010 3 00000011 4 00000100 5 00000101 6 00000110 7 00000111 8 00001000 9 00001001 10 00001010 11 00001011 12 00001100 13 00001101 14 00001110 15 00001111 20 00010100 100 01100100 200 11001000 255 11111111 Figure 2-87: 8 Bit Binary Codes

As an example, consider the sampling of one cycle of the color burst which was shown in Figure 2-86. The color burst has a peak-to-peak amplitude from –20 to +20 IRE. From figure 2-87 these values carry the quantized numbers ranging from 27 to 87. Taking into account the sample values at the proper phase of the burst signal, the sample quantized values are 82.6 (83), 74.7 (75), 37.4 (37 and 44.3 (44). The binary words corresponding to these levels are shown in Figure 2-88.

4 3

Figure 2-88 Binary Words for Burst Samples

Serial or Parallel Transmission The bits in a code word can be transmitted in a serial or parallel manner. For satellite transmission, parallel (8) paths would be impractical although it would take only one- eight of the required bandwidth for serial operation. For serial operation, the 8 bits in each word are stored in a shift register and read out sequentially to a single circuit. As indicated earlier (Figure 2-85) the overall bit rate for NTSC with sampling frequency of 14.5 MHz (4fsc) is 114.5 Mb/s. The required bandwidth for transmission is about one- half the bit rate (approximately 57 MHz minimum), which as indicated earlier makes satellite transmission unattractive.

Compression Figure 2-89 below provides the bit rates for video which is not yet compressed . As can be noted, these bit rates are extremely high.

ITU-R-601 270 Mbps VGA 220 Mbps NTSC 186 Mbps

Figure 2-89: Uncompressed Video Data Rates

Compression consists of both Intra Frame (within the frame) and Intra Frame (between frames) coding. Intra frame coding is characterized by: • First frame in a Group of Pictures (sequence of frames) • Coded completely within the frame • Coded similarly to JPEG ( used in computer photos) • Discrete Cosine Transform ( DCT) • DPCM or run-length Coding • Variable quantization levels • Huffman variable length Coding • It’s the only time you have a “real” picture • Editing/channel switching must be done on “I” frames

Whereas Inter-frame coding is characterized by: • Coding is done over the Group of Pictures • Combination of forward Prediction (P) and Bi-directional (B) interpolation • Generally P frames are predicted and B frames provide mostly “difference” information • Uses motion compensation vectors, coded image difference information • Complicates editing, channel switching

In general, Figure 2-90 shows how inter frame coding is accomplished using I, P and B frames.

Forward Motion Compensation GOP =9

II B B P B B P B B

Spatially Coded I Frame

Bidirectional Time Motion Compensation

Figure 2-90: Motion Compensation in MPEG 2

Bit rate and the attendant bandwidth reduction are possible in large part due to the nature of the television image and the human vision system. There is a great deal of statistical redundancy in a television image, even one with considerable motion as in sports events. Furthermore, the human vision system has limitations that mask certain distortions or artifacts. Compression technology utilizes sophisticated mathematical tools (algorithms), coding techniques and components (hardware) to exploit these factors.

. Even though there is a great deal of redundancy in a television signal, the statistics of this signal are enormously complex. Each frame of the image represents a new set of statistics. Even if the image is relatively stationary, panning or zoom of the camera creates an entirely new frame. Powerful signal processing chips and massive storage chips have become available at reasonable cost. These semiconductor LSI’s have allowed the sophisticated mathematical tools to analyze and process the image and remove the redundancy and thereby reduce the bit rate and the required bandwidth.

Copyright 2006 125 All rights reserved Skjei Telecom, Inc. Compression Techniques Figure 2-91 shows in a general manner what needs to be accomplished to perform digital video compression encoding. Simplistically speaking, the main task as shown in the first two blocks is to remove time and space redundancies, and then to perform quantization and transformation on the signal, followed by entropy reduction. There are many methods and approaches to video compression taken by researchers over the years. In this document, only those methods used in modern methods are discussed. The discussion here is limited to generalization and simple examples because the theory and implementation is beyond the scope of this text. In practice, current implementation uses a cascade of several methods.

Transform Remove Remove and temporal spatial quantizer Entropy redundancy redundancy reduction

Figure 2-91 Encoding Process Simplified

Pre-Processing and Redundancy Removal Pre-processing removes information that is relatively unimportant to visual quality and is most difficult to code. This is accomplished by a combination of spatial and temporal filters. Pre-processing is an inexact science and its effectiveness depends largely on source material. It is more effective on “canned” material, and less effective on live event material.

Prediction and Motion Compensation Because the difference in signal level of adjacent pixels in a line is usually small, a smaller number of bits per word can be used to represent the difference in levels. This is known as differential PCM. Modern compression systems, utilize the fact that video sequences are highly correlated in time. That is, each frame is quite similar to the preceding and following frames. Therefore, coding the difference means that a lot less information needs to be sent.

The predictive method used is to break a frame down to small blocks (typically 16 x 16 pixels) and searching at nearby positions of the previous frame. It is therefore possible

Copyright 2006 126 All rights reserved Skjei Telecom, Inc. to find a good predictor block so that only the position of predictor block and the current block needs to be sent. This process greatly reduces the bit rate when portions of a scene are in motion. Hence, a great deal of motion compensation is produced.

Figure 2-90 shows how MPEG 2 uses I, B and P frames to achieve motion compensation. I frames are complete frames and are suitable for editing. P frames are predicted frames based on the last I frame and the encoding algorithm. B frames are interpolated among both I and P frames.

Transformation–Frequency Decomposition The object of a transformation process is to find a frequency representation or the signal representing the motion compensation residual frame. The exact mathematical approach is beyond the scope of this text. Instead, we discuss the purpose and provide a simple example of a transform.

The motion compensated signal is analyzed into two dimensional (spatial and temporal) frequency components. The signal usually has most of its energy concentrated in a narrow band, so that fewer bits need be sent at the unimportant frequencies. Furthermore, the transformation process mirrors the human visual system and allows quantization to be tailored to the sensitivity of the human visual system to frequency content.

The predominant transform currently used is the discrete cosine transform (DCT). The DCT takes a block of the motion compensation residual (16 x 16 pixels) and converts it to a corresponding set of coefficients representing different frequency components. As indicated earlier, this is an exceedingly complex procedure, whereby a set of values is converted to another set of values that can be transmitted with less data by utilizing a series of equations.

As a simple example, assume the values A,B,C and D are transformed to the values W,X,Y and Z by the equations: W=A X=B-A Y=C-A Z=D-A W,X,Y and Z are transmitted, Since X, Y and Z are difference values, they are smaller than the original values and do not require as many bits per word.

Quantization In a previous section, we discussed the quantization process whereby the amplitude of a sample was given an eight bit word that can identify 256 discrete levels. Non linear quantization (Companding) and/or course linear quantization can reduce the bit rate at

Copyright 2006 127 All rights reserved Skjei Telecom, Inc. the expense of quality or noise. Judicious companding of the unimportant frequency components of the decomposition process can reduce the bit rate.

“Vector Quantization” applies the quantization process to more than one co-efficient simultaneously. A block of pixels (or a set of coefficients) are considered as a vector, and a search is made to find one of a small set of representative vectors that is close to the input vector. The representative vectors are resident in a codebook and its index (number) in the codebook is transmitted. The number of bits required for transmission is therefore less than if the input vector was transmitted.

Entropy Reduction Bit rate reduction can also be accomplished by exploiting the fact that some image values are less frequently encountered than others. By applying a longer code word to the less frequently encountered values, and a shorter code word to the frequently encountered values, bit rate reduction can be realized. A commonly used code in video compression in the “Huffman Code”.

The quantized frequency domain coefficients tend to have the value zero at many different frequencies and large groups of zeros are clustered at the higher frequencies. A run-length of many zeros gets assigned a short word in the Huffman Code.

Algorithms The algorithm is the basic tool for specifying the mathematical operations to be performed for bandwidth reduction. Since modern compression systems use a combination of the methods described above, the operations are extremely complex. Figure 2-92 shows the various elements of MPEG-2 encoding. Motion compensation, DCT frequency decomposition, Companding and Huffman Coding are employed.

MOTION DISCRETE STATISTICAL HUFFMAN COMPENSATION COSINE QUANTIZATION CODING TRANSFORM

NTSC VIDEO INPUT MPEG-2 ALGORITHM COMPRESSED DIGITAL VIDEO OUT

Figure 2-92: Basic Elements in MPEG 2 Encoder Once encoded, the encoded audio signal and any data signals are added to the formatted digital bit stream known as a “transport stream” by a process which is shown in Figure 2-93, Figure 2-94, and Figure 2-95.

44 Byte Byte PacketPacket Header Header 188 Byte Packet 184 Byte Payload (incl. optional Adaptation Header)

video TEXT video audio 1 video video audio 2 video video PGM GD video

• All packet types carry one type of data, identified by a PID • Various data types are multiplexed into the packet stream • PID eliminates backward compatibility problems - receivers ignore packet types that they cannot process

www.skjeitelecom.com 703-917-4077

Figure 2-93 MPEG Transport Packet Stream

Figure 2-94 MPEG Transport Stream Packet

control TheThe Packet Packet Scheduler Scheduler permits permits packets packets audio 1 audio 1 intointo the the bit bit stream stream according according to to needneed and and priority. priority. This This allows allows dynamic dynamic allocation of the channel. audio 2 audio 2 allocation of the channel.

Video Video Video Video Video Packet Scheduler aux data aux data From source encoders

Video Video audio 1 Video audio 2 aux data control

To Transmitter

Figure 2-95 MPEG Transport Stream Packet Multiplexing Decompression – Decoding The decompression processing is not nearly as complex as the compression process. In fact, several semiconductor manufacturers are providing decoders on a single chip. The process involves the inverse of the compression process to produce the signal in the same format as the input to the encoder. Figure 2-96 shows a conceptual view of the Packet Demultiplexing process and Figure 2-97 shows a block diagram of the de- compressor. All signals to activate the elements are imbedded in the transmitted data stream.

Packet Demultiplexing . . . unrecognized packets are discarded . . .

De-mux Initialization Reads PID's and sorts packets control CPU Video audio 1 Video audio 2 aux data control Packet De-Mux Packets from RF demodulator Video audio 2 Video Decoder

aux data audio 1 Audio Decoder

Digital Broadcast Receiver TheThe Path Path to to ExtensibilityExtensibility

Figure 2-96 Packet Demultiplexing

RECON- ERROR HUFFMAN DCT FREQ. STRUCTED CONCEAL- DECODER COMPOSITION FRAME MENT VIDEO OUT INCOMING DATA STREAM MOTION COMPENSATION REFERENCE

Figure 2-97 Basic Elements of the MPEG-2 Decoder

Representative Digital Distribution Process

AMC-3 Satellite, Transponder

ABC Television SA 9110 Digital Radio Encoder AT&T CBS Television Uplink Remote Cable SA 9110 Scientific Cable Digital Operator Radio Atlanta Fiber Encoder Digital Multiplexer Optic NBC Television SA 9110 Model D9130 Interface Digital Radio Encoder QPSK Fox Television Mod- Up- HPA SA 9110 ulator Converter Digital Encoder PBS Television Encryptor SA 9110 Digital Integrated Encoder Receiver Decoder SA 9110 Digital Radio Encoder Decryptor

Data Services SA 9110 Digital Radio Encoder

Television to Cable Plant

Figure 2-98: Complete Digital System Standardization Compression technology has advanced very rapidly. The International Standard Setting organization (ISO) and the Motion Picture Engineering Group (MPEG) has worked diligently toward establishing a standard for compression of entertainment quality television. The MPEG-2 standard was adopted in November 1993. Figure 2-99 shows the Levels and Profiles adopted for the MPEG 2 standard. Looking at this figure shows that not only is the “main level main profile” shown, which is the standard for video backhauling and other applications, but MPEG 1, 4:2:2 profile and even HDTV is shown in this figure.

Satellite delivery is currently operating both multiplexed (several TV channels) on a carrier (MCPC), and a single TV channel on a carrier (SCPC). One carrier will normally contain one transport stream, which is capable of multiplexing together a large number of video, audio and data signals.

The MPEG-2 system is not a fixed bit rate system. The bit rate is determined by the quality desired. There are no current objective standards. Only subjective descriptions are currently used. It is generally agreed that ½ inch VCR quality is obtained at bit rates in the range 1.5 to 2.5 Mb/s. Broadcast quality is obtained with bit rates from 2.5 to 8.0 Mb/s depending on program content. In MCPC transmission, adaptive bit rate multiplexing can be employed by borrowing bits from one program with little or no motion and assigning them to a program with a lot of motion.

Current Standards for Satellite Transmission of Digital Television The following digital video standards exist for the more common video formats, and are listed in approximate chronological order;

• MPEG 1 • MPEG 2 • 4:2:0 profile • 4:2:2 profile • DVB • ATSC (also known as DTV, ATV): HDTV and SDTV • MPEG 4 (also now known as MPEG 4-2) • AVC (also known as H.264, H.26L, MPEG 4-10) • Windows Media 9/ SMPTE VC-9 • JPEG 2000 In previous sections above, some basic aspects of digital television and modern compression technology (MPEG 2 – ISO/IEC 13818-1) were discussed. In this section, we discuss additional standards activity since the adoption of the MPEG 2 standard.

Profiles

Sim p le Main Professional SN R Sp acially High Leve ls I,P I,P,B I,P,B I,P,B I,P,B I,P,B 4:2:0 4:2:0 4:2:2 4:2:0 4:2:0 4:2:2 Non- Non- Non- SN R Sp acially SN R, scalable scalable scalable scalable scalable spacially scalable High Grand <1920x1152 Alliance < 100 Mbps 60 frames/ HDTV second < 80 Mbps

High-1440 <1440x1152 <60 Mbps < 60 Mbps < 80 Mbps 60 frames/ second

Main MLMP Professional <720x576 < 15 Mbps < 15 Mbps Profile < 15 Mbps < 20 Mbps 30 frames/ < 50 Mbps second

Low MPEG-1 <352x288 < 4 Mbps < 4 Mbps 30 frames/ second

www.skjeitelecom.com 703-917-4077

Figure 2-99 MPEG-2 Levels and Profiles

Digital Television Standard (DVB) The European Broadcast Union has generated standards for both terrestrial broadcast and satellite distribution of digital TV in a comprehensive project called DVB. For satellite transmission, interoperability tests were run between manufacturers of codecs (coders and decoders). As a result of these tests an approved list of compatible coders and decoders has been developed. From an uplink earth station operator standpoint it is prudent to purchase coders that are “DVB compatible”. Receivers in the network should also have DVB compatible decoders. If the program transmitted has conditional access encryption, the receivers in the network should be equipped with appropriate decryption. The conditional access process is unique and not part of the standard.

The DVB standards are based on MPEG 2 and on Forward Error Correction (FEC) coding. It goes beyond MPEG 2 however by adding standards not included in MPEG 2 for • Modulation • FEC • Interleaving • Conditional Access/Encryption • Data Services encapsulation

A powerful FEC coding is used that consists of a combination of Reed Solomon (block), Viterbi (convolution), and interleaving coding. In addition, the baseband filter shape (alpha factor) is standardized.

As indicated earlier in the text , MPEG-2 is not a fixed data rate standard. Therefore, the data rate is a function of the quality desired in the television reproduction. Variables are: 1) symbol rate, FEC rate, and modulation level (m).

The defining relation is:

Usable Information Rate Ru = Sx(m)x(188/204)x(FEC)

Where, S is the symbol rate (selected in the modulator), (m) is the modulation factor (for QPSK, m = 2) 188/204 is the Reed Solomon block inner code, and FEC is the Viterbi convolution code

A popular MCPC (Multiple Channel per Carrier) utilizes a full (36 MHz) satellite transponder with QPSK modulation, ¾ rate FEC, and a composite Symbol Rate of 19.5 Mb/s. This provides a usable Information rate of 27 Mb/s. This amount of information can be shared with a multiplicity of TV channels by the use of adaptive multiplexing.

DVB-S and DVB-S2 A brief comparison of DVB-S and DVB-S2 shows that DVB-S2 is essentially a significant expansion of the transmission method prescribed in DVB-S: • DVB-S – Published in 1993 – Modulation: primarily QPSK and BPSK – Convolutional codes concatenated with RS codes • DVB-S2 – Published in 2003

Copyright 2006 136 All rights reserved Skjei Telecom, Inc. – Modulation: QPSK, 8-PSK (broadcast applications), 16-APSK and 32-APSK (professional applications) – Backward-compatibity with existing DVB-S receivers – Reasonable receiver complexity – Interactivity (i.e., Internet access) – Best transmission performance • LDPC codes were concatenated with BCH codes • Variable and adaptive coding and modulation (recovers rain margin) • Approximately 30 % capacity increase compared to DVB-S – Maximum flexibility • framing structure • variable and adaptive coding and modulation • can operate in any existing satellite transponder • accommodates any input stream format (188-byte MPEG-2 transport streams (packets), continuous bit streams, IP, ATM) Emerging Encoding Methods: MPEG 4 and JPEG 2000

MPEG 4 Part 10 and SMPTE VC-9 • H.26L or AVC or MPEG- 4-10: – continuation of MPEG standards process – takes advantage of more powerful processing – probably 50% reduction in data rate • Windows Media 9 – similar to H.26L but proprietary thus far – serious attempt by Microsoft to enter broadcast video market – Being standardized by SMPTE as VC-9

Characteristics of MPEG 4, also known a H.264, include: • By 2007, broadcast quality at 1 Mbps • Enhanced motion compensation prediction – Quarter pixel interpolation – Optional smaller block sizes (16x16 to 4x4) – Option to search across many more frames – Option for more “B” frames • Allows Adaptive Field or Frame encoding on macro block basis • Loop filter smoothes block edges to minimize blocking • Improved Entropy Coding – Context Adaptive Binary Arithmetic Coding (CABAC) • Computationally intensive • Gives 15% improvement in compression

Copyright 2006 137 All rights reserved Skjei Telecom, Inc. – Context Adaptive Variable Length Coding (CAVLC) • Refined version of Huffman coding – These are the first defined MPEG encoder functions but are optional

The encoding process is shown in Figure 2-100. Compare this process with that previously shown for MPEG 2 to observe the improvements. Simplified H.264 Encoding Process

Remove Remove Transform and Entropy temporal spatial reduction redundancy redundancy quantizer

Loop Filter Inverse Inverse Transform Quantizer

Figure 2-100 MPEG 4-10 Encoding Process

JPEG 2000 • JPEG2000 is the successor to JPEG • State of the art compression technology based on Wavelet Technology • Overall goals: – Better compression then original DCT (Discrete Cosine Transform) based JPEG – Scalability, extract multiple resolution images from a single high resolution master – Softer Artifacts – Compression scheme that worked well in network environments (trade off bandwidth for resolution, quality JPEG 2000 uses Wavelet compression that transforms images into wavelet subbands and resolutions. The general process for this is as follows:

Copyright 2006 138 All rights reserved Skjei Telecom, Inc. -Image is sent to a set of wavelet filters -pixel information is transformed into wavelet coefficients -these are then grouped into several sub-bands that describe H and V frequencies -Lower frequencies remain in first transform level -Higher frequencies contained in higher transform levels Figure 2-101, shows graphically more on how this is accomplished.

LL2 HL2 LL2 HL2 LL1 HL1 LL1 HL1 LL1 HL1 LH2 HH2 LH2 HH2 Original 1st Pass

LH1 HH1 LH1 HH1 LH1 HH1

A BCD

Figure 2-101: JPEG 2000 Process

High Definition Television Currently three standards exist throughout the world for transmission of HDTV. These are shown in Figure 2-102. System Adopters Modulation/Other ATSC US, Mexico, Canada, 8 VSB Argentina, South Korea ISDB Japan, Brazil COFDM Contains DAB capability DVB Everybody else COFDM Figure 2-102 HDTV Standards and Implementation Advanced Television Standards Committee (ATSC) The ATSC has generated a standard (DOC. A/80) that builds on the MPEG-2 and DVB standards specifically for satellite transmission. A copy of this standard can be obtained on the Internet at http://www.atsc.org/standards.html.

This document provides for optional higher modulation levels of 8PSK and 16 QAM, while QPSK is considered mandatory. It also contains more detailed information about digital television than is contained in this textbook. The student is encouraged to download it. Furthermore, the interested student should consider obtaining the references contained in ATSC-A/80.

The ATSC DTV standard consists of a single or multiple television channel or multiple television channel carrying HDTV and/or SDTV. Encapsulated data may be transmitted as well. It consists of :

• Video Formats – Interlaced & Progressive • Video Compression – MPEG-2 • Audio Compression – Dolby AC-3 • Packetized Data Transport It can be thought of as defining the signal in three layers, as shown in Figure 2-103 below:

1920 x 1080 Picture MultipleMultiple Picture Picture Formats Formats Layer 1280 x 720 and Frame Rates 60,30, 24 Hz and Frame Rates

Video MPEG-2 Compression compression syntax Layer Data Motion Chroma and Luma Headers Vectors DCT Coefficients Variable Length Codes

Packet Headers FlexibleFlexible delivery delivery of of data data Transport Layer Video packet Audio packet Video packet Aux data MPEG-2 packets

Transmission 6 MHz Channel 8-VSB Layer

Figure 2-103 : ATSC Digital Television Layers

ATSC Video Compression is characterized by : • MPEG-2 Compliant • Main Profile @ High & Main Levels • 19.3 Mbps Information Stream • 188 byte transport packetization • ATM Compatible

Whereas the ATSC Audio Signal does not use MPEG 2 audio but instead can employ Dolby 2.0, PCM or Dolby AC-3. AC-3 characteristics include: • Motion Picture System • Surround Sound • 5.1 channels • 384 Kbps

When transmitted terrestrially, the ATSC signal employs: • 8-VSB Modulation • 10.76 M Symbols/sec • 3 Bits/Symbol • Forward Error Correction – Trellis coding (2/3 rate) – Reed-Solomon (T=10) • 19.4 Mbps information rate • Offset QAM, 16 VSB also defined

Figure 2-104 below shows four HDTV Formats currently defined. The two formats in most common use today are 1920 x 1080 Interlaced (30 frames/sec) and 1280 x 720 P (60 frames/sec): Resolution Scan Frames/Sec Possible Applications 1920 x 1080 P 24 or 30 Highest Spatial Resolution 1920 x 1080 I 30 Scenes shot with an interlace camera 1280 x 720 P 24 or 30 Complex film scenes, graphics, possibly reduced data rate 1280 x 720 P 60 Sports, concerts, animation, graphics, up converted NTSC Figure 2-104: Four currently defined ATSC HDTV formats

There are several SDTV picture formats defined. Two of these formats include: • 480 x 704 Resolution – 16:9 & 4:3 – Progressive @ 60, 30, 24 frames/second – Interlace @ 60 fields/second • 480 x 640 Resolution – 4:3, square pixel spacing – Progressive @ 60, 30, 24 frames/second – Interlace @ 60 fields/second

Satellite Transmission of Compressed Television Current satellite transmission practice for compressed television utilizes QPSK modulation as a good compromise between spectrum and power efficiency. Higher

Copyright 2006 142 All rights reserved Skjei Telecom, Inc. modulation levels can be used such as 8 PSK and 16 QAM. ATSC has defined standard satellite transmission formats that generally employ DVB-S modulation. Other proprietary digital formats such as Digicipher II are still in use however and employ their own transmission systems. Video file formats and asset management systems employing MXF asset management will increasingly be used.

The threshold bit error rate is on the order of 10-10 to 10-12. The FEC used is a concatenation of two separate coding schemes (Reed-Solomon, which is a block or polynomial code, and Viterbi, which is a convolutional code). Typically a 188/204 Reed Solomon is used with a concatenated code. If Rate ¾ concatenated coding is used, this results in an effective FEC rate of 0.69

Typically, a Nyquist rolloff factor of 1.35 is used for occupied bandwidth calculation.

For example, one SCPC implementation has a (digital video) information rate of 2.6 Mb/s, and a rate .69 FEC. The transmission rate is 3,770,000 transmitted bits. This is a symbol rate of 1.88 mega symbols per second for QPSK, which requires a bandwidth of approximately 2.54 MHz assuming the rolloff factor mentioned above. This is calculated as follows: • Bandwidth Required = (Symbol Rate) X (Roll off Factor) – Roll off factor typically 1.33 TO 1.35 but can be as small as 1.25 – Equipment limitations (minimum frequency step size) can add to bandwidth requirement • Symbol Rate = (Usable Info Rate)/(M) x (FEC) – Where M= Modulation factor =2 for QPSK, 3 for 8 PSK and 4 for 16 QAM, – FEC= Total Forward Error Correction – inner plus outer code

Concerning the question of number of compressed digital signals that can be transmitted in a satellite transponder,: • Need to distinguish between “contribution”, “distribution” and “direct to home” type of service. • Encoding bit rates will continue to decrease as they have in the past • Generally, Contribution>Distribution>Direct to home bit rates • Number of signals per transponder varies depending upon: – Type of service/bit rate – Modulation – Multiplexing (MCPC or SCPC), including statistical multiplexing – Receive Antenna Size – Transponder bandwidth, gain step, power • Today, most digital video signals tend to be bandwidth limited but 16 QAM could change that. A representative link budget for compressed digital video via satellite is shown in Figure 2-105 below.

REQUIREMENTS SATELLITE ------Availability (%): 99.866 Satellite GE 3 Required Eb/No (dB): 5.50 Satellite West Long : 87.0 Bit Error Rate : QEF Transponder LINEARIZED 60 W Modulation Type : QPSK Usable Trnspndr BW (MHz): 36.00 Info. Rate (Kbps): 3000.00 SFD @ 0 dB/K (dBW/M^2): -93.10 FEC Rate : 0.69 Transponder Atten (dB): 8.0

TRANSMIT E/S RECEIVE E/S ------North Lat: 48.0 West Long: 73.0 North Lat: 30.0 West Long: 100.0 Frequency (GHz): 14.25 Frequency (GHz): 12.10 Satellite G/T (dB/K): 4.30 Satellite EIRP (dBW): 46.00 Antenna Diameter (m): 5.0 Antenna Diameter (m): 3.0 Antenna Gain (dBi): 55.10 Antenna Gain (dBi): 49.20 Antenna Elevation (Deg): 33.23 Antenna Elevation (Deg): 52.30 Carrier EIRP (dBW): 55.31 LNA Noise Temp (K): 125.00 Power Control (dB): 0.00 Loss betw.LNA & Ant.(dB): 0.10 Output Circuit Loss (dB): 3.00 System Noise Temp. (K): 175.67 Path Loss (dB): 207.22 Station G/T (dB/K): 26.75 Other Losses (dB): 0.70 Path Loss (dB): 205.48 (other loss = atm,pol,ant point) Other Losses (dB): 0.60

INTERFERENCE ------C/Io Adj Sat U (dB-Hz): 83.42 C/Io Intermod (dB-Hz): 86.44 C/Io Adj Sat D (dB-Hz): 87.30 C/No Thermal Up (dB-Hz): 80.29 C/Io Crosspol (dB-Hz): 150.00 C/No Thermal Dn (dB-Hz): 80.87 C/Io Adj Channel (dB-Hz): 90.92 C/Io Total (dB-Hz): 79.90 C/Io Adj Trans (dB-Hz): 91.31 C/No Therm Total (dB-Hz): 77.56 C/Io Microwave (dB-Hz): N/A C/No Total (dB-Hz): 75.57

RAIN ATTENUATION ------Overall Link Margin (dB): 5.29 Rain Model : CRANE Uplink Availability (%): 99.934 Rain Margin (dB): 5.29 Uplink Rain Zone : D2 Dnlink Availability (%): 99.932 Rain Margin (dB): 6.08 Dnlink Rain Zone : D3 G/T Degradation (dB): 3.51

TRANSPONDER H.P.A ------Number of Carriers : MULTIPLE Number of Carriers : 1.0 Total OPBO (dB): 3.50 Total HPA OPBO : 0.00 Total IPBO (dB): 7.78 HPA Power/Carrier (dBm): 33.21 Carrier OPBO (dB): 14.40 Required HPA Size (dBW): 3.21 Carrier IPBO (dB): 18.68 Required HPA Size (W): 2.10

FCC Req: 1) Uplink Flange Density (dBW/4kHz): -27.14 (@50.0) 2) Downlink EIRP Density (dBW/4kHz): 8.25 Transponder BW Used Per Carrier (x1.35) (%): 8.15 Transponder Power Used Per Carrier (%): 8.13 Transponder Bandwidth Allocation (MHz): 2.935 Figure 2-105 Compressed Video Link Budget (outputs in italics)

High Definition (HD) Transmission Over Satellite Many terrestrial stations from network feeds distributed by satellite currently broadcast HD. In addition, cable and Direct to Home (satellite) companies can deliver HD programs to subscribers if they are equipped with HD receivers.

In most cases of current satellite distribution, the ATSC-A/80 standard is used. QPSK modulation with a data rate of 19.4Mb/s and ¾ rate FEC is established for an HD transmission. This signal can be transmitted in either a full or half (36 or 54mHz) transponder or a full 27 MHz transponder. In one implementation, two 19.4 Mb/s streams are multiplexed into a 39 Mb/s ¾ rate signal and transmitted to a 36 MHz satellite transponder. Frequently for distribution not directly to the end user, 32-38 Mbps data rate is used for a single HDTV signal.

Direct Broadcast Satellite Systems Figure 2-106 provides a comparison of the satellite transmission bands used for the Fixed Satellite Service (FSS) and the Direct Broadcast Service (BSS) FSS BSS Polarization Linear Circular Satellite Spacing 2 degrees 9 degrees Dish Size > 1 m < 1 m Uplink 14 GHz 17 GHz Downlink 11.7‐12.2 GHz 12.2‐12.5 GHz FCC Authorization By orbital arc location By location and channel (transponder) Access Method Various MCPC

Figure 2-106: Difference between Ku FSS and BSS

Figure 2-107 provides comparative information on the two primary DBS systems Standard Used Conditional Private Networks Access DirecTV DSS NDS Few Echostar (Dish) DVB (except audio) Nagra Some Figure 2-107 Comparison of DBS Systems

Figure 2-108 provides representative spot beam coverage for the DirecTV system.

25 8 3 Three Transponders 15

DIRECTV 4S Spot Beam Coverage (estimated maximum coverage area per spot beam)

Figure 2-108 Representative DirecTV Spot Beam Coverage

In very general terms, a complete satellite earth station consists of the following basic elements: Uplink ¾ Baseband Equipment ¾ Modulating Equipment ¾ Upconverters ¾ Intermediate Power Amplifier/High Power Amplifier (IPA/HPA) Channelization Multiplexer Downlink ¾ Low Noise Amplifier (LNA) or LNB ¾ Power Divider ¾ Downconverter ¾ Demodulation Equipment ¾ Baseband Equipment Common Equipment ¾ Antenna, Feed and Duplexer ¾ Interfacility Link (IFL) ¾ Monitor and Control System Figure 3-1 shows how these basic elements form a complete earth station.

Figure 3-1 Composite Satellite Earth Station

Copyright 2006 147 All rights reserved Skjei Telecom, Inc. In many applications, two or more of these basic elements are lumped together to form an integrated unit. In some cases, a very high level of integration exists. Furthermore, not all of these basic elements need to exist in some applications. In addition, as indicated in Figure 3-1, the low level (uplink) equipment and downlink (less LNA) equipment is sometimes called Ground Communications Equipment or GCE.

These elements will be discussed in this chapter in a functional manner. Important features from a safety and interference avoidance standpoint are highlighted. Specifics on a particular piece of equipment are not included.

Figure 3-2 shows a more detailed view of a larger earth Ku earth station, showing other earth station components such as monitoring and control, etc.

Figure 3-2 Large Ku Band Earth Station

Uplink Equipment Uplink Ground Communications Equipment Modern uplink GCE for use in U.S. domestic service television transmission frequently consists of a single rack mounted unit called an exciter. Baseband, modulation and upconversion functions are usually performed with plug-in modules. The interconnection between modules is either on a printed circuit or wired backboard. For other services, packaging can cover a wide range of configurations.

In analog SCPC, a modulator contains both baseband and modulation functions and a separate upconverter contain tight tolerance frequency oscillators. In digital SCPC systems, the modulation and demodulation function is performed in a single package called a modem, and the RF function of up and down conversion is also combined in a single package. A narrowband VSAT will contain a RF Head (also called the Outdoor Unit) which includes the up/down conversion plus the added function of power amplifier and low noise amplifier. The RF head will interface directly with the antenna feed horn in an outdoor enclosure.

Some functional details are given here for the more commonly used services. The uplink operator must be constantly aware of the fact that the frequency determining elements are contained in the uplink GCE regardless of the service or configuration. The operator must be familiar with all of the elements that determine the uplink frequency and never radiate a signal from the antenna that is not on the authorized frequency. Whenever frequency adjustment or changes are made, either disconnect the GCE from subsequent amplifier stages, disable the amplifier stages, or radiate into a dummy load. Always carefully monitor frequency adjustment or changes.

Television Exciters/Uplink Video Equipment

Analog Exciter Figure 3-3 shows a typical analog television exciter in block diagram form. The configuration shown is for a non-redundant, non-scrambled, NTSC video transmission, with FM subcarriers, subcarrier ATIS and dual conversion for frequency agility. Redundant configurations will have a pair of exciters fed from a video/audio distribution amplifier (DA), and the outputs feeding separate power amplifiers. In some configurations, the exciter outputs can be switched to either single string power amplifiers or redundant power amplifiers.

Baseband Circuits Reference to figure 3-3 shows several functional elements in the baseband portion of a TV Exciter. In some exciters, the baseband video is band limited by a low pass (roofing) filter. This is good practice, since a highly saturated color signal may have frequency

Baseband audio is frequency modulated on a subcarrier and combined with the video waveform. The composite baseband signal is then fed to an FM modulator.

An Automatic Transmitter Identification System (ATIS) is required on all wideband FM/TV transmission. The method mandated by the FCC is one where a subcarrier at 7.1 MHz is used. This ATIS is discussed in more detail below.

Figure 3-3 Basic Elements of an Analog TV Exciter

Modulation and Upconversion The frequency modulation is usually accomplished at a high frequency in the neighborhood of 1.0 GHz in order to obtain good linearity over a reasonably wide bandwidth. The signal is then down converted to a 70 MHz IF. A band-pass filter is then used for the dual purpose of: 1) elimination of image and harmonies in the downconversion, and 2) limit the modulation components to the necessary bandwidth and guard against over-deviation.

Dual conversion to the ultimate RF output is usually employed to provide frequency agility. Figure 3-4 shows how frequency agility over a 500 MHz band is obtained using fixed tuned filters and by varying the second local oscillator frequency. By selecting the upper sideband, the modulation sense remains erect throughout the conversion

Figure 3-4 Dual Conversion Process –

Transmitter Identification As indicated above, the FCC has mandated an ATIS for wideband video transmission. The method chosen is one where a low level subcarrier is continuously modulated by an international Morse Code message. Other methods may be used if they were implemented before March 1, 1991 if a waiver is requested. The message to be transmitted consists of:

The FCC assigned earth station call sign; A telephone number providing immediate access to personnel capable of resolving ongoing interference or coordination problems with the station; A unique ten digit serial number or random code programmed into the ATIS device in a permanent manner such that it cannot be readily changed by the operator on duty; Additional information, provided that the total message length, including ATIS does not exceed 30 seconds.

Figure 3-5 is a block diagram of the subcarrier ATIS. The subcarrier frequency is 7.1 MHz and the injection level is -26 dB referenced to the unmodulated microwave main carrier.

Figure 3-5 Subcarrier ATIS-Block Diagram

Digital Exciter A block diagram of a digital exciter is shown in Figure 3-6. Digital exciters are not as common as analog exciters but are frequently used where cost and space are at a premium. If the video and audio signals are not already in digital format, they are first converted to digital using an A/D or analog to digital converter. They are then MPEG (typically) encoded and combined into an MPEG transport stream. This transport stream is typically digitally modulated by a QPSK or 8 PSK modulator and then upconverted to RF for transmission to the satellite.

SCPC Uplinks There are many SCPC or partial transponder applications ranging from one-way (point- to-multipoint) to two-way services or combinations such as teleconferencing. Both analog and digital modulation, with a wide range of data rates and occupied bandwidth are in use. Equipment packaging varies greatly depending on the type of network involved. Figure 3-7 shows the configuration of a generic SCPC uplink in block diagram form.

In both analog and digital networks the modulators have frequency agility to allow operation at any frequency within a transponder bandwidth, and the upconverters have agility to operate in any transponder.

VIDEO IN RF OUT

A to D AUDIO CONVER- ENCODER TER

AUDIO IN Figure 3-6 Digital Exciter

Figure 3-7 Simplified Block Diagram of Digital SCPC Uplink If an upconverter is used to carry more than one carrier as might be the case for several SCPC channels from the same uplink station, the levels out of the converter should be

Copyright 2006 153 All rights reserved Skjei Telecom, Inc. carefully monitored and controlled to preclude or limit intermodulation components to a tolerable level. Figure 3-8 shows two audio channels using a single upconverter.

Figure 3-8 Two or more SCPC Channels Feeding a Common Upconverter

Satellite operators normally require that intermodulation components emanating from an uplink station be more than 30 dB below the carrier level (C/IM> 30 dB). This will entail de-rating the output power level. The amount of output backoff will depend on the saturation and intermodulation characteristics of the particular upconverter used. The manufacturer will supply data or specifications relating to intermodulation. This data can be: 1) Saturation Level, 2) Third Order Intercept Level, or 3) Actual 2 Tone Intermodulation data. The actual data can be given as a single point or a curve. Regardless of the data or specifications supplied, it is recommended that actual measurements be made with two equal level carriers. The input level of each carrier should be adjusted so that the output C/IM is greater than 45 dB. The actual power level of each carrier should then be measured (dBm) with the spectrum analyzer. A check with a power meter can be made, keeping in mind that the power meter reads twice the power in each carrier. A typical value of carrier level to meet this requirement is - 27 dBm for upconverters used in this service.

If more than 2 carriers are passed through the upconverter, the input level should be further reduced in accordance with (10 log N) dB where N is the number of carriers.

Power Amplifiers Power amplifiers used in U.S. domestic satellite service range from 1 watt output power to over 3,000 watts capacity. The power amplifier used depends on the application. Solid state amplifiers are usually used for requirements up to about 20 watts at C band

Copyright 2006 154 All rights reserved Skjei Telecom, Inc. and up to about 10 watts at Ku band. Traveling wave tube amplifiers are generally used for power requirements up to about 700 watts at C band and up to about 600 watts at Ku band. Klystron tube amplifiers are used for power requirements above that. Commonly used Klystron amplifiers have 3,000 watt capacity at C band and 2,500 watt capacity at Ku Band.

Solid state amplifiers and traveling wave tube amplifiers are broadband, meaning they have reasonably flat response over the full 500 MHz of the C or Ku band. Klystron amplifiers are narrowband, meaning they have reasonably flat response over a transponder bandwidth. Modern Klystron amplifiers can be pre-tuned to a number of channels corresponding to a number of transponders.

Some power amplifiers consist of two distinct stages, especially the higher power TWT and Klystron amplifiers. The first stage is an Intermediate Power Amplifier or IPA, and the second or final stage is a High Power Amplifier or HPA. This two stage arrangement is necessary because the nominal output power from an upconverter is usually about - 40 dBW. To obtain 1,000 watts (+30 dBW) a gain of 70 dB would be required. Although this is possible, a single amplifier with this much gain in one unit would be unstable. The higher power klystron amplifiers usually have the IPA included in a single package. These units are sometimes called KPA’s.

Figure 3-9 Shows characteristics of various types of power amplifiers

TWTA SSPA Klystron

Bandwidth 500 MHz 500 MHz 70 MHz

Linearity Fair (but can be Good Fair-Poor linearized) Max Power 3000 W (but 200 W 3000 W 600W more common) Cost/ Watt Medium Medium-High Low

Life Medium Longest Long

Figure 3-9 Characteristics of Different Power Amplifiers

If a power amplifier is used to amplify more than one carrier simultaneously, consideration should be given to intermodulation in a manner similar to the common upconverter discussed above.

Copyright 2006 155 All rights reserved Skjei Telecom, Inc. It is good practice to have plenty of reserve power for flexibility to operate with any satellite. For most Ku band satellites a Power Amplifier with 300 watts is adequate for most SCPC applications.

Multiplexers and Switches

Switches The discussion on uplink hardware to this point implies operation with a single satellite using a single antenna. There are a variety of major earth stations where the same uplink hardware may be used to access more than one satellite. These major stations are known as teleports. A teleport may have a multiplicity of antennas each dedicated to a single satellite or it might have an antenna known as a TORUS that has a single reflector and a multiplicity of feeds. The station might have a combination of the two. In order to time share equipment on more than one antenna or feed, the station usually has a switch matrix on the output of the power amplifiers. Figure 3-10 shows a simple case of how a single uplink chain can be fed to more than one antenna. This is accomplished by using waveguide switches capable of handling the high power usually required. The switches can also be used to feed signals into either of the cross polarized ports of an antenna.

Great care should be taken if switches are used to feed multiple antennae or cross polarized ports. The switches should be clearly labeled to preclude wrong positioning. They should be located in an area with restricted access. If the switches are controlled remotely, they should be interlocked by faultless logic to preclude wrong positioning. Bells and/or alarms should sound for wrong positioning.

Multiplexers Most major earth stations will transmit on more than one transponder in a satellite on the same polarization. This multiple transmission will probably be required simultaneously. Multiplexing can be accomplished in a number of ways. The best way depends on the circumstances that exist in a particular installation. Two major components are used for multiplexing. They are 1) Filters and 2) Hybrids. Use of filters is recommended for use with a specific transponder or a similar transponder on another satellite. Generally, a filter type multiplexer will present less than 1.0 dB loss to any one channel. Hybrid type multiplexers present 3 dB loss every time the number of channels doubles. In some cases, a combination of filters and hybrids is required.

Figure 3-11 shows how a filter type multiplexer is used to inject several channels onto a single waveguide manifold, with relatively low loss. This example shows 6 channels being fed to the horizontal polarization port of an antenna on their way to a satellite, where the uplink vertical polarization channels have odd numbers (1,3,5…). It should be noted that every other odd channel is multiplexed onto this single manifold. This is necessary since the channels are contiguous, and a guard band is needed for practical purposes.

Figure 3-10 Sharing an Uplink with More Than One Antenna. Consider a signal entering the channel #21 filter. It passes through with less than 0.5 dB loss. It then enters a circulator that is a passive device that allows energy to travel in only one direction (arrow denotes direction). The signal then enters the output of the filter for channel 17 where it is reflected back to a circulator and so on until it is fed to an antenna transmit port. Each pass through the circulator presents a loss of less than 0.1 dB.

Now consider a similar case using hybrid multiplexing as depicted in Figure 3-11. The hybrid is a 3 dB directional coupler. Energy entering ports A or B is split equally between ports C & D. Half the power from A & B must be absorbed by the dummy load on port D. In the case shown here, more than 9 dB is lost on the way to the antenna and a lot of power is wasted. An element of flexibility is afforded in this case because any one of the 12 channels available to a certain polarization can be used. Figure 3-12 shows how more than 6 channels on a given polarization can be handled with a combination of filters and hybrid.

Figure 3-11 Six Channel Filter Diplexer Multiplexer

Figure 3-12 Six Channel Hybrid Multiplexer

Figure 3-13 Twelve Channel Multiplexer Uses Filters and Hybrid

Satellite Simulator–Non Radiation Tests It is considered good practice to include a switch in the transmission line to an antenna. This switch can be used to dump transmitter power into a dummy load and/or feed a satellite simulator. Satellite operators usually require such a switch in the line to the antenna. It is also a good practice to include a directional coupler in this line to monitor transmission at all times. Figure 3-14 shows such an arrangement.

The satellite simulator is a commercially available unit that serves as a wideband downconverter. At C band, input frequency from 5925 to 6425 MHz is converted to 3700 to 4200 MHz. At Ku band, the 14,000 to 14,500 MHz band is converted to 11,700 to 12,200 MHz. Care must be exercised in the design of directional couplers and level controls to preclude overload levels in the simulator and receivers. Component ratings should be such as to easily handle the transmit power levels required.

The usefulness of this apparatus should be instantly obvious. Before radiating any signal, the internal workings of the station can be checked, for proper frequency and occupied bandwidth. Furthermore, if the antenna is pointed at the appropriate satellite, the received signal can indicate if the satellite is occupied on the frequency or transponder to which the local station is tuned to transmit.

Figure 3-14 Monitor and Non Radiation Test Apparatus

Downlink Equipment An uplink station should have the capacity to monitor its own transmission from baseband to baseband even if it is in a one-way (point-to-multipoint) network. In a two- way network, of course, the receiver function is used for beneficial purposes. We have already discussed the fact that in some applications transmit (uplink) and receive (downlink) equipment are combined in a functional unit such as modems in a digital network and RF heads in VSAT networks. In this section, we discuss the downlink in a functional manner.

Low Noise Amplifiers/Converters Figure 3-15 shows the differences between LNAs, LNBs, and LNCs. In a previous section, a device called an LNA was introduced as part of a receiver chain. It was shown that this component is a contributor to system noise. The LNA is a pre-amplifier in the receiving system. In most applications, it is the major contributor to system noise.

LNA’s can take on many physical forms. In older systems (pre 1976) most earth stations used parametric amplifiers either cooled (liquid helium) or un-cooled. The range of noise temperature for paramps was 15˚ K (cooled) to 85˚ K for un-cooled ones.

Copyright 2006 160 All rights reserved Skjei Telecom, Inc. Although some may still be in use, the paramp has virtually disappeared from use in U.S. domestic satellite service, being replaced by amplifiers using gallium arsenide devices. From 1976 to now, gallium arsenide LNA performance has evolved from noise temperature of 500 ْK to about 65 ْK (30˚ K for selected units) at C band. Current Ku Band LNA performance is about 110˚ K or 75 ْK for selected units. However, many lower noise temperature devices apparently are achieved only by increasing the output VSWR of these devices. This is a “false economy” because poor VSWR itself increases the system noise temperature by reflected and lost power transfer.

Early television receive only stations for homes at use combined the low noise function and downconverter function to produce an output frequency of 70 MHz which was then used to feed an indoor unit to perform the demodulation function. This is called an LNC. A new de-facto standard has evolved for virtually all television receiver functions where the RF (C band or Ku band) is converted to the frequency range 950 to 1450 MHz. This device is called an LNB, which denotes a low noise broadband converter. Some SCPC applications are using LNB’s. In very narrowband applications, this places a requirement for tight tolerance on the local oscillator part of the converter. The tight tolerance requirement in this application is met with the use of an oscillator that is phase locked to a very stable reference oscillator.

Figure 3-15 General Configuration of LNA’s, LNB’s and LNC’s

Copyright 2006 161 All rights reserved Skjei Telecom, Inc. It should be remembered from a previous section , that loss between the antenna and LNA/LNB should be held to a minimum. This is because this loss enacts an inordinately high increase in the system noise temperature. Therefore, the LNA should be located outdoors as close to the antenna as possible.

Figure 3-16 provides a conversion chart from L band frequencies to RF frequencies, assuming that the standard local oscillator frequencies of 5150 ( C Band) and 10750 (Ku Band) are used. It should be noted that DBS receive dishes use a different format. L-band C-band Ku-band Freq. (MHz) Freq. (MHz) Channel Freq. (MHz) 950 4200 11700 970 4180 24 11720 990 4160 23 11740 1010 4140 22 11760 1030 4120 21 11780 1050 4100 20 11800 1070 4080 19 11820 1090 4060 18 11840 1110 4040 17 11860 1130 4020 16 11880 1150 4000 15 11900 1170 3980 14 11920 1190 3960 13 11940 1210 3940 12 11960 1230 3920 11 11980 1250 3900 10 12000 1270 3880 9 12020 1290 3860 8 12040 1310 3840 7 12060 1330 3820 6 12080 1350 3800 5 12100 1370 3780 4 12120 1390 3760 3 12140 1410 3740 2 12160 1430 3720 1 12180 1450 3700 12200 Figure 3-16: L Band to C and Ku Conversion Chart

Copyright 2006 162 All rights reserved Skjei Telecom, Inc. Power Dividers When more than one channel is received by an earth station, the output of the LNA/LNB must be divided so that each demodulator will have an input. In this case, channelization is easily performed with passive dividers. The downconverters and demodulators have filters to discriminate against unwanted signals. Furthermore, in most cases, the gain of the preamplifier (LNA) will overcome the losses from a level and noise standpoint.

In rare cases where the line loss from LNA to power divider is high, and many channels are equipped, it may be necessary to include a post amplifier before the power divider. This might typically be required in SCPC service where each carrier level is low compared to a carrier that saturates a satellite transponder. Figure 3-17 depicts this situation. In this example, the power delivered to the channel receiver is - 87 dBm without a post amplifier. Most receivers require levels between - 30 to - 65 dBm. A post amp with at least 30 dB gain is required to provide margin against gain degradation in the LNA. If the channel receiver has a noise figure of 15 dB (typical), without a post amp, the second stage contribution of noise would be 23° K. With the 30 dB gain post amp having a 10 dB noise figure, the total contribution would be less than 1° K.

Figure 3-17 Example Where Post Amplifier is Required to Boost Levels and Decrease Noise

Copyright 2006 163 All rights reserved Skjei Telecom, Inc. Downconverter/Demodulator Downconverters and/or demodulators perform the functions of channel separation and demodulation to a useful baseband signal.

Modern television receivers generally operate at the 950 to 1450 MHz input frequency and have a single conversion to an IF where demodulation takes place. Threshold extension demodulators are almost universally in use. The noticeable distortion threshold usually occurs at a system carrier to noise ratio in the range 7 to 9 dB, depending on the program content. Noticeable distortion is generally defined as the onset of sparkles in a television picture. These sparkles are a result of noise peaks which are of greater amplitude than the carrier at a given instant of time. In FM/SCPC transmissions of radio or other audio networks, this dynamic threshold results in audible clicks in the receiver output. In digital systems, peaks of noise cause false triggering of the decision circuits in the regeneration of the digital data (1’s and 0’s). As indicated earlier, there are many methods of coding and/or synchronization methods to extend the useful threshold (desired Bit Error Rate).

Integrated Digital Receiver-Decoder Figure 3-18 shows the block diagram of a digital MPEG video receiver called an IRD or Integrated Receiver Decoder. It is roughly the digital counterpart of an analog video receiver. It receives an L Band signal from an LNB, demodulates, and decodes the signal, delivering analog or digital video and audio outputs, as well as data in some IRDs.

Figure 3-18: Integrated Receiver-Decoder The IRD frequently will have a conditional access capability for descrambling and decrypting a protected digital signal. Additionally, the IRD will normally provide a DC voltage at its input port to provide power to the LNB over the connecting coaxial cable. Antennas, Duplexer and IFL The antenna and its duplexer are common to both uplink and downlink in a satellite earth station. Another item generally treated as a common element is the interfacility link (IFL). Actually, the IFL is nothing more than the transmission lines connecting the outdoor and indoor equipment. In addition to the transmission lines, it can include

Copyright 2006 164 All rights reserved Skjei Telecom, Inc. power and control wiring for the various outdoor items that require it. The antenna and its duplexer deserve a good deal of description and we will do so in this section.

Duplexer A duplexer is a device or a means of connecting a transmitter and a receiver to a common element such as an antenna. Sometimes the term diplexer is used to describe this functional element, however the use of the term diplexer in this context is actually not correct. A diplexer is a means of connecting two transmitters or two receivers to a common element such as an antenna. In our earlier discussion of the means for connecting more than one transmitter to an antenna we used the generic term multiplexer. A diplexer is a special case of a two channel multiplexer. A three channel multiplexer could be called a triplexer and so on.

On the transmit side, a duplexer may contain a high-pass filter and/ or a band-pass filter. The high pass filter is not normally needed if the final power amplifier is a narrowband one such as a klystron (KPA). Sometimes the KPA will have a band pass filter built in on the output in which case, the band-pass would not be required. The function of the band-pass is to eliminate harmonics from being transmitted.

The function of the high pass filter is not quite so straightforward. In the case of a wideband power amplifier such as TWT’s and SSPA’s, their output may contain noise or other spurious outputs at the receive frequency band. This is especially true of TWT amplifiers in both C band and Ku band. Isolation may not be adequate in the antenna transducer depending on the level of receive band noise transmitted. The high-pass filter consists of a length of waveguide that is beyond cutoff at the receive frequency band.

On the receive side, a band reject filter is included in a duplexer. Only in rare circumstances is the component not required. Its purpose is to keep the transmit band from entering the receive side. Even if the LNA used has some rejection to the transmit frequencies, the possibility of desensitization and the concomitant gain reduction is highly probable.

The type of antenna feed transducer to be used in the duplexer will depend on the overall antenna function. If only transmission and reception on a single polarization is required, a simple 2 port device can be used. If either transmit or receive function or both are required on two polarizations, a 3 or 4 port device is required. These 3 or 4 port transducers are quite complex (and costly) and it is beyond the scope of this training course for explanation.

Antennas–Radiating Elements In earlier sections of this text, antenna radiation patterns (main lobe and sidelobe) have been discussed and their importance to link performance has been shown. In this section, we will discuss antennas in more detail. Performance verification, some common geometries, and mechanical factors which affect performance are delineated.

Gain and Sidelobe Performance Verification Since the sidelobe requirements of satellite earth stations are mandatory and carry the force of law, it would seem prudent for the uplink operator to be assured of compliance. A subcommittee of an industry FCC advisory committee has recommended a two-step approach toward verification. The two steps are: 1) Manufacturer’s Production Qualifications and 2) On Site Verification. In Step 1, a detailed set of data is supplied to the FCC and would be a prerequisite for an earth station license application. The second step would consist of less detailed tests on site to verify the performance.

Considering the legal requirements, and the technical consequences of non compliance, good judgment in the selection of an uplink antenna and periodic checks are warranted regardless of the ultimate FCC rules adopted. The prudent buyer of an uplink antenna should review the qualification data submitted to the FCC, as well as the manufacturer’s specifications. Mechanical specifications should be scrutinized carefully so that there is reasonable assurance that the electrical performance will hold under the climatic conditions that are likely to exist at fixed locations. The mechanical ruggedness of the antenna in transportable applications should be considered. Finally, in applications where the antenna is likely to be disassembled and reassembled, the means for doing so and maintaining required tolerances should be of paramount importance.

Antenna Geometry–Feed Systems Commonly used earth station antennas use a paraboloidal reflector as the (secondary) radiating element. The primary radiating element is a flared waveguide aperture (feed horn) or a combination of horn and a second (sub) reflector. The parabolic reflector has the virtue of providing a uniform phase over the aperture when it is fed by the primary radiator from its focal point (center of phase). It is worth noting that the focal point of antenna is independent of frequency, and depends only upon the geometry of the parabola.

Figures 3-19 and 3-20 show the commonly used geometries. Each has its advantages and disadvantages.

Figure 3-19 Prime Focus and Dual Reflector Geometry

Figure 3-20 Offset Fed Antenna Geometry

Copyright 2006 167 All rights reserved Skjei Telecom, Inc. The ultimate performance of an antenna depends on a combination of feed design and dimensional tolerances of the main reflector. In general, the design must be a compromise between efficiency (on axis gain) and sidelobe gain. It is desirable to have high on axis gain and low sidelobe gain. Unfortunately, the two do not go hand in hand. A feed system which provides a uniform amplitude over the main reflector aperture will provide high gain and high sidelobes. A design for a large amplitude taper over the aperture will produce low sidelobes and a concomitant low gain. Various feed designs exist for a compromise. In a prime focus design, the feed horn will have corrugations (circular metal rings) or other means to shape the primary pattern. Some flexibility in design exists in a combination of horn and focal length to diameter ratio (f/d). In dual reflector systems, a combination of sub-reflector parameters, and/or horn design afford the designer a good deal of flexibility.

In some large antennas, the main reflector is intentionally designed in a non-parabolic shape so that the combined feed and reflector produce the desired result of high gain and low sidelobes.

In circularly symmetric designs shown in Figure 3-19, the feed or sub-reflector and the necessary support struts produce a significant blockage of the aperture. This blockage has only slight effect on the gain, but it can have significant effect on sidelobes. Offset fed designs as shown in Figure 3-20 can mitigate against degradation due to blockage. Due to the mechanical complexity of offset fed designs, they are only practical in relatively small apertures. The horn antenna provides extremely low sidelobes and low noise because of negligible spillover. It is however very bulky, difficult to steer, and very expensive even in relatively small sizes.

Mechanical Features Mechanical aspects of fixed earth station antennas could be greatly simplified if the operator of the station was to be assured that the satellite of choice would always be at the same orbital position. In that case the reflector could be imbedded in concrete (or even made of concrete with a reflective surface), and made a part of the earth. Unfortunately, FCC guidelines do not allow this. An earth station antenna must be able to point to any satellite in the entire orbital arc in U.S. domestic service. This requirement gives rise to a variety of mounting arrangements with a wide range of pointing adjustment of approximately 65° degrees in elevation and more than 120 in azimuth. The entire structure must withstand a wide range of climatic conditions without significant degradation.

The earth station operator should be aware of these degrading factors in order to maintain appropriate performance.

Copyright 2006 168 All rights reserved Skjei Telecom, Inc. Dimensional Tolerances From the discussion above dealing with feeds and geometries, it can be seen that earth station antennas are quasi-optical devices. The main reflector is a mirror that should provide a uniform phase over its aperture. Deviations from proper optics and or surface roughness will detract from the ideal conditions. Errors are either random or regular.

Random errors arise generally from manufacturing tolerances or installation errors in the main reflector. These errors are statistical in nature. The net effect of this type of error is to increase sidelobe energy and decrease main beam energy. Specifications for surface tolerance are usually included by the manufacturer. A reasonable tolerance for C band to maintain specified gain and sidelobe specifications is in the range of 0.05 to 0.08 inches RMS with a maximum deviation of 0.125 inches over the entire surface. At Ku band, since the frequency of operations is more than twice that of C band, the appropriate tolerance is in the range of 0.015 to 0.025 inches RMS with maximum excursion of 0.060 inches over the entire surface. These are not trivial requirements. Since the reflecting surface is a relatively thin metallic skin, there is a requirement for a strong backup structure to maintain these tolerances.

Regular errors arise from reflector surface deformations or feed displacements. This type of error generally affects only the phase over the aperture. The entire main beam can be made asymmetric, tilted (mis-pointed), reduced in amplitude (gain), or a combination of all. The sidelobes can be raised in amplitude, blurred (no deep nulls) and asymmetrical. These deformations and displacements can occur gradually, or they can be induced by temperature variations, wind and snow loads and self induced by changing position. Manufacturer specifications include the range of environmental factors in which the antenna will operate and survive.

Never operate an earth station if winds in excess of the specifications are anticipated. Drive the antenna to “Stow Position” and lock in place. Even with this precaution, evacuate all personnel and/or other inhabitants who could be affected by destruction of the antenna.

It is recommended that when installing a large earth station, or even modest size ones, the manufacturer be contracted to install and provide proof of performance tests. This data can be used to compare subsequent periodic checks on antenna performance to denote degradation for subsequent correction.

Foundations, Mounts and Motor Drives Earth station mounts come in a wide diversity of types and design. Reputable manufacturers supply foundation designs and when required by building code, will supply structural design data. A prudent buyer should obtain structural data regardless of whether it is required for local construction permit or not.

Receive Only Earth Station Figure 3-21 shows the block diagram of an analog and digital receive only earth station or TVRO. This low cost earth station, with an L-band inter-facility link (IFL) can provide simultaneous reception of analog and digital video and audio. Data capability (not shown) can be readily added. For simplicity, analog reception is shown on one polarity and digital on another, but in an actual terminal, either polarity can receive and process either or both types of video, and multiple signals as well.

Figure 3-21: Video Receive Only Earth Station

Interfacility Link (IFL) As indicated earlier, the IFL consists of nothing more than the transmission lines and control wiring for outdoor (antenna) equipment. However, the waveguide used in the uplink portion from HPA output to antenna input must be treated with care. In all cases, some sort of pressurization system should be included to maintain a clean dry (air or nitrogen) atmosphere in this line. Moisture and/or other contaminants can cause degradation or serious damage to expensive antenna, multiplexer, or power amplifier components. In major earth stations a compressor/desiccant system is suggested. For smaller stations, dry nitrogen from a small tank is suggested.

Physical damage to this waveguide line (dents, dings, etc.) must be avoided, and care should be exercised when installing or modifying this line. Discontinuities can cause reflections that can cause distortion in transmission.

Finally, this waveguide line should be carefully designed and installed with thermal considerations in mind. All components used must be rated for the expected power. If necessary, cooling of some sort (convection, forced air, or water cooling) must be included. A component, say a filter for example, with 0.5 dB loss would dissipate over

Copyright 2006 170 All rights reserved Skjei Telecom, Inc. 300 watts with 3000 watts on the input. If this heat is not removed, it could cause serious damage. Dummy loads used in hybrid type multiplexers or in non-radiating testing should be especially considered. Dummy loads usually have heat dissipation fins but the heat needs to be carried away through convection or forced air.

In some major stations, it might be necessary to have a closed system with heat exchanger for this interfacility link.

Power Systems Backup power systems are recommended for uplink stations. It is an absolute necessity for major uplink stations. The complexity and expense expended on standby power depend on the philosophy of the ownership and the mission of the station. Generally speaking, the most unreliable element in a satellite earth station is commercial power, especially if it is located in a rural area.

Backup power can be uninterruptible power supply (UPS), where batteries are used for prime power until generators can be brought on line. In any case, some economies can be obtained by splitting the load into essential and non-essential loads. Some utility power for minimal lighting and test equipment outlets should be included in the essential load. If the operating equipment requires a closed air conditioning system or heat exchangers, this should also be included in the essential load. Maintenance Programs Practical considerations warrant the institution and close adherence to a preventative maintenance schedule. The kind of program instituted depends of course on the nature of the earth station and its mission. Listed herein are only a few suggested items to be included in the program:

1. Corrosion Control: All outdoor equipment should be regularly inspected for corrosion and treated before actual corrosion occurs. This includes grounding as well as antennas, mounts, LNA’s, and waveguide runs and their support structures.

2. Lubrication and exercise of mechanical moving parts throughout the system. This includes antenna drives, as well as any cooling fans or heat exchangers of HPA and/or waveguide components.

3. Close scrutiny of expendables, such as fuel for backup power, battery conditions, etc.

4. Close monitoring and log of major electronic units. This should include daily checks of:

a. Power amplifier voltage and current:

Copyright 2006 171 All rights reserved Skjei Telecom, Inc. b. Radiated signal with spectrum analyzer. The uplink operator should know and recognize the normal or proper spectral signature of his own transmission. Any changes or frequency shift in the signature should be investigated and corrected. c. Levels (gain) of all components in the uplink chain. d. Modulation depth.

5. Occasional sweep checks of all RF units, including the RF portion of modulator, upconverters and power amplifiers. These sweep checks should use reflectometer techniques using leveled input signals, with comparison to inherent test equipment variation with frequency and comparison with previously taken sweep tests. These data should be recorded in X-Y plots, oscilloscope photographs or computer memory.

6. Periodic monitoring of antenna performance (gain, sidelobes and cross polarization isolation).

An important part of a preventative maintenance program should be the rapid repair of failed units, or to have shelf spares of major units in addition to redundancy units.

Earth Station Licensing The FCC requires licensing of transmitting earth stations and permits licensing of receive only (RO) earth stations. It is normally desirable for an operator to license a C- band RO earth station, since licensing protects the station from future interference from domestic microwave systems.

The FCC Rules and Regulations are ever evolving therefore it is strongly suggested that the FCC be contacted before filing to obtain the latest regulations and regulatory procedures effective at the time of the filing. Licensing information is available on the internet at: www.fcc.gov .

The FCC Rules and Regulations, Part 25, form the basis of the applicable documents which must be followed for the planning and implementation of any FSS band satellite communication system.

The FCC established precedents for the minimum diameter apertures and sidelobe gain envelopes for earth station antennas operating in the FSS bands at the beginning of these services in the early 1970s to minimize interference between terrestrial systems and satellite systems and between satellite systems. These precedents have been modified through the years as the use of satellite services has increased. The more significant recent rulings pertaining to earth station antenna performance have resulted in improved antenna radiation patterns in the close-in sidelobe region and have established maximum radiated power densities.

Copyright 2006 172 All rights reserved Skjei Telecom, Inc. The FCC Rules and Regulations Part 25.209 pertaining to antenna gain envelopes are mandatory for all transmit antennas. Excerpts from this standard follow (refer to the current rules publication for the entire text): a) The gain of any antenna to be employed in transmission from an earth station in the fixed satellite service shall lie below the envelope defined below: 1. In the plane of the geostationary satellite orbit as it appears at the particular earth station location:

[29-25*log(θ)]dBi 1˚ < θ < 7˚ + 8 dBi 7˚ < θ < 9.2˚ [32-25*log(θ)] dBi 9.2˚ < θ < 48˚ -10 dBi 48˚ < θ < 180˚ where θ is the angle in degrees from the axis of the main lobe and dBi refers to the dB relative to an isotropic radiator. For the purposes of this section, the peak gain of an individual sidelobe may not exceed the envelope defined above for θ between 1˚ and 7˚. For θ greater than 7˚, the envelope may be exceeded by 10% of the sidelobes, but no individual sidelobe may exceed the envelope by more than 3 dB.

2. In all other directions: Outside the main beam, the gain of the antenna shall lie below the envelope defined by :

[32-25*log(θ)] dBi 1˚ < θ < 48˚ -10 dBi 48˚ < θ < 180˚ where θ is the angle in degrees from the axis of the main beam and dBi refers to dB relative to an isotropic radiator. For the purpose of this section, the peak gain of an individual sidelobe may be reduced by averaging its peak level with the peaks of the nearest sidelobes on either side, or with the peaks of the two nearest sidelobes on either side, provide that the level of no individual sidelobe exceeds the gain envelope given above by more than 6 dB. b. The off-axis cross-polarization isolation of any antenna to be employed in transmission at frequencies between 5925 and 6425 MHz from an earth station to a space station in the domestic fixed-satellite service shall be defined by:

[19-25*log(θ)] dBi 1.8˚< θ < 7˚ -2 dBi 7˚ < θ < 9.2˚ c. Any antenna licensed for reception of radio transmission from a space station in the fixed-satellite service shall be protected from radio interference caused by other space stations only to the degree to which harmful interference would not be expected to be caused to an earth station employing an antenna conforming to the standards defined in paragraphs a. and b. of this section.

Copyright 2006 173 All rights reserved Skjei Telecom, Inc. d. The operations of any earth station with an antenna not conforming to the standards of paragraph a. and b. of this section shall impose no limitations upon the operation, location and design of any terrestrial station, any other earth station, or any space station.

e. An earth station with an antenna not conforming to the standards of paragraphs (a) and (b) of this section will be routinely authorized upon a finding by the Commission that unacceptable levels of interference will not be caused under conditions of uniform 2˚ orbital spacings. This provision is generally applied to antennas with diameters smaller than 4.6 m at C band and 1.0 m at Ku Band, because these antennas are not able to meet the sidelobe requirement. To invoke this provision and license smaller antennas, it is necessary to overlay the transmit power density expressed in dBW/4 kHz to show that the power presented to the antenna feed at Ku band, for example, is less than 15-25 log (θ) per 4 kHz of transmitted spectrum.

f. The antenna performance standards of small antennas operating in the 12/14 GHz band with diameters as small as 1.2 meters starts at 1.25˚ instead of 1˚ as stipulated in paragraph (a) of this section. The FCC further acknowledged that the envelope defined above is only a reference envelope in the receive band. Receiving antennas do not have to conform to this envelope to be eligible for licensing. Facilities with performance worse than the reference envelope must, however, accept higher interference levels.

Frequency Coordination C Band earth stations require frequency coordination prior to licensing because the band is shared with the fixed microwave service. Normal Ku Band earth stations do not require coordination because this band is dedicated to satellite communications. Coordination is normally performed by a commercial organization such as Comsearch.

CHAPTER 4: UPLINK OPERATION

In this chapter, we discuss operation of the uplink earth station with emphasis on the initial line-up procedures. Much of what is given in this chapter has already been covered in previous chapters, but it is felt that repetition is needed for emphasis.

Safety is also discussed in this chapter. The operator has a responsibility to himself (or herself) as well as to others. It is important that he (or she) be aware of certain dangers in operating an earth station.

Operating Responsibilities The operator of a satellite uplink is responsible for assuring that no interference is being created to other satellite systems. While the operator’s first concern may be to provide the desired transmission for the sponsor, it is equally important that the service being provided through other satellite networks not be harmed.

Human Error 19.2%

Adjacent Satellites 0.8%

Terrestrial 3.2%

Cross Pol 25.6%

Equipment Malfunction 16.8%

Unknown Source 34.4%

Figure 4.1 Causes of Interference (source: SUIRG)

Similarly, if the desired signals are not being transmitted properly, it is possible that the received signals will be harmed by the signals of other satellite networks that are operating perfectly within coordinated specifications. These problems will become more severe as additional satellites are placed in the geostationary orbit. It is therefore the technical and moral responsibility of all satellite uplink operators to assure proper orientation and operation of their equipment.

There are also legal obligations that accompany the transmitting license issued by the FCC. Negligence can result in severe penalties, including fine, jail terms, or loss of the owner’s station operating license.

Operator Controls The uplink operator must be familiar with all equipment controls. Special attention should be given to antenna pointing and polarization controls, frequency (operating channel) controls and bandwidth (modulation depth) controls. Once set, access to those controls should be limited physically to prevent accidental readjustment.

Test Equipment and Calibration At a minimum, a properly equipped uplink station should have the ability to: 1) Monitor its own signal through the various stages on the uplink; 2) Monitor its own signal in the downlink, and 3) Monitor the entire satellite frequency band of the downlink. This means that downlink equipment is required for its own signal and a wideband access point is available on the downlink–usually the output of an LNA or LNB.

In the case of unmanned small station (VSAT or other), the responsible service person should be properly equipped and the control (HUB) station should carefully monitor the transmission with appropriate means.

The SPECTRUM ANALYZER is a powerful tool, and is the single item of test equipment capable of performing the minimum routines for operation of an uplink station. This does not mean that it is the only item of test equipment needed for all aspects of uplink maintenance, repair and operation. A decent quality spectrum analyzer can perform the minimum monitoring functions listed above. It, therefore, follows that the person in control of an uplink station fully understands the functions, limitations, and proper use of spectrum analyzers and in particular the one, which is used by that person. The uplink operator should know and understand various types of “spectrum signatures”, especially that signature associated with transmission from the station he (or she) controls.

The balance of test equipment provided is a function of maintenance and operational philosophy of the uplink station owner and is dependent on the application to which the uplink station is to be used. However, since the stakes are rather high and harsh penalties can be meted out to improperly operating stations which interfere with other services, it would seem prudent that owners equip adequately and keep the equipment in good working order and in calibration at all times.

Access Procedures The satellite operators of U.S. domestic satellites have formed a committee entitled “Satellite Users Interference Reduction Group.” This committee has generated a simple one-page document dealing with a standard “Access Procedure.” It is included in this textbook as Appendix E. This procedure is general and applies mostly to analog TV transmission or other full transponder service.

Each uplink operator should be thoroughly familiar with the procedure for the particular service and satellite to be accessed; a copy of the written details should be available at each uplink site.

Should the uplink station initiate a new class of service for any satellite, the access procedure should be worked out (in advance of actual rollout of service) between the satellite operator and the user.

The following paragraphs will be helpful to gaining access.

Establish Contact with Satellite Operators Contact shall be maintained with the satellite operator during the initial phases of any transmission to ascertain that predicted signal levels are being received and distributed by the satellite system. This contact shall be accomplished through the switched public telephone system. Transportable stations should be equipped with cellular telephones if located remotely from telephone access.

Some Satellite News Gathering (SNG) vehicles are equipped with voice and other communications features. This method of operation will be discussed later in this chapter.

Locate and Verify Identity of Proper Satellite Before transmitting any signal, the uplink station operator should be certain the station antenna is pointing at the satellite to which it will be communicating. There should be a clear, unobstructed line of sight path to this satellite. The operator should know the approximate geographic coordinates of the station. This information combined with the knowledge of the satellite orbital location, is sufficient to calculate the azimuth and elevation angle of the antenna. The satellite operator can assist in the calculation or verify the accuracy of the calculation.

Antenna pointing can be facilitated by the proper use of inclinometer and compass or antenna angular readout, if it is so equipped. A spectrum analyzer monitoring the wideband downlink test port should be used to recognize the “spectrum signature” of the proper satellite. If there is any doubt at all, the uplink operator should request an

© Skjei Telecom, Inc. 2005 177 All rights reserved identifying signal from the satellite operator. Once positive identification is made, the earth station operator should move the antenna in both directions in azimuth and elevation to make certain that the main lobe and not a sidelobe is pointing at the proper satellite.

Antenna Optimization and Pre-Transmission Adjustments As indicated above, each satellite operator has (or should have) procedures for uplink access. This procedure will have certain pre-transmission requirements. These pre- transmission requirements can be facilitated by the satellite operator providing a steady (usually an unmodulated) carrier on the frequency that the uplink station will operate. The uplink station can use this signal to perform antenna pointing and polarization adjustment and other antenna tests as might be requested by the satellite operator or mandated by FCC rules or guidelines. Other pre-transmission adjustments should include a non-radiating check by the uplink station of carrier frequency and modulation parameters. In the case of video transmission, the modulation parameters should include deviation of the main carrier by video and subcarrier(s) (if used), deviation of subcarrier (s) by audio. See Appendix D for video and subcarrier deviation adjustment procedures.

Transmission When initiating transmission, the uplink station should transmit at a greatly reduced power level of at least 20 dB below its normal power output. The power level should be increased under direction from the satellite operator. These adjustments might include “fine tuning” of antenna polarization, carrier frequency, or modulation parameters.

The uplink operator should log various values and settings and continuously monitor and maintain a log of the values and settings. Changes and/or adjustments to critical parts or parameters should be coordinated with the satellite operator.

Satellite News Vehicles–(SNG) The television broadcast industry has embraced satellite usage from the earliest days of satellite operations. Network distribution has gradually drifted away from terrestrial service to where virtually all distribution is by satellite.

The official FCC nomenclature for mobile earth stations is “Temporary Fixed Earth Stations.” Although these stations are mobile, they operate in the Fixed Satellite Service (FSS) frequency range.

© Skjei Telecom, Inc. 2005 178 All rights reserved Evolution of SNG Vehicles Early implementation of SNG vehicles was at C band. This usually involved a rather large trailer with a 4.5 or 5-meter antenna on a trailer hauled by a hefty truck. These mobile earth stations were cumbersome, difficult to get to remote locations, and difficult to assemble on site. They also require frequency coordination to operate in each new location, which eliminated the ability to move them quickly to a new location.

When Ku band satellites became available, this frequency band became the natural choice for SNG and now virtually all operate at Ku band (14 GHz transmit, 12 GHz receive).

A) Frequency coordination with terrestrial microwave systems is not required. A station can be located at virtually any site. B) The antenna can be much smaller than C Band ones. The main reflector need not be sectionalized. It can be an integral part of the vehicle used to transport the station.

With the advent of Ku band, most broadcast stations have invested in SNG facilities to enhance their local news gathering capabilities. Entities other than broadcast such as educational institutions and governments have also used transportable earth stations for a variety of applications.

Vendors have responded to this demand with compact earth stations in a variety of configurations ranging from large trucks with extensive production facilities to smaller trucks (or vans) and “flyaway packages” in suitcases. These vendors can configure the earth station to customer requirements based on several standard vehicle or trailer models.

Most of the earlier implementations and some of the modern ones utilize moderately large truck bodies. If the aggregate gross weight of these vehicles exceeds 10,000 pounds, they are subject to regulations of the U.S. Department of Transportation (DOT). More about these regulations are given below.

A resurgence is also being seen in C Band SNG vehicles, particularly for HDTV sports applications. These vehicles have greater reliability than Ku band vehicles due to their decreased rain fade attenuation. Furthermore, the location of the uplink is generally known far enough in advance to accomplish the required frequency coordination, which remains valid for a period of 6 months.

Pertinent DOT Regulations Time limits placed on the driver.

No more than 10 hours driving following at least 8 consecutive hours off duty. No driving after 15 hours on duty following 8 consecutive hours off duty. Waivers are not generally given.

Duty status records (log books) There are specific regulations for driver record keeping (49 CFR section 395). Driver status must be logged in duplicate for each 24-hour period if the driver travels beyond a 100-mile radius from normal location. If within 100-mile radius, a “time card” can be considered sufficient. Specific log entries:

“Off Duty” “Sleeping Berth” “Driving” “On Duty, Not Driving”

Sanctions and penalties

If record keeping violations are found, the carrier can be fined up to $500 for each violation and the driver may be placed “out of service” for 8 consecutive hours. Each driving time violation may be fined up to $ 1,000. Violations causing injury or death may be fined up to $ 10,000. Patterns of violations may also be fined up to $ 10,000.

Other Driver and truck requirements

Drivers must pass a written test on regulations. Drivers must pass a medical examination by an approved physician, and be re- examined every two years. The truck must undergo a simple daily inspection. Trucks are not allowed on roadways that are designated as “Parkways.” There are many streets in metropolitan areas that have restricted truck access. While not a rule, watch for signs that say “low clearance.”

Analog or Digital Early SNG vehicles used analog FM/TV modulation. The modulation parameters are such that either full or half transponder bandwidth is used. With the advent of standardized digital compression techniques, most vehicles have added digital transmission to the capabilities. New vehicles can be equipped with both analog and digital.

© Skjei Telecom, Inc. 2005 180 All rights reserved Digital transmission from a transportable usually operates in “Single (TV) Channel Per Carrier” (SCPC) mode. From a satellite usage standpoint, this means that a multiplicity of TV channels can be transmitted to a transponder. Satellite cost can therefore be reduced. Satellite operators typically sell 4.5 MHz blocks of bandwidth for occasional use. With a 36 MHz transponder, 8 TV channels can be accommodated if each is contained in a 4.5 MHz bandwidth.

There are a variety of modulation parameters that are in current use for both analog and digital transmission. Before starting out on any use of the transportable, the operator must have a clear understanding of:

Work order number Satellite (orbital slot) Transponder If analog: Full or half transponder Deviation of main carrier by video Deviation of main carrier by the subcarrier(s) Deviation of the subcarrier(s) by audio

If digital Frequency (slot) Bandwidth (symbol rate) FEC rate

Voice Communications As indicated in paragraph 4.4 of this textbook, voice communications with the network controller MUST be established before attempting to access satellites. The most common method used in the U.S. is by cellular phone. If the location is in a cellular dead zone (unlikely), the station should be equipped with a global “mobile satellite phone.” Vehicle vendors can supply this item as an option.

In some cases of SNG operations, additional phone connection to the home TV station may be required for producer/director communication with a reporter or the driver/operator on the scene.

SNG Priorities

1. Safety a. Driving and vehicle inspections b. Safe operation of electronic equipment c. Personal safety (section 4.6 below) 2. Interference a. To others in the same transponder b. To others in the same satellite c. To others in different satellites 3. Courtesy with satellite control 4. Communications 5. On-Air operations Safety Workers in any job or industry are subject to hazards and satellite uplinking is no exception. In this section, we highlight a few of the safeguards that a prudent worker should follow. Above all, the operator should have a healthy respect for electricity and should avoid direct contact with it. In making all measurements, insulated probes must be used. A few unique safety aspects of satellite uplinking need emphasis.

One frequent safety problem which can easily cause death is deploying a SNG or ENG antenna into a power line overhead. Operators must always look before deploying any antennas to ensure clearance from power lines and other objects.

A carbon monoxide detector is advisable due to potentially long periods operating on generator..

Microwave Radiation Hazards Microwave energy like all electromagnetic energy is non-ionizing. That is, it does not change the molecular structure of any material. It can damage tissue or any material by heating that material or tissue. The following precautions should be taken in this regard.

1. Turn off the power amplifier when working on the waveguide transmission line, multiplexers, switches and other components between HPA and the antenna. 2. Never work inside the antenna when RF power is in the feed. This area inside the antenna aperture is the only area, which exceeds the environmental protection agency guidelines for safe exposure. 3. Access by the public to the antenna must be restricted in some manner.

© Skjei Telecom, Inc. 2005 182 All rights reserved Power Amplifier and Power Supply TWT and Klystron power amplifiers have power supplies of greater than 1,000 volts with some as high as 8,000 volts. NEVER WORK ON A HIGH VOLTAGE SUPPLY ALONE. A LARGE WOOD ROD MUST BE KEPT NEARBY TO REMOVE A FELLOW WORKER FROM A HIGH VOLTAGE SUPPLY. NEVER TOUCH A WORKER WHO IS IMMOBILIZED ON A HIGH VOLTAGE SUPPLY. USE THE WOOD ROD IMMEDIATELY. FINALLY, A TRAINING COURSE IN CPR IS STRONGLY RECOMMENDED.

Equipment Layout and Housekeeping Ordinary common sense dictates that equipment should be arranged in a neat manner, with interconnecting cables and test equipment arranged to preclude accidents when working on earth station equipment. In some states, Occupational Safety and Health Administration (OSHA) standards exist for the layout and spacing between electronic equipment racks. The uplink operator should make a serious attempt to adhere to these standards. A neat working environment is conducive to neat orderly work, which can cut down or eliminate errors.

Interference Management Interference management and coordination is appropriately performed between satellite operators and users of that satellite, or between satellite operators. In extreme cases, the FCC may be involved. Nasty legal consequences can arise if users either on the same satellite or on different satellites attempt to resolve interference cases. All coordination between users should be done by the satellite operator in the case of mutual interference or between satellite operators in the case of interference between different satellites.

Uplink operators should be prepared to cooperate with all reasonable requests of the satellite operator in the search for sources of interference. Remember, the uplink station only has a contractual relationship with the satellite operator (or carrier) with whom it is operating.

An operator should promptly notify the satellite operator of any anomalies in their transmission. Be prepared to give details and cooperate with the satellite operator or the FCC.

Review of Common Operator Errors

• Antenna mispointing • Antenna polarization misadjustment • Wrong carrier frequency • Overdeviation (or excessive occupied bandwidth) • Oversaturation (or power too high in multicarrier per transponder operation) • Wrong polarization insertion of carrier • Radiating while moving antenna from one satellite to another • Poor assembly of antennas • Not checking for antenna clearance before deploying

Review of Critical Equipment Items

1. Antenna mechanical damage 2. Frequency determining devices (oscillators, synthesizers) 3. Polarization injection devices (Switches, diplexers) 4. Bandwidth determining devices (modulators, baseband amplifiers) 5. Amplifiers (which might oscillate)

Careful monitoring of the transmission, and/or proper access procedures can eliminate or minimize interference due to equipment malfunction.

REFERENCES

1. Introduction to Satellite Communications, Bruce R. Elbert. Artec House, 1987, Chapters 5 and 7.

2. Satellite Communications, Emanual Fthenakis. McGraw-Hill, 1984, Chapters 3 and 4.

3. International Satellite Directory, Design Publishers (yearly publication).

4. Electromagnetic Wave Propagation through Rain, Robert K. Crane, Wiley & Sons, 1996.

5. Digital Communications, Satellite/Earth Station Engineering. Kamilo Feher, Prentice Hall, 1993.

6. ITU – RP 618-7.

7. Satmaster Pro. (Arrowe Services, U.K.) available over the Internet. Author Derek Stephens on www.arrowe.co.uk.

8. ATSC Standard A/80. “Modulation and Coding requirements for Digital TV (DTV) Applications over Satellite,” July 17, 1999.

9. NAB Satellite Uplink Operator’s Course Text and Classroom Notes, Norman Weinhouse, Norman Weinhouse Associates, April 2004

10. Satellite Users Interference Reduction Group , various publications on www.suirg.org

APPENDICES

PAGE APPENDIX A: SAMPLE LINK BUDGETS A-1

APPENDIX B: TUTORIAL IN THE USE OF DECIBELS (DB) B-1

APPENDIX C: FORMS FOR LINK BUDGET ANALYSIS C-1

APPENDIX D: FM MODULATION ADJUSTMENT PROCEDURES D-1

APPENDIX E: SUIRG SPACECRAFT ACCESS PROCEDURES E-1

APPENDIX F: C+N/N MEASUREMENT AND Eb/No CALCULATION F-1

APPENDIX G: GLOSSARY OF SATELLITE TERMS G-1

Appendix A

Appendix F: C+N/N MEASUREMENT AND Eb/No CALCULATION

E-1

The below Eb/No calculation from the C+N/N measurement assumes the use of a Hewlett Packard or equivalent operation spectrum analyzer. The measurements will be made with the carrier at the normal operational level in unmodulated (CW) mode at the input to the receiver. The spectrum analyzer will use a resolution bandwidth of 100 kHz.

# Parameter Value Units

A. Measured C+N ______dBm

B. Measured Noise Level ______dBm

C. Corrected Noise Level, B + 2.5 ______dBm

D. (C+N)/N, A – C ______dB

(D/10) E. C/N, 10 Log10[10 -1] ______dB

F. Bandwidth Conversion, 10 Log10[1.2 x Resolution Bandwidth] 50.8 dB-Hz

G. C/No, E + F ______dB-Hz

H. Data Rate Conversion, 10 Log10[data rate] ______dB/Hz

I. Calculated Eb/No, G – H ______dB

Note: Step F assumes the spectrum analyzer is set to 100 kHz resolution bandwidth when the C+N and Noise measurements are made in steps A and B respectively.

APPENDIX G: GLOSSARY OF SATELLITE TERMS

8 PSK: 8 state phase shift keying. A modulation method in which each symbol represents 3 bits

16 QAM: 16 state quaternary amplitude modulation. A modulation method in which each symbol represents 4 bits

A...

Amplitude Modulation (AM) The baseband signal is caused to vary the amplitude or height of the carrier wave to create the desired information content.

Amplifier A device used to boost the strength of an electronic signal.

Analog A form of transmitting information characterized by continuously variable quantities, as opposed to digital transmission, which is characterized by discrete bits of information in numerical steps. An analog signal is responsive to changes in light, sound, heat and pressure.

Analog-to-Digital Conversion (ADC) Process of converting analog signals to a digital representation. DAC represents the reverse translation.

ANIK The Canadian domestic satellite system that transmits Canadian Broadcasting Corporation’s (CSC) network feeds throughout the country. This system also carries long distance voice and data services throughout Canada as well as some transborder service to the U.S. and Mexico.

Antenna A device for transmitting and receiving radio waves. Depending on their use and operating frequency, antennas can take the form of a single piece of wire, a di-pole a grid such as a yagi array, a horn, a helix, a sophisticated parabolic-shaped dish, or a phase array of active electronic elements of virtually any flat or convoluted surface.

Aperture A cross sectional area of the antenna which is exposed to the satellite signal.

Apogee The point in an elliptical satellite orbit which is farthest from the surface of the earth. Geosynchronous satellites which maintain circular orbits around the earth are first launched into highly elliptical orbits with apogees of 22,237 miles. When the communication satellite reaches the appropriate apogee, a rocket motor is fired to place the satellite into its permanent circular orbit of 22,237 miles.

Apogee Kick Motor (AKM) Rocket motor fired to circulate orbit and deploy satellite into geostationary orbit.

ATSC Advanced Television Systems Committee- the organization specifying Digital Television (DTV) , including HDTV and SDTV, standards.

Attenuation The loss in power of electromagnetic signals between transmission and reception points.

Attitude Control The orientation of the satellite in relationship to the earth and the sun.

Audio Subcarrier The carrier between 5 MHz and 8 MHz containing audio (or voice) information inside of a video carrier.

Automatic Frequency Control (AFC) A circuit which automatically controls the frequency of a signal.

Automatic Gain Control (AGC) A circuit which automatically controls the gain of an amplifier so that the output signal level is virtually constant for varying input signal levels.

ATIS- Automatic Transmit Identification System- a Morse code signal added to an analog FM TV carrier to identify the transmitting station. Required by the FCC for all analog television uplinks.

AVC: Advanced Video Codec- a second generation digital encoding method improving on MPEG 2

AZ/EL Mount Antenna mount that requires two separate adjustments to move from one satellite to another;

Azimuth The angle of rotation (horizontal) that a ground based parabolic antenna must be rotated through to point to a specific satellite in a geosynchronous orbit. The azimuth angle for any particular satellite can be determined for any point on the surface of the earth giver the latitude and longitude of that point. It is defined with respect to due north as a matter of easy convenience.

B...

B-Frames: MPEG frames which basically provide difference information from frames before or after it

B-Mac A method of transmitting and scrambling television signals. In such transmissions MAC (Multiplexed Analog Component) signals are time-multiplexed with a digital burst containing digitized sound, video synchronizing, authorization, and information.

Backhaul A terrestrial communications channel linking an earth station to a local switching network or population center.

Backoff The process of reducing the input and output power levels of a traveling wave tube to obtain more linear operation.

Band Pass Filter An active or passive circuit which allows signals within the desired frequency band to pass through but impedes signals outside this pass band from getting through.

Bandwidth A measure of spectrum (frequency) use or capacity. For instance, a voice transmission by telephone requires a bandwidth of about 3000 cycles per second (3KHz). A TV channel occupies a bandwidth of 6 million cycles per second (6 MHz) in terrestrial Systems. In satellite based systems a larger bandwidth of 17.5 to 72 MHz is used to spread or “dither” the television signal in order to prevent interference.

Baseband The basic direct output signal in an intermediate frequency based obtained directly from a television camera, satellite television receiver, or video tape recorder. Baseband signals can be viewed only on studio monitors. To display the baseband signal on a conventional television set a “modulator” is required to convert the baseband signal to one of the VHF or UHF television channels which the television set can be tuned to receive.

Baud The rate of data transmission based on the number of signal elements or symbols transmitted per second. Today most digital signals are characterized in bits per second.

Beacon Low-power carrier transmitted by a satellite which supplies the controlling engineers on the ground with a means of monitoring telemetry data, tracking the satellite, or conducting propagation experiments. This tracking beacon is usually a horn or omni antenna.

Beamwidth The angle or conical shape of the beam the antenna projects. Large antennas have narrower beamwidths and can pinpoint satellites in space or dense traffic areas on the earth more precisely. Tighter beamwidths thus deliver higher levels of power and thus greater communications performance.

Binary: referring to the basic digital encoding of “1”s and “0”s

Bit A single digital unit of information

Bit Error Rate (BER) The fraction of a sequence of message bits that are in error. A bit error rate of 10-6 means that there is an average of one error per million bits.

Bit Rate The speed of a digital transmission, measured in bits per second.

Blanking An ordinary television signal consists of 30 separate still pictures or frames sent every second. They occur so rapidly, the human eye blurs them together to form an illusion of moving pictures. This is the basis for television and motion picture systems. The blanking interval is that portion of the television signal which occurs after one picture frame is sent and before the next one is transmitted. During this period of time special data signals can be sent which will not be picked up on an ordinary television receiver.

Block Down Converter A device used to convert the 3.7 to 4.2 KHz signal down to UHF or lower frequencies (1 GHz and lower).

BOi: input backoff see backoff

BOo- output backoff- see backoff

BPSK: Binary Phase Shift Keying System of modulating a satellite signal such that each symbol contains one transmitted bits. Also called Biphase modulation

Broad beam A single large circular beam that covers a large geographic area

Broadcast The sending of one transmission to multiple users in a defined group (compare to unicast).

Business Television Corporate communications tool involving video transmissions of information via satellite. Common uses of business television are for meetings, product introductions and training.

BSS: Broadcast Satellite Service: e.g. DirecTV, Echostar,

C...

C Band This is the band between 4 and 8 GHz with the 6 and 4 GHz band being used for satellite communications. Specifically, the 3.7 to 4.2 GHz satellite communication band is used as the down link frequencies in tandem with the 5.925 to 6,425 GHz band that serves as the uplink.

Carrier to Noise Ratio (C/N) The ratio of the received carrier power and the noise power in a given bandwidth, expressed in dB. This figure is directly related to G/T and S/N; and in a video signal the higher the C/N, the better the received picture.

Related: C/No: Carrier to Noise Spectral Density- ratio of total carrier power to noise in 1 Hz of bandwidth

Carrier The basic radio, television, or telephony center of frequency transmit signal. The carrier in an analog signal is modulated by manipulating its amplitude (making it louder or softer) or its frequency (shifting it up or down) in relation to the incoming signal. Satellite carriers operating in the analog mode are usually frequency modulated.

Carrier Frequency the main frequency on which a voice, data, or video signal is sent. Microwave and satellite communications transmitters operate in the band from 1 to 14 GHz (a GHz is one billion cycles per second).

Cassegrain Antenna The antenna principle that utilizes a subreflector at the focal point which reflects energy to or from a feed located at the apex of the main reflector.

CCIR or CCITT: terminology formerly used for branches of the ITU

CDMA Code division multiple access. Refers to a multiple-access scheme where stations use spread-spectrum modulations and orthogonal codes to avoid interfering with one another.

Channel A frequency band in which a specific broadcast signal is transmitted. Channel frequencies are specified in the United States by the Federal Communications Commission. Television signals require a 6 MHz frequency band to carry all the necessary picture detail.

CIF: a video display format

Circular Polarization Unlike many domestic satellites which utilize vertical or horizontal polarization, the international Intelsat satellites transmit their signals in a rotating corkscrew-like pattern as they are down-linked to earth. On some satellites, both right-hand rotating and left-hand rotating signals can be transmitted simultaneously on the same frequency; thereby doubling the capacity of the satellite to carry communications channels.

Clamp A video processing circuit that removes the energy dispersal signal component from the video waveform.

Clarke Orbit That circular orbit in space 22,237 miles from the surface of the earth at which geosynchronous satellites are placed. This orbit was first postulated by the science fiction writer Arthur C. Clarke in Wireless World magazine in 1945. Satellites placed in these orbits, although traveling around the earth at thousands of miles an hour, appear to be stationary when viewed from a point on the earth, since

the earth is rotating upon its axis at the same angular rate that the satellite is traveling around the earth.

C/No Carrier-to-noise ratio measured either at the Radio Frequency (RF) or Intermediate Frequency (IF)

Codec Coder/decoder system for digital transmission.

COFDM: Coherent orthogonal frequency division multiplex. A form of modulation employing multiple small carriers combined together to reduce intersymbol interference

Co-Location Ability of multiple satellites to share the same approximate geostationary orbital assignment frequently due to the fact that different frequency bands are used.

Color Subcarrler A subcarrier that is added to the main video signal to convey the color information. In NTSC systems, the color subcarrier is centered on a frequency of 3.579545 MHz, referenced to the main video carrier.

Common Carrier Any organization which operates communications circuits used by other people. Common carriers include the telephone companies as well as the owners of the communications satellites, and others. Common carriers are required to file fixed tariffs for specific services. Today, most services are sold on a non- common carrier basis.

Companding A noise-reduction technique that applies single compression at the transmitter and complementary expansion at the receiver.

Composite Baseband The unclamped and unfiltered output of the satellite receiver’s demodulator circuit, containing the video information as well as all transmitted subcarriers.

Concatenated Coding: two or more different types of coding sequentially applied to a signal

CONUS Contiguous United States. In short, all the states in the U.S. except Hawaii and Alaska.

Co-Pol- co-polarized; on the same polarization or on the primary polarization or on the polarization being used

Cross-Pol or XPOL: cross polarized; on the opposite (orthogonal) polarization or on the polarization which is not providing primary service

Cross Modulation A form of signal distortion in which modulation from one or more RF carrier(s) is imposed on another carrier.

CSU Channel service unit. A digital interface device that connects end-user equipment to the local digital telephone loop. CSU is frequently coupled with DSU (see below) as CSU/DSU.

C/T Carrier-to-noise-temperature ratio.

CW- Continuous Wave. An unmodulated carrier signal. A sine wave such as would be generated by a signal generator.

D...

DAMA Demand-Assigned Multiple Access - A highly efficient means of instantaneously assigning telephony channels in a transponder according to immediate traffic demands.

DBS Direct broadcast satellite. Refers to service that uses satellites to broadcast multiple channels of television programming directly to home mounted small-dish antennas.

DCT- Discrete Cosine Transform: a way of transforming a digital signal into a frequency like domain in which it can be compressed and digitally manipulated

Decibel (dB) The standard unit used to express the ratio of two power levels. It is used in communications to express either a gain or loss in power between the input and output devices. dBi The dB power relative to an isotropic source. dBW The ratio of the power to one Watt expressed in decibels

Declination The offset angle of an antenna from the axis of its polar mount as measured in the meridian plane between the equatorial plane and the antenna main beam.

Decoder A television set-top device which enables the home subscriber to convert an electronically scrambled television picture into a viewable signal. This should not be confused with a digital coder/decoder known as a CODEC which is used in conjunction with digital transmissions.

De-emphasis Reinstatement of a uniform baseband frequency response following demodulation.

De-interleaver: see Interleaver

Delay The time it takes for a signal to go from the sending station through the satellite to the receiving station. This transmission delay for a single hop satellite connection is very close on one-quarter of a second.

Delta Modulation: a very early form of digital encoding. Not used extensively today

Demodulator A satellite receiver circuit which extracts or “demodulates” the “wanted “signals from the received carrier.

Deviation The modulation level of an FM signal determined by the amount of frequency shift from the frequency of the main carrier.

Digital Conversion of information into bits of data for transmission through wire, fiber optic cable, satellite, or over air techniques. Method allows simultaneous transmission of voice, data or video.

Digicipher or Digicipher II:A proprietary digital video audio and data encoding system invented by General Instruments ( Now Motorola)

Digital Speech Interpolation DSI - A means of transmitting telephony. Two and One half to three times more efficiently based on the principle that people are talking only about 40% of the time.

Diplexer (or Filter Diplexer): Device used to combine multiple HPA outputs by adding filtered outputs together

Duplexer: Device used to combine transmitter and receiver to a common antenna.

Dolby AC-3 or Dolby E: two types of advanced audio encoding for broadcasting

Downlink The satellite to earth half of a 2 way telecommunications satellite link. Often used to describe the receive dish end of the link.

DSU Data service unit. A device used in digital transmission that adapts the physical interface on a DTE device to a transmission facility such as T1 or E1. The DSU is also responsible for such functions as signal timing. DSU is frequently coupled with a CSU (see above) as CSU/DSU.

DTV Digital Television

Dual Spin Spacecraft design whereby the main body of the satellite is spun to provide altitude stabilization, and the antenna assembly is despun by means of a motor and bearing system in order to continually direct the antenna earthward. This dual-spin configuration thus serves to create a spin stabilized satellite.

Duplex Transmission Capability for simultaneous data transmission between a sending station and a receiving station.

DVB Digital Video Broadcasting - The European-backed project to harmonize adoption of digital video. Also, an open standard digital Video, Audio and Data encoding system.

DVB-S DVB Satellite specifications.

DVB-S2: Newly revised DVB satellite specifications giving many enhanced characteristics.

E...

E1 Wide-area digital transmission facility used predominantly in Europe that carries data at a rate of 2.048 Mbit/s.

E3 Wide-area digital transmission facility used predominantly in Europe that carries data at a rate of 34.368 Mbit/s.

Earth Station The term used to describe the combination or antenna, low-noise amplifier (LNA), down-converter, and receiver electronics. used to receive a signal transmitted by a satellite. Earth Station antennas vary in size from the.2 foot to 12 foot (65 centimeters to 3.7 meters) diameter size used for TV reception to as large as 100 feet (30 meters) in diameter sometimes used for international communications. The typical antenna used for INTELSAT communication is today 13 to 18 meters or 40 to 60 feet.

Eb/No: Energy per bit divided by Noise Spectral Density: The crucial figure of merit for digital signals, comparable to signal to noise ratio

Echo Canceller An electronic circuit which attenuates or eliminates the echo effect on satellite telephony links. Echo cancellers are largely replacing obsolete echo suppressors.

Echo Effect A time-delayed electronic reflection of a speaker’s voice. This is largely eliminated by modern digital echo cancellers.

Edge of Coverage Limit of a satellite’s defined service area. In many cases, the EOC is defined as being 3 dB down from the signal level at beam center. However, reception may still be possible beyond the -3dB point.

EIRP Effective Isotropic Radiated Power - This term describes the strength of the signal leaving the satellite antenna or the transmitting earth station antenna, and is

used in determining the C/N and S/N. The transmit power value in units of dBW is expressed by the product of the transponder output power and the gain of the satellite transmit antenna.

Elevation The upward tilt to a satellite antenna measured in degrees required to aim the antenna at the communications satellite. When. aimed at the horizon, the elevation angle is zero. If it were tilted to a point directly overhead, the satellite antenna would have an elevation of 90 degrees.

Encoder A device used to electronically alter a signal so that it can only be viewed on a receiver equipped with a special decoder.

ENG- Electronic News Gathering: usually refers to use of microwave transmission for local area news operations, normally from an ENG truck equipped with a microwave transmitter and mast..

EOL End of Life of a satellite.

Equatorial Orbit An orbit with a plane parallel to the earth’s equator.

ESC Engineering Service Circuit - The 300-3,400 Hertz voice plus teletype (S+DX) channel used for earth station-to-earth station and earth station-to-operations center communications for the purpose of system maintenance, coordination and general system information dissemination. In analog (FDM/FM) systems there are two S+DX channels available for this purpose in the 4,000-12,000 Hertz portion of the baseband. In digital systems there are one or two channels available which are usually convened to a 32 or 64 Kbps digital signal and combined with the earth station traffic digital bit stream. Modern ESC equipment interfaces with any mix of analog and digital satellite carriers, as well as backhaul terrestrial links to the local switching center.

Exciter or Television Exciter: a piece of equipment containing: baseband encoder, modulator and up converter. May be analog or digital

F...

FCC: Federal Communications Commission- a branch of the government that regulates satellite communications, particularly antennas and orbital arc locations.

F/D Ratio of antenna focal length to antenna diameter. A higher ratio means a shallower dish.

FDMA Frequency division multiple access. Refers to the use of multiple carriers within the same transponder where each uplink has been assigned frequency slot

and bandwidth. This is usually employed in conjunction with Frequency Modulation.

Feed This term has at least two key meanings within the field of satellite communications. It is used to describe the transmission of video programming from a distribution center. It is also used to describe the feed system of an antenna. The feed system may consist of a subreflector plus a feedhorn or a feedhorn only.

Focal Length Distance from the center feed to the center of the dish.

Focal Point The area toward which the primary reflector directs and concentrates the signal received.

Footprint A map of the signal strength showing the EIRP contours of equal signal strengths as they cover the earth’s surface. Different satellite transponders on the same satellite will often have different footprints of the signal strength. The accuracy of EIRP footprints or contour data can improve with the operational age of the satellite. The actual EIRP levels of the satellite, however, tends to decrease slowly as the spacecraft ages.

Forward Error Correction (FEC) Adds unique codes to the digital signal at the source so errors can be detected and corrected at the receiver.

Frequency The number of times that an alternating current goes through its complete cycle in one second of time. One cycle per second is also referred to as one hertz; 1000 cycles per second, one kilohertz; 1,000,000 cycles per second, one megahertz: and 1,000,000,000 cycles per second, one gigahertz.

Frequency Coordination A process to eliminate frequency interference between different satellite systems or between terrestrial microwave systems and satellites. In the U.S. this activity relies upon a computerized service utilizing an extensive database to analyze potential microwave interference problems that arise between organizations using the same microwave band. As the same C-band frequency spectrum is used by telephone networks and CATV companies when they are contemplating the installation of an earth station, they will often obtain a frequency coordination study to determine if any problems will exist.

FSS; Fixed Satellite Service- normal point to point satellite service between fixed locations

G...

Gain A measure of amplification expressed in dB.

Geostationary Refers to a geosynchronous satellite angle with zero inclination. so the satellite appears to hover over one spot on the earth’s equator.

Geosynchronous The Clarke circular orbit above the equator. For a planet the size and mass of the earth, this point is 22,237 miles above the surface.

Gigahertz (GHz) One billion cycles per second. Signals operating above 3 Gigahertz are known as microwaves. above 30 GHz they are know as millimeter waves. As one moves above the millimeter waves signals begin to take on the characteristics of Iightwaves.

Global Beam An antenna down-link pattern used by the Intelsat satellites, which effectively covers one-third of the globe. Global beams are aimed at the center of the Atlantic, Pacific and Indian Oceans by the respective Intelsat satellites, enabling all nations on each side of the ocean to receive the signal. Because they transmit to such a wide area, global beam transponders have significantly lower EIRP outputs at the surface of the Earth as compared to a US domestic satellite system which covers just the continental United States. Therefore, earth stations receiving global beam signals need antennas much larger in size (typically 10 meters and above (i.e.30 feet and up).

“Goodnight” Notification of cessation of transmissions

Gregorian Dual-reflector antenna system employing a paraboloidal main reflector and a concave ellipsoidal subreflector.

G/T A figure of merit of an antenna and low noise amplifier combination expressed in dB. “G” is the net gain of the system and “T” is the noise temperature of the system. The higher the number, the better the system.

Guard Channel Television channels are separated in the frequency spectrum by spacing them several megahertz apart. This unused space serves to prevent the adjacent television channels from interfering with each other.

H...

H.264 or H.26l: See AVC. An improved video encoding method

Half Transponder A method of transmitting two TV signals through a single transponder through the reduction of each TV signal’s deviation and power level. Half-transponder TV carriers each operate typically 4 dB to 7 dB below single- carrier saturation power.

Headend Electronic control center - generally located at the antenna site of a CATV system - usually including antennas, preamplifiers, frequency converters, demodulators and other related equipment which amplify, filter and convert incoming broadcast TV signals to cable system channels.

HDTV:High Definition Television. One of several formats (1080I, 720P, etc) providing high definition television under the DTV group of specificications.

Hertz (Hz) The name given to the basic measure of radio frequency characteristics. An electromagnetic wave completes a full oscillation from its positive to its negative pole and back again in what is known as a cycle. A single Hertz is thus equal to one cycle per second.

Hub The master station through which all communications to, from and between micro terminals must flow. in the future satellites with on-board processing will allow hubs to be eliminated as MESH networks are able to connect all points in a network together.

Huffman Coding: a type of entropy coding employed in MPEG to reduce bandwidth required

I...

I Frames: MPEG frames employing only intra frame coding. These frames are complete within themselves and are similar to JPEG frames in many ways

Inclination The angle between the orbital plane of a satellite and the equatorial plane of the earth.

Inclined Orbit: Normally refers to a satellite which has ceased North-South stationkeeping and is no longer in a “geostationary” orbit because it exceeds the .01 degree orbital assignment “box”.

INMARSAT The International Maritime Satellite Organization operates a network of satellites for international transmissions for all types of international mobile services including maritime, aeronautical, and land mobile.

INTELSAT The International Telecommunications Satellite Organization operates a network of satellites for international transmissions.

Interframe: within the same frame

Intraframe: relating to previous or subsequent frames

Interleaver: a way of separating a normal bit stream so that sequential burst errors in transmission do not overpower the FEC decoder

Intermodulation: unintended spurious signals generated when two or more signals are passed through a non liner process

Interference Energy which tends to interfere with the reception of the desired signals, such as fading from airline flights, RF interference from adjacent channels, or ghosting from reflecting objects such as mountains and buildings.

IRD An integrated receiver and decoder for reception of a transmission of voice, video and data.

ISDN - Integrated Services Digital Network. A CCITT standard for integrated transmission of voice, video and data. Bandwidths include: Basic Rate Interface - BR (144 Kbps - 2 B & 1 D channel) and Primary Rate - PRI (1.544 and 2.048 Mbps).

Isotropic Antenna A hypothetical omnidirectional point-source antenna that serves as an engineering reference for the measurement of antenna gain.

ITU International Telecommunication Union.

J...

Jammer - An active electronic counter-measures (ECM) device designed to deny intelligence to unfriendly detectors or to disrupt communications.

JPEG ISO Joint Picture Expert Group standard for the compression of still pictures.

JPEG 2000: A digital encoding format used for very high resolution image compression such as digital cinema.

K...

Ka Band The frequency range from 18 to 31 GHz.

Kbps Kilobits per second. Refers to transmission speed of 1,000 bits per second.

Kelvin (K) The temperature measurement scale used in the scientific community. Zero K represents absolute zero, and corresponds to minus 459 degrees Fahrenheit

or minus 273 Celsius. Thermal noise characteristics of LNA are measured in Kelvins.

Kilohertz (kHz) Refers to a unit of frequency equal to 1,000 Hertz.

Klystron A microwave tube which uses the interaction between an electron beam and the RF energy on microwave cavities to provide signal amplification. The klystron operates on principles of velocity modulation very similar to those in a TWT except that klystron interaction takes place at discrete locations along the electron beam. Common types of klystrons are the reflex klystron (an oscillator having only one cavity), two-cavity klystron amplifiers and oscillators, and multi- cavity klystron amplifiers.

Ku Band The frequency range from 10.9 to 17 GHz.

L...

L-Band The frequency range from 0.5 to 1.5 GHz. Also used to refer to the 950 to 1450MHz used for mobile communications.

LDPC: Low Density Parity Check: a type of high powered FEC used in DVB-S2

Leased Line A dedicated circuit typically supplied by the telephone company.

Low Noise Amplifier (LNA) This is the preamplifier between the antenna and the earth station receiver. For maximum effectiveness, it must be located as near the antenna as possible, and is usually attached directly to the antenna receive port. The LNA is especially designed to contribute the least amount of thermal noise to the received signal.

Low Noise Block Downconverter (LNB) A combination Low Noise Amplifier and downconverter built into one device attached to the feed.

Luminance: the “black and white” signal quality of a video signal-

M...

M&C: Monitoring and Control: usually an electronic systems that permits remote equipment monitoring and controlling

Margin The amount of signal in dB by which the satellite system exceeds the minimum levels required for operation.

Master Antenna Television (MATV) An antenna system that serves a concentration of television sets such as in apartment buildings, hotels or motels.

Megahertz (MHz) Refers to a frequency equal to one million Hertz, or cycles per second.

Microwave Line-of sight, point-to-point transmission of signals at high frequency. Many CATV systems receive some television signals from a distant antenna location with the antenna and the system connected by microwave relay. Microwaves are also used for data, voice, and indeed all types of information transmission. The growth of fiber optic networks have tended to curtail the growth and use of microwave relays.

Microwave Interference: Interference which occurs when an earth station aimed at a distant satellite picks up a second, often stronger signal, from a local telephone terrestrial microwave relay transmitter. Microwave interference can also be produced by nearby radar transmitters as well as the sun itself. Relocating the antenna by only several feet will often completely eliminate the microwave interference.

Modulation The process of manipulating the frequency or amplitude of a carrier in relation to an incoming video, voice or data signal.

Modulator A device which modulates a carrier. Modulators are found as components in broadcasting transmitters and in satellite transponders. Modulators are also used by CATV companies to place a baseband video television signal onto a desired VHF or UHF channel. Home video tape recorders also have built-in modulators which enable the recorded video information to be played back using a television receiver tuned to VHF channel 3 or 4.

MODEM: a combination MODulator and DEmodulator

MPEG The Moving Pictures Experts Group, the television industry’s informal standards group.

MPEG-2 The agreed standard covering the compression of data (coding and encoding) for digital television.

MPEG-2 MP@HL (also known as 4:2:2)Main Profile at High Level - The agreed much higher bit-rate system adopted to provide high definition television in wide screen format.

MPEG 4-2 or MPEG 4 or MPEG 4 ASP: a video encoding method designed mainly for IP video encoding and web video. Uses video objects to permit video at very low bit rates

MPEG 4-10: see AVC. An improved (over MPEG-=2) video encoding method

MSS: Mobile Satellite Service: between mobile platforms. Typically at L or S Band

Multicast Multicast is a subset of broadcast that extends the broadcast concept of one to many by allowing “the sending of one transmission to many users in a defined group, but not necessarily to all users in that group.”

Multiplexing Techniques that allow a number of simultaneous transmissions over a single circuit.

N...

Noise Any unwanted and unmodulated energy that is always present to some extent within any signal.

Noise Figure (NF) A term which is a figure of merit of a device, such as an LNA or receiver, expressed in dB, which compares the device with a perfect device.

NTSC - National Television Standards Committee A video standard established by the United States (RCA/NBC} and adopted by numerous other countries. This is a 525-line video with 3.58-MHz chroma subcarrier and 60 cycles per second.

O...

Orbital Period The time that it takes a satellite to complete one circumnavigation of its orbit.

P...

P Frames: MPEG frames containing prediction motion vectors and difference information

Packet Switching Data transmission method that divides messages into standard- sized packets for greater efficiency of routing and transport through a network.

PAL - Phase Alternation System The German developed TV standard based upon 50 cycles per second and 625 lines.

Parabolic Antenna The most frequently found satellite TV antenna, it takes its name from the shape of the dish described mathematically as a parabola. The function of the parabolic shape is to focus the weak microwave signal hitting the surface of the dish into a single focal point in front of the dish. It is at this point that the feedhorn is usually located.

PCM- Pulse Code Modulation- a method of digitally encoding an analog signal

Phase-Locked Loop (PLL) A type of electronic circuit used to demodulate satellite signals.

Polarization A technique used by the satellite designer to increase the capacity of the satellite transmission channels by reusing the satellite transponder frequencies. In linear cross polarization schemes, half of the transponders beam their signals to earth in a vertically polarized mode; the other half horizontally polarize their down links. Although the two sets of frequencies overlap, they are 90 degree out of phase, and will not interfere with each other. To successfully receive and decode these signals on earth, the earth station must be outfitted with a properly polarized feedhorn to select the vertically or horizontally polarized signals as desired.

In some installations, the feedhorn has the capability of receiving the vertical and horizontal transponder signals simultaneously, and routing them into separate LNAs for delivery to two or more satellite television receivers. Unlike most domestic satellites, the Intelsat series use a technique known as left-hand and right-hand circular polarization.

Polarization Rotator A device that can be manually or automatically adjusted to select one of two orthogonal polarizations.

Polar Mount Antenna mechanism permitting steering in both elevation and azimuth through rotation about a single axis. While an astronomer’s polar mount has its axis parallel to that of the earth, satellite earth stations utilize a modified polar mount geometry that incorporates a declination offset.

Polar Orbit An orbit with its plane aligned in parallel with the polar axis of the earth

PTT - Post Telephone and Telegraph Administration Refers to operating agencies directly or indirectly controlled by governments in charge of telecommunications services in most countries of the world.

Pulse Code Modulation A time division modulation technique in which analog signals are sampled and quantized at periodic intervals into digital signals. The values observed are typically represented by a coded arrangement of 8 bits of which one may be for parity.

Q...

QPSK - Quadrature Phase Shift Keying System of modulating a satellite signal such that each symbol contains two transmitted bits.

Quantization: The individual levels of steps used to digitally encode an analog signal

R...

Rain Outage Loss of signal at Ku or Ka Band frequencies due to absorption and increased sky-noise temperature caused by heavy rainfall.

Receiver (Rx) An electronic device which enables a particular satellite signal to be separated from all others being received by an earth station, and converts the signal format into a format for video, voice or data.

Receiver Sensitivity Expressed in dBm this tells how much power the detector must receive to achieve a specific baseband performance, such as a specified bit error rate or signal to noise ratio.

Router Network layer device that determines the optimal path along which network traffic should be forwarded. Routers forward packets from one network to another based on network layer information.

S...

Satellite A sophisticated electronic communications relay station orbiting 22,237 miles above the equator moving in a fixed orbit at the same speed and direction of the earth (about 7,000 mph east to west).

Scalar Feed A type of horn antenna feed which uses a series of concentric rings to capture signals that have been reflected toward the focal point of a parabolic antenna.

Scrambler A device used to electronically alter a signal so that it can only be viewed or heard on a receiver equipped with a special decoder.

Secam A color television. system developed by the French and used in the USSR. Secam operates with 625 lines per picture frame and 50 cycles per second, but is incompatible in operation with the European PAL system or the U.S. NTSC system.

SDTV Standard Definition Television. Normal (not high definition) television as defined by the DTV group of specifications.

SMPTE: Society of Motion Picture and Television Engineer. A standardization body and professional organization

SFD - Saturation Flux Density The power required to achieve saturation of a single repeater channel on the satellite.

Sidelobe Off-axis response of an antenna.

Signal to Noise Ratio (S/N) The ratio of the signal power and noise power. A video S/N of 54 to 56 dB is considered to be an excellent S/N, that is, of broadcast quality. A video S/N of 48 to 52 dB is considered to be a good S/N at the headend for Cable TV.

Simplex Transmission Capability for transmission in only one direction between sending station and receiving station.

Single-Channel-Per-Carrier (SCPC) A method used to transmit a large number of signals over a single satellite transponder.

Skew An adjustment that compensates for slight variance in angle between identical senses of polarity generated by two or more satellites.

Slant Range The length of the path between a communications satellite and an associated earth station.

Slot That longitudinal position in the geosynchronous orbit into which a communications satellite is “parked”. Above the United States, communications satellites are typically positioned in slots which are based at two to three degree intervals.

SNG Satellite news gathering usually with a transportable uplink truck.

Snow A form of noise picked up by a television receiver caused by a weak signal. Snow is characterized by alternate dark and light dots appearing randomly on the picture tube. To eliminate snow, a more sensitive receive antenna must be used, or better amplification must be provided in the receiver (or both).

Solar Outage Solar outages occur when an antenna is looking at a satellite, and the sun passes behind or near the satellite and within the field of view of the antenna. This field of view is usually wider than the beamwidth. Solar outages can be exactly predicted as to the timing for each site.

Spectral Regrowth- regeneration of spectral sidelobes (and thereby increasing the signal occupied bandwidth) due to passing the signal through a non-linear device such as an HPA or SSPA not operating in the linear range.

Spectrum The range of electromagnetic radio frequencies used in transmission of voice, data and television.

Spectrum Analyzer: A piece of test equipment that permits visualization of the frequency spectrum and the signals contained therein.

Spillover Satellite signal that falls on locations outside the beam pattern’s defined edge of coverage.

Spin Stabilization A form of satellite stabilization and attitude control which is achieved through spinning the exterior of the spacecraft about its axis at a fixed rate.

Splitter A passive device (one with no active electronic components) which distributes a television signal carried on a cable in two or more paths and sends it to a number of receivers simultaneously.

Spot Beam A focused antenna pattern sent to a limited geographical area. Spot beams are used by domestic satellites to deliver certain transponder signals to geographically well defined areas such as Hawaii, Alaska and Puerto Rico.

Spread Spectrum The transmission of a signal using a much wider bandwidth and power than would normally be required. Spread spectrum also involves the use of narrower signals that are frequency hopped through various parts of the transponder. Both techniques produce low levels of interference Between the users. They also provide security in that the signals appear as though they were random noise to unauthorized earth stations. Both military and civil satellite applications have developed for spread spectrum transmissions.

SSPA Solid state power amplifier. A VSLI solid state device that is gradually replacing Traveling Wave Tubes in satellite communications systems because they are lighter weight and are more reliable.

Stationkeeping Minor orbital adjustments that are conducted to maintain the satellite’s orbital assignment within the allocated “box” within the geostationary arc.

Subcarrier A second signal “piggybacked” onto a main signal to carry additional information. In satellite television transmission, the video picture is transmitted over the main carrier. The corresponding audio is sent via an FM subcarrier. Some satellite transponders carry as many as four special audio or data subcarriers whose signals may or may not be related to the main programming.

SUIRG: Satellite Users Interference Reduction Group. An industry organization dedicated to reducing satellite interference

Synchronization (Sync) The process of orienting the transmitter and receiver circuits in the proper manner in order that they can be synchronized . Home television sets are synchronized by an incoming sync signal with the television cameras in the studios 60 times per second. The horizontal and vertical hold controls on the television set are used to set the receiver circuits to the approximate sync frequencies of incoming television picture and the sync pulses in the signal then fine tune the circuits to the exact frequency and phase.

T...

TT&C: Telemetry, tracking and control: A system used to collect and transmit to earth the spacecraft status, and to permit earth personnel to control the spacecraft.

T1 The transmission bit rate of 1.544 millions bits per second. This is also equivalent to the ISDN Primary Rate Interface for the U.S. The European T1 or E1 transmission rate is 2.048 million bits per second.

T3 Channel (DS-3) In North America, a digital channel which communicates at 45.304 Mbps.

TDMA Time division multiple access. Refers to a form of multiple access where a single carrier is the shared by many users. Signals from earth stations reaching the satellite consecutively are processed in time segments without overlapping.

TI - Terrestrial Interference to satellite reception caused by ground based microwave transmitting stations.

Transmitter An electronic device consisting of oscillator, modulator and other circuits which produce a radio or television electromagnetic wave signal for radiation into the atmosphere by an antenna.

Transponder A combination receiver, frequency converter, and transmitter package, physically part of a communications satellite. Transponders have a typical output of five to ten watts, operate over a frequency band with a 36 to 72 megahertz bandwidth in the L, C, Ku, and sometimes Ka Bands or in effect typically in the microwave spectrum, except for mobile satellite communications. Communications satellites typically have between 12 and 24 onboard transponders although the INTELSAT VI at the extreme end has 50.

Turbo Coding: a high powered FEC technique

TVRO Television Receive Only terminals that use antenna reflectors and associated electronic equipment to receive and process television and audio communications via satellite. Typically small home systems.

Tweaking The process of adjusting an electronic receiver circuit to optimize its performance.

TWT (Traveling-wave tube) A microwave tube of special design using a broadband circuit in which a beam of electrons interacts continuously with a guided electromagnetic field to amplify microwave frequencies.

TWTA (Traveling-wave-tube amplifier) A combination of a power supply, a modulator (for pulsed systems), and a traveling-wave tube, often packaged in a common enclosure.

U...

Unicast A unicast application transmits a copy of every packet to every receiver.

Uplink The earth station used to transmit signals to a satellite

UPS Uninterruptible Power Supply. A battery or rotating device designed to ensure continuity of prime power to equipment

V...

V.35 ITU-T standard describing a synchronous, physical layer protocol used for communications between a network access device and a packet network. V.35 is most commonly used in the United States and in Europe, and is recommended for speeds up to 48 Kbit/s.

VC-2 Videocypher 2- an analog scrambling method for NTSC television

VC-9: SMPTE designation of the Windows Media 9 digital encoding standard. Similar to AVC.

VGA: the means whereby video is shown on computer display

Viterbi Decoding: a method of decoding convolutional codes which has been shown to have certain desirable characteristics

VSWR Voltage Standing Wave Ratio. A measurement of mismatch in a cable, waveguide, or antenna system.

VSAT Very small aperture terminal. Refers to small earth stations, usually in the 1.2 to 2.4 meter range. Small aperture terminals under 0.5 meters are sometimes referred to Ultra Small Aperture Terminals (USAT’s)

W...

Waveguide A metallic microwave conductor, typically rectangular in shape, used to carry microwave signals into and out of microwave antennas.

X...

X.25 A set of packet switching standards published by the CCITT. < P>