CALIFORNIA STATE UNIVERSITY, NORTHRIDGE
SPACE COMMUNICATION THEORY AND PRACTICE
A graduate project submitted in partial satisfaction of the requirements for the degree of the Masterr of Science 1n
Engineering
by
Kristine Patricia Maine
January, 1983 The project of Kristine Patricia Maine is approved:
CDr. Ste~en A. Gad~ski)
(Dr. ~y H. Pettit, Chair)
California State University, Northridge
ii ACKNOWLEDGEMENTS
A deep sense of gratitude is expressed to my academic advisor, Dr.
Ray Pettit, for his invaluable counsel and advisement during the course of this study. In addition, I express thanks to The Aerospace
Corporation for the use of their facilities and personnel for the completion of this document.
iii TABLE OF CONTENTS
Title Page
Acknowledgement iii
Table of Figures v
List of Tables vii
List of Symbols viii
Abstract xi
I. Introduction 1
II. Space Communication System Components 2
III. Theory of Link Analysis 8
IV. Applications of Link Analysis 21 v. Summary 41
Appendix A--Signal Losses 45
Appendix B--Space Shuttle Design Model 54
Appendix C--Communication Satellite Systems 62
Footnotes 67
Bi biliography 70
iv TABLE OF FIGURES
Figure Number Title
1 Simplified Transponder Block Diagram 3 Showing a Single Channel Transponder
2 TWT Characteristics 4
3 Typical Antenna Beamwidth Pattern 7
4 Satellite Communication System 9
5 Uplink Model 10
6 Downlink Model 13
7 Satellite Communication Transmitter 16 to-Receiver with Typical Loss and Noise Sources
8 Zenith Attenuation Values Expected to 18 be Exceeded 2% of the Year at Mid latitudes
9 Probability of Error for Binary Digital 20 Modulation Schemes
10 Jupiter Atmospheric Loss for a 10 Bar 32 Pressure Level
11 Link Margin as a Function of Periapsis 35 Distance
12 Bit Error Probability vs Energy-per 42 Bit to Noise Density Ratio
13 Typical AM/PM Conversion Phase Shift 47
14 Model of AM/PM Nonlinearities in a 48 TWT Amplifier
15 CW Phase-Locked Loop 55
16 Costas Loop 56
17 Equivalent Baseband Phase Error Model 57 of Shuttle Phase-Locked Receiver
v 18 Estimated Phase Noise Loop Density 61
19 Evolution of INTELSAT Space System 63
vi LIST OF TABLES
Table Number Title
1 High Definition Television 23 Satellite Broadcasting
2 DBS Satellite Link Analysis 25
3 Satellite-to-Satellite Link 27
4 Terminal Performance Characteristics 29 Shipboard Terminal
5 Terminal Performance Characteristics 29 COMSAT Labs Terminal
6 Ship-to-Satellite-to-Earth Link Budget 30 Calculation
7 Jupiter Probe Communication System 36 Parameters
8 Hypothetical Military Communication 38 Uplink Calculation
vii LIST OF SYMBOLS
saturated flux density
P* outpower corresponding to the saturated flux density s RF transmitter power
input backoff value
output backoff value
G power gain
transmitter antenna gain
satellite antenna gain (uplink)
G receiver gain r satellite gain (downlink)
uplink wavelength
wavelength
downlink wavelength
antenna aperature efficiency
physical area of antenna
A effective area of satellite antenna e R uplink distance u downlink distance
range
L additional uplink losses u additional downlink losses
viii L space loss s transmitting polarization and filter losses 1 tpf receiving polarization and filter losses L rpf N noise density 0 23 k Boltzmann's constant, 1.38 x lo- J/°K
system effective temperature T s T antenna temperature a T receiver temperature r NF noise figure
C/N carrier-to-noise density ratio 0 C/N carrier-to-noise power ratio
B receiver bandwidth
bit rate
bit-energy-to-noise density ratio
bit-energy-to-jammer noise density ratio
error probability
M link margin
J/S jammer power-to-signal power ratio w transmission bandwidth
PG processing gain
low frequency
frequency
high frequency
constant for flicker noise
white noise component
flicker noise frequency
A(t) input envelope
ix 8(t) phase noise variation
G(t) VCO phase estimation
phase noise fluctuation
receiver phase noise
input signal amplitude n ( t) white gaussian noise g cp phase error
F(p) loop filter transfer function
K open loop gain
probability density function
mean squared value of phase error
X ABSTRACT
SPACE COMMUNICATION THEORY AND PRACTICE
by
Kristine Patricia Maine
Master of Science in Engineering
Link power budgets for estimating the quality of signal
transmission have been developed for space communication.
Essentially, a link power budget can be set up, a balance sheet of gains and losses for the signal transmission path, as a tool to analyze the performance of the entire communication link. Examples in
this paper illustrate the uses of link power budget analysis for geostationary communication satellites, deep space orbiting probes,
satellite-to-satellite crosslinks, and ship-satellite-shore communication links.
xi SPACE COMMUNICATION THEORY AND PRACTICE
I. Introduction
Present-day space communications utilize microwave engineering, a distinguished technology, that has greatly advanced since World War
II. During this same period, the developement of statistical communications provided the means for understanding the fundamental relationships of transmission bandwidth, carrier-to-noise ratio, and attenuation. The development of communication link performance analysis based on statistical communication theory has been applied to space communications. Essentially, signal power, antenna gain, noise, and signal loss are analyzed over the signal transmission path. The signal transmission path starts at an earth ground station which transmits a signal through space. A satellite or spacecraft receives the signal and transmits it to another earth station in the network.
A "link power budget" is commonly set up as a balance sheet of gains and losses for the transmission path and is used to analyze the performance of the entire communication link. Link analysis is primarily used to predict link performance by a carrier-to-noise density ratio, a value indicating the strength of the signal over the
1 2 ' . noise encountered along its path. In order to understand the parameters used in a link analysis, a brief description of components found in space communication systems is given in the next section.
II. Space Communication System Components
Most communication satellites contain several (four or more) parallel transponders with several narrow beam antennas for different classes of users (mobile terminals or fixed ground stations). Figure
1 illustrates a model of a single channel transponder. Multiple input sinusoidal waves which carry digital information enter the transponder in frequency band fu and exit in band fd. The frequency bands are sufficiently separated to prevent "ring-around" oscillation in the transponder. Ring-around oscillation is a term used to describe the effect of fu being indistinguishable from fd implying fd will be included with the other uplink signals. A bandpass filter allows the f frequency band to enter the transponder and excludes any other u frequency. Following the filter, a preamplifier strengthens the signal which travels into a frequency converter changing f to u fd. The traveling wave tube (TWT), a power amplifier, strengthens fd significantly so that it may be detected at a ground station.
Most TWTs utilize vacuum-tube technology with nonlinear power characteristics, as shown in Figure 2. The flux density necessary to saturate the TWT is denoted by Q* and corresponding output power u is P* • Frequently, the satellite is operated backed off from s saturation mode to avoid nonlinear distortions. The input and output MULTIPLE INPUT SIGNALS f \t/ u
BANDPASS BANDPASS FILTER r ___ -, AMPLIFIER LOW NOISE 1 X I PREAMP I LIMITER I I r--__..___ _, TRAVELING I 1 LOCAL I WAVE TUBE I OSCILLATOR 1 t_: ______~ INPUT OUTPUT FREQUENCY CHANNEL CHANNEL CONVERTER ~ ~ lllf fu fd
Figure 1. Simplified Transponder Block Diagram Showing a Single Channel Transponder.l
w --BOI 20 10 0 0 BOo -::-0..Vl 22. 0 SINGLE CARRIER~ s ;g 12.0 10 l 0 r- r--4
lo.... Q) 2.0 20 > 0 0.. 0::: LLJ -8.0
-_J SINGLE CARRIER <( CQ SATURATION 0 _J (.!)
- 113. 7 - 103. 7 - 93. 7 - 83. 7 - 73. 7 2 dBW I m AT MINIMUM ATTENUATOR SETTING, .Q:~ 2 Figure 2. TWT Characteristics
+:'-
"' 5
backoff values are respectively denoted as B0 and B0 • 1 0 Satellite transponders are often multichannel in configuration.
The transponder is channelized by a frequency-selector filter to allow different frequency bands to be h~ndled by separate amplifiers and antennas. Transponder channelization is usually applied in order to increase the total downlink power by using parallel power amplifiers
(TWTs) inside the satellite. The number of signals handled by a single TWT can then be decreased, reducing the nonlinear effects.
However, one of the most important features of the transponder is its ability to isolate signals from small mobile terminals and large fixed 3 ground stations by using separate channels inside one transponder.
The multiple sinusoidal waves that may appear at the TWT input can cause significant intermodulation products in the output channel bandwidth unless the power is backed off along the power curve shown in Figure 2. Intermodulation products refer to the nonlinear effects created by a cross-product of two or more input signals. Because the same antenna can be used for transmitting and receiving, the TWT must be heavily filtered to prevent cross-products from falling in the receiver frequency band and saturating the preamplifier.
The antenna of a ground station or a satellite acts as a transducer between free-space propagation and the guided-wave propagation. A measure of the ability of an antenna to concentrate energy in a particular direction is called "gain". Power gain u defined as
G = maximum radiation intensity from subject antenna (1) radiation intensity from (lossless) isotropic source with same power input. 6
The relationship between the gain and the beamwidth of an antenna depends on the distribution of the area across the aperture. A typical beamwidth pattern in polar coordinates for a parabolic antenna
(the most common type of antenna used in space communications) is illustrated in Figure 3.
Another useful antenna parameter related to gain is the effective receiving aperture or effective area. The gain G and the effective 4 area A of a lossless antenna are related by e
G = 4nAe_ = 4np 8 A (2) A2 A2 where
A = wavelength
p antenna aperature efficiency a = A = physical area of antenna
The direction of polarization of the signal from an antenna 1s defined as "the direction of the electric field vector which is either 5 vertical or hortizontal." Polarization may also be elliptical or circular. Elliptical polarization may be considered as the combination of two linearly polarized waves of the same frequency traveling in the same direction and perpendicular to each other. If the amplitudes of the two waves are equal and 90 0 out of phase, the polarization is circular. It is common procedure in space communications to have the two opposing transmission paths be polarized in different directions when the same antenna is used for transmitting and receiving. 7
6 Figure 3. Typical Antenna Beamwidth Pattern 8
III. Theory of Link Analysis
A general model of a satellite connnunication system is shown in
Figure 4. The terrestrial system and interface are hooked up to the earth station which transmits the signal to the satellite through space; the satellite receives the signal and transmits it to the appropriate earth station. Figure 5 shows the uplink model elements required for sending a signal from the ground to the satellite. The earth station parameters contain the RF transmitted power (Pt in watts) and the transmitted antenna gain, Gt. R represents uplink u distance from the ground station to the satellite (in nautical miles or kilometers) which takes into account the angle of the antenna relative to the earth's surface. L represents other uplink losses u in the form of bandlimiter loss, radome loss, pointing-error loss, filter loss, or polarization loss. The signal power received at the . . 7 sate 111te ts
c = ,Qu Ae Watts (3)
2 = nu 2.s~u 41T where
n = the flux density at the satellite, u (4)
A effective area of the satellite antenna e = G satellite antenna gain su = A wavelength on the uplink. u = I SATELLI~-E I-
EARTH (/ ~)._ EARTH STATION \ STATION ,
TERRESTRIAL l~TERfACE TERRESTRIAL I TERRESTRIAL------·] ! SYSTEE I SYSTE~1 I CSER -·~-~~-- --j-- I;.JTERFACE
r ~~-£~ IUSE9 ! :
3 Figure 4 . .S ate ll1te. commun1cat1on. . . s ystem
1..0
... Gsu
2 47TR PT u .. RF
' -r-· EARTH STATION PROPAGATION ~lEDIUH ANTENNA GAIN FLUX DEN I STY
9 Figure 5. Up link !-lode 1
...... 0
... 11
When the TWT is taken into consideration, the flux density becomes
the saturated flux density at the satellite (see Figure 2):
(5)
The noise of the system is made up of thermal noise, atmospheric-
induced noise, and earth background noise. Generally, noise is
assumed to have a flat spectral density of N over the receiving 0 2 bandwidth measured in W/Hz • Expressing noise density as a function
of system effective temperature,
(6) where 23 k = the Boltzmann's constant (1.38 x 10 - J/°K)
T = the system effective temperature of the s satellite in (°K).
The system temperature is calculated from the antenna temperature
(T ) and the receiver temperature [T (NF- 1)*290 °K.] a r =
Ts = Ta + (NF - 1)* 290 °K. (7)
Noise figure (NF) is the ratio of the input signal-to-noise ratio to
the output signal-to-noise ratio. It tells the amount of noise in the amplifier of the receiver compared to the source; however, it does not
provide the absolute measure of the amplifier noise.
Combining equations (3), (4), and (6) yields: 12
2
C/N0 = (PtGt) [ _lu_ J 1 (8) 41TRu k where
C/N = carrier-to-noise density ratio 0 R = uplink range. u The same equation can be expressed 1n decibels (dB) by
C/N0 up(dB) = lOlog (PtGt) - 20log 41TRu +10 log Q.su_ Au Ts + lOlog Lu - lOlog (k) (9)
The downlink communication equation is a duplicate of the uplink equation (9).
C/N0 down(dB) = lOlog (PsGsd) - 20log 41TRd +10 log Qr_ Ad Ts
+ lOlog Ld - lOlog (k); (10) where,
satellite EIRP; p s G s d = G /T earth station G/T; r s = = additional downlink losses described for L (See u Figure 6.)
Equation (9) and (10) are the basic uplink and downlink performance equations.
Three important expressions in equations (9) and (10) are used as design criteria for a communication satellite and ground station.
First, the effective isotropic radiated power (EIRP) is equivalent to the product of the transmitted power and the transmitter antenna gain 13
~ ~ > 1-1 tlJ u w :::.::
"t1 z 0 z<: z z tlJ H "t1 E-< <: e, ~ z e, <: 1 _t cGU) t:JW "t1 ::J::U) >--1 f- r-'-- 0:: w E-< E-< H :4 en U) A. z ~ E-< '-r-- 14 (PtGt). This value is used to indicate the power and directivity of the transmitter and the transmitting antenna. A large EIRP, PtGt' value implies that a strong signal of great intensity and low power coming through the sidelobes of the antenna reaches the satellite. The second important parameter, G /T or G su s r IT s (abbreviated G/T ), is a number quoted as the amount of gain per s degree Kelvin used to judge the signal strength over the noise of the receiver. A high G/T ratio indicates that the antenna gain is s greater than the noise and allows the receiver to detect weaker signals than a receiver with a lower G/T value. Ground stations s have higher EIRP and G/T values than satellites; therefore, a high s power signal needs to be sent to the satellite and a high gain antenna is required on the ground for detecting the weaker signals from the satellite. Higher EIRP and G/T values also imply that the s transmitter, receiver, and antennas must be large to produce the large power and gain. The transmitter, receiver, and antennas on the satellite are small in size compared to the ground stations. The third important parameter in equation (9) is the expression 20log (4TIR /A ) representing the uplink space loss or path loss u u (L )--the amount of power lost along the uplink transmission path. s A similar expression 20log(4TIRd/Ad) appeared in equation (10) representing the downlink space loss. A more practical expression for space loss is: L 92.796 + 20log f + 20log R , dB (11) s = where f = frequency in gigahertz; 15 R = length of uplink or downlink transmission path in nautical miles. Losses occur when a portion of the signal is diverted, scattered, 11 or reflected from the intended route. Figure 7 lists the noise sources and losses that affect the performance of satellite communication link. Some of the losses are combined and represented by the terms Lu and Ld in equations (9) or (10): bandlimiter loss, polarization loss, radome loss, filter loss, or pointing error loss. Atmospheric attenuation, background noise, and receiver noise are important features in the link budget and are accounted for in the basic carrier-to-noise density equation [equations (9) and (10)] in the analysis of the link performance. On the other hand, AM/PM conversion, intermodulation products, and phase-noise error were considered at the detailed design level and will not be included in link performance analysis. Definitions of each loss listed in Figure 7 can be found in Appendix A. Antenna efficiency is used to indicate the amount of signal energy lost due to reradiation, scattering, or spillover from the antenna. The efficiency is measured relative to the signal radiated from an ideal antenna compared with the actual radiated signal. Radome loss is due to the attenuation of the signal as it passes through the protective radome. Plumbing or circuit loss not mentioned in Figure 7 is a finite loss found in the transmission lines connecting the output of the transmitter to the antenna. At the lower frequencies, the circuit loss is a small and can be neglected. However, for applications at ® CRDsnAU fROM OTH(JI CHUIIlLS POI.rlfiG POIIITIIIG r------,LOSS LOSS •r----- LOU ---T, 1 CH.UUL 1 I I G) IIADDIIl I I I LOSS I I I I I I I IIMULATIOII XIIT I I I@ MULTI·CAIIIII(JI I I IIITUIMOD IIDISE I 10 ® I ~::::)=:=>1 I GALACTIC, ITA II, I TllllltrTIAL.IIOISt• •lEY SOLIIICU Of IIOIU OU;RAOATIOII ~0 0 L.O. I'IIASl IIOISl UGEIID ------[:;!:=::::] II GliAL LOU lllfDRIIATIOII ~ IIOIU SOUIIC£ SOURCE TU.IISIIITTIIIG TUIIIIIAL m IOTII IIECEIYIIIII TliiMIIIAL Figure 7 Satellite Communication Transmitter-to Receiver with Typical Loss and Noise Sources 12 f-0 0\ 17 high frequency, this loss is large and must be taken into consider ation. Polarization loss is the result of mismatching the polarized signal between the transmitting and receiving antennas. The pointing error loss is a decrease in signal strength when the antenna is pointed off from boresight; filter loss is due to attenuation as the signal passes through the various filters in the transponder. One of the principle causes of signal loss through the atmosphere is rain attenuation. Absorption and scattering due to water vapor, heavy rain clouds, or fog cause the signal strength to vary. The estimation of rain attenuation is based on the statistics derived from meterological data taken at different locations, path geometries, and at different times of the year. The statistical variations of rainfall intensity and attenuation along a path depend on the number, type, and intensity of rainstorms that occur and must be obtained empirically. The method for calculating the rain attenuation can be found in "Prediction of Attenuation by Rain" by Robert K. Crane (see Bibiliography). Figure 8 shows an example of the relative magnitude of absorption due to water vapor, a liquid water cloud, and rain that is expected to be exceeded during less than 2% of the year on the zenith path (a path directly above the observer). The overall carrier-to-noise ratio is a calculation of the ratio of the received carrier power to the noise density ratio described in equations (9) or (10). The carrier-to-noise density ratio for the 13 overall link is 18 02 3 CLOUD 0. 2 g I m 2.5 l 2 km VERT/ PATH~ co "'0 z 2.0 0 1- <:: ::::>z L1.J 1- 1- <:: RAINlmm/h ::r: 1.5 1- 2. 5 km z VERTICAL PATH L1.J N ...... 2cr VARIATION 0 OF GASEOUS 1'-z H o ABSORPTION L1.J 2 z 1.0 (global) 0 a.. :E 0 u 2CJ VARIATION ABOUT L1.J _J MODEL FOR GIVEN co <:: SURFACE CONDITIONS a:: 0.5 <:: > o~~~~--~--~--~---J---~--~--~~. 0 I 0 20 30 40 50 60 70 80 FREQUENCY. GHz Figure 8. Zenith Attenuation Values Expected to be14 Exceeded 2% of the Year at Midlatitudes. 19 = (C/N)up (C/N)down (12) (C/N)up + (C/N)down The C/Nall ratio is the basis of analysis for space communication. For a noise bandwidth B, the C/(Nall x B) can be written as C/N, known as carrier-to-noise power ratio. The required carrier-to-noise density ratio for digital signals at a given bit rate Rb is related to bit energy-to-noise (Eb/N ) as 0 R = required C/N (13) C/N . (dB) (14) o requ1red = Eb/N can be determined from an error probability (Pe) curve for 0 binary signals as shown in Figure 9. Normally, a desired P value e is chosen for the type of binary signal used; therefore, the Eb/N 0 value is found directly from the curve. Eb/N established from 0 the P value is sometimes called the threshold. Link carrier- e to-noise density ratio must exceed this threshold by a factor the numerical value of which is determined by the probability of signal detection. Link margin (M) is a value used to measure the link performance. It represents the maximum allowable signal attenuation due to all causes. A generalized expression using the margin, in decibels, is C/N0 up or down(dB) = EIRP + G/T - Ls - k - M + Lu or d; (15) where L d represents the summation of losses other than space u or loss (described earlier) and k is the Boltzmann's constant (-228.6 ·rcOHERENT / NONCOHERENT ASK FSK 10- l -- 10-2 NONCOHERENT ASK 10-3 p e 10-4 COHERENT PSK 5 l 0- DPSK 10-6 COHERENT FSK l o- 7 0 5 10 15 20 2 S/ N = A Tb/ (2T)) ,. I I Figure 9. Probability of Error for Binary Digital Modulation SchemeslS N 0 21 dB/ 0 K/Hz). The decibel equation for the data rate is derived from equations (13) and (15) R(dB) = EIRP + G/T - Ls - k - M + 1u or d - Eb/N0 (16) IV. Applications of Link Analysis There are several methods for calculating the link power budget based upon the theory presented above. Variations of these methods employed depend on the types of losses considered, ways of calculating EIRP or G/T, and the type of parameter to be used for measuring performance. For example, link performance for a high definition television (HDTV) receiver was developed using a direct broadcasting satellite to the home receivers as weii as for community 16 reception. The community reception antenna has a diameter larger than one meter, but the home viewer may employ an antenna of one meter on a rooftop, chimmey side, or at ground level. Link analysis presented in Table 1 is for the one-meter antenna home receiver. 17 High Definition Television Link Analysis The link power budget of Table 1 is set up as a tally sheet in order to determine the carrier-to-noise density ratio of the uplink and downlink using equations (9) and (10). Rain attenuation is assumed to be 14 dB, based upon the requirements to accommodate the worst-case signal from remote locations in wet regions of the United States. The rain attenuation is determined from statistical rain 22 averages for the worst month of rain in the area of interest. The downlink calculation of Table 1 is designed to determine the carrier-to-noise density ratio under worst conditions. The peak EIRP (end-of-life) is the expected value of the satellite EIRP near the end of its life. Edge of coverage gain is a loss of signal at the edge of the antenna beamwidth. Taking into account space loss, the received carrier power is determined. Next, the noise parameters as well as additional losses contributing to the link are included in the calculations. The uplink noise contribution is from the ring-around effects. Atmospheric absorption or attenuation value is based upon attenuation of a signal in a clear atmosphere free from rain or heavy moisture. Finally, polarization loss is estimated and included in the received carrier-to-noise density ratio yielding a value of 94.1 dB. Adverse tolerance values are estimated from the variations of the transmitter power and receiver antenna gain. Total tolerance of 1.2 dB accounts for a variation of the C/N from 92.9 dB to 95.3 dB. 0 Further downlink analysis is carried out for the worst rain attenuation expected in the area of satellite coverage. The value of 5 dB rain attenuation is attributed to the statistical worst rain month and water absorption curves at the downlink frequency. In addition, performance of the home receiver deteriorates under increased sky noise conditions. This deterioration of signal leads to a decrease in the G/T ratio. The overall C/N is computed based on 0 equation (12). 18 Direct Broadcasting Satellite Link Calculation In the near future, another design of a direct broadcasting 23 TABLE 1 HIGH DEFINITION TELEVISION SATELLITE BROADCASTING UPLINK Description Value Adverse Tolerance Earth Station EIRP 87.0 dBW Space Loss (17.3-17.8 GHz) -208.0 dB Rain Attenuation -14.0 dB Satellite G/T 8.2 dB/°K Boltzmann's Constant 228.6 dB/°K/Hz Uplink C/N0 101.8 dB-Hz DOWNLINK Peak EIRP, End of Life 63.4 dBW Relative Gain, Edge of Coverage -2.0 dB Net EIRP to Terminal 61.4 dBW + 0.7 dB. Space loss ( 12.5 GHz) -206.4 dB - Received carrier power -145.0 dBW Terminal G/T 12.0 dB/°K + 0.5 dB Boltzmann's Constant 228.6 dBW/°K/Hz Uplink Noise Contribution -0.2 dB (worst case) Atmospheric Absorption -1.0 dB Polarization Loss -0.3 dB Received C/N ,dB-Hz composite 94.1 dB + 1.2 dB 0 - In the Eresence of 5 dB rain attenuation Atmospheric attenuation -5.0 dB Terminal G/T 10.7 dB/°K. Overall C/N 87.8 dB-Hz + 1.2 dB 0 - 24 satellite (DBS) will be used for home reception. Home receiver-only antennas are designed to be small (0.75 meters) to receive a downlink frequency band of 12.1 to 12.7 GHz. Two lists of link calculations allow direct comparison of the numbers for clear atmospheric and rain attenuation conditions. The link analysis is used to determine the margin available in clear atmosphere and heavy rain conditions based upon the overall C/N • A C/N of 10 dB is set as a guideline based upon the 0 0 desired bit-error probability and the Eb/N values. Referring to 0 Table 2, the overall C/N values are determined to be 87.9 dB-Hz for 0 clear atmosphere conditions and 82.0 dB-Hz for a 5-dB rain attenuation. The C/N value is reduced further when the decibel 0 value of the bandwidth (16 MHz) is used to determine the carrier- to-noise power ratio (C/N). Finally, the margin is found from the overall carrier-to-noise power (C/N) for a 16 MHz bandwidth and the threshold. A 5.9 dB margin exists for any extraneous effects on the transmission path that may cause unexpected fading not accounted for by the link calculations under clear skies. However, if any unexpected fading occurs during heavy rain conditions, the signal will be lost because zero margin exists under those conditions. If the communication engineer wishes to have greater margin value under rain attenuation, he may compensate by increasing the transmitting power or building a low noise receiver to increase the margin. 19 Sate 111te. Cross 1·1n k Ana 1 ys1s . Transmission of satellite signals includes a crosslink to another 25 TABLE 2 DBS SATELLITE LINK ANAYLSIS Uplink Earth Station EIRP 86.6 dBw Free Space Loss -208.9 dB (17.6 GHz,48°elev.) Rain Attenuation (assumed) -12.0. dB Satellite G/T 7.7 dB/°K Boltzmann's constant 228.6 dB/°K/Hz Uplink C/N0 102.0 dB-Hz Downlink Atmosphere Condition 5-dB Clear Rain Attenuation Satellite EIRP 57.0 dBW 57.0 dBW Free Space Loss (12.5 GHz) -206.1 dB -206.1 dB Atmospheric Attenuation -0.14 dB -5.o dB Home Receiver G/T (0.75 m) 9.4 dB/°K 8.1 dB/°K Receiver Pointing Loss (0.5° error) 0.6 dB 0.6 dB Boltzmann's Constant 228.6 dB 228.6 dB/°K/Hz Polarization Mismatch Loss (avg) 0.04 dB 0.04 dB Downlink C/N0 88.1 dB-Hz 82.0 dB-Hz Overall C/N0 87.9 dB-Hz 82.0 dB-Hz Overall C/N (16 MHz bandwidth) 15.9 dB 10.0 dB Reference threshold C/N 10.0 dB 10.0 dB Margin Over Threshold 5.9 dB 0.0 dB 26 satellite in addition to uplink and downlink transmissions; however, atmospheric losses or fading loss between satellites are not required in the analysis. Table 3 shows the link calculations for 40,000 km (21,598 nautical miles) transmission path for a data rate of 100 6 kbps. The bit error rate is 10- for a BPSK modulated signal with 6 rate 1/2 convolutional coding. At P = 10- , the required e Eb/N is found to be 10 dB. In this link analysis, the purpose is 0 to find the antenna gain required for satellite-to-satellite link but no margin value is included. The required carrier-to-noise density ratio at the receiving satellite is found to be 60 dB based upon the Eb/N and R values expressed in equation (10). In order to find 0 the antenna gain, the terms used in the link equation for Table 3 are C/N0 (dB) = Pt(dB) + Gt(dB) - Ltpf(dB) - Ls(dB) (17) - Lrpf(dB) - k(dB) -T8 (dB) where Ltpf and Lrpf represent the transmitting and receiving pdlarization and filter losses. The value T is the noise system s temperature, for the noise figure value of 7 dB [see equation (6)]. The result is a 84 dB antenna gain needed to reach a satellite 40,000 km away. 20 Ship-Satellite-Laboratory Link Analysis An experimental ship-to-shore satellite communication link was initiated by Communication Satellite Corporation (COMSAT) and Cunard Line, Ltd •• The purpose of the analysis was to determine the link margin of a good-quality voice link between the earth terminal and a commercial ship without modifying the existing equipment. In this 27 TABLE 3 SATELLITE-to-SATELLITE LINK Required Eb/N0 10.0 dB (BER = 10-6) Desired data rate 100 kps (50 dB-Hz) Required C/N0 60.0 dB-Hz Available transmitter Power(RF) 28.0 dBm Transmitter Filter and Polarization Losses -2.0 dB Path Loss (40,000 km,37GHz) -216.0 dB Receiver Filter and Polarization Losses ~1.0 dB k(dB) + Ts(dB)(noise figure= 7dB) -167.0 dBm-Hz Resulting C/N0 -24.0 dB-Hz Required Antenna Gain [60 dB -(-24dB)] 84.0 dB 28 case, the margin is determined by deriving the difference between the required EIRP needed for the the link and the available EIRP from the existing terminals. Two beams from the satellite were used for the link: (1) a global beam for the laboratory-to-ship link, which allowed constant coverage of the traveling ship, and (2) a spot beam for the ship-to-laboratory link. Tables 4 and 5 show the numbers used to derive the EIRP for both terminals. Transmitter power is given at the point where the signal enters the antenna; transmitter gain is given as its peak value along the axis of the beam. Pointing error allowance of 0.75° applies to the ship board terminal because of the constant pitching motion of a ship at sea. In order to incorporate the most accurate value for the margin, nominal values (after the losses are considered) of EIRP and G/T are used for the link budget calculations in Table 6. The type of signal used is a delta modulated PSK signal, with a data rate of 28 kbps and a bandwidth of 80 kHz. Referring to Table 6, the carrier-to-noise power ratio (C/N not C/N ) value (required) 0 within the bandwidth has a predetermined threshold set at 4.8 dB based 4 upon a bit-error probability P = 10- (see Footnote 20). e Satellite parameters are given for each type of beam, including the satellite saturation flux density called Q* for the TWT. The u required satellite EIRP required for the labs-to-ship link is 11.8 dB. Equation (5), converted to decibels, becomes 2 Q 2 EIRP + B0 - 10 log 4TIR {dB). u 1 {L effects already have been accounted for in Tables 4 and 5). For u 29 TABLE 4 TERMINAL PERFORMANCE CHARACTERISTICS A. Shipboard Terminal Transmit Frequency 6,040.0 MHz Transmit Power at Feed,8W 9.0 dBW Transmit Gain (peak of beam) 40.0 dB Peak EIRP 49.0 dBW Pointing Error Allowance -3.0 dB (+ 0.75°) Nominal EIRP 46.0 --dBW Receive Frequency 4,189.6 MHz Receive Gain 37.0 dB System Noise Temperature (300°K) -24.8 dB/°K Peak G/T 12.2 dB/°K Pointing Error Allowance -2.0 dB(+ 0.75°) Nominal G/T 10.2 dB/K- TABLE 5 TERMINAL PERFORMANCE CHARACTERISTICS B. COMSAT Labs Terminal Transmit Frequency 6,414.5 MHz Transmit Power at Feed,250W 24.0 dBW Transmit Gain 46.0 dB EIRP 70.0 dBW Receive Frequency 3,815.0 MHz Receive Gain 43.0 dB System Noise Temperature (141°K) -21.5 dB/°K Nominal G/T 21.5 dB/°K 30 TABLE 6 SHIP-TO-SATELLITE-TO-EARTH LINK BUDGET CALCULATION Channel Transmission Characteristics Modulation and Coding 28-kbps Delta modulated voice, rate 1/2 convolutional coding, 2-phase CPSK C/N at Pe=lo-4 4.8 dB C/N0 required (including BW) 54.6 dBW-Hz Satellite Parameters G/T -16.0 EIRP Global Beam 23.2 dBW Spot Beam 35.0 dBW 5"2u* -77.2 dBW/m2 Labs-to-Ship Parameters C/N0 required 54.6 dBW-Hz Path Loss 196.0 dB Ship Terminal G/T (nominal) 10.2 dB Boltzmann's constant -228.6 dB/°K/Hz Satellite EIRP required 11.8 dBW Lab Station EIRP available 70.0 dBW Lab Station EIRP required (see text) -69.0 dBW Margin 1.0 dB Ship-to-Labs Parameters C/N0 required 54.6 dBW-Hz Path Loss 196.0 dB Labs Station G/T 21.5 dBW/°K Boltzmann's constant -228.6 dBW/°K/Hz Satellite EIRP required 0.5 dBW Ship Station EIRP available (nominal) 46.0 dBW Ship Station EIRP required -45.4 dBW Margin 0.6 dB 31 2 R = 40,000 km, BOl = 6 dB, Q*u = -77.2 dBW/m , EIRPlab is 70.4 dB. However, for a global beam transmission from satellite- to-ship, the satellite EIRP is 11.4 dB larger than required (23.2 - 11.8 = 11.4). Therefore, the actual required EIRPlab is 70.4 - 11.4 = 69 dB. Similarly, for the spot beam transmission from satellite to laboratory, the required satellite EIRP of 0.5 dBW is 34.5 larger than required (35.0 - 0.5 34.5). From equation (5) EIRP h" 79.9 dB = s lp = with the corresponding actual required EIRP h" becoming 79.9 - 34.5 s lp = 45.4 dB. The difference between the required EIRP and the available EIRP represents the margin for the link. Both margins are small, therefore, the communication link will be very weak under rain or fading conditions. 21 Jupiter Probe Example Link analysis was carried out for data transmission from a probe entering the atmosphere of Jupiter, from a spacecraft in a trajectory past the planet. In this case environmental factors such as atmospheric attenuation, ionospheric scintillation, and noise temperature are entirely different from those found on Earth. Attenuation of a radio signal in Jupiter's atmosphere, for a bandwidth from 400 to 2400 MHz, is the result of absorption by ammonia and water vapor. Defocusing of the signal causes an incremental amount of additional loss; however, the majority of the loss is the absorption loss from ammonia. Data from Pioneer 11 indicated that the atmosphere corresponds to a model represented by Figure 10. The results of an atmospheric attenuation analysis were estimated by a descent profile yielding from 0 to 1.6 dB loss for the frequency range 32 4 a:l 3 "'0 Vl- Vl 0 ...... J u ~ w COOL! DENSE\ / ::r: 2 0.... / Vl / 0 :2: / I- / <( ...... J <( I- 0 I- l \_NOMINAL 0 0.2 0.6 1.0 1.4 1.8 2.2 2.6 FREQUENCY, GHz Figure 10. Jupiter Atmospheric Loss for a 10 Bar Pressure Leve122 33 of 400 to 2400 MHz (nomial line in Figure 10). Signal fading losses, as the result of transmission through the ionsphere of Jupiter, had been based upon processing the radio signal from Pioneer 10 data, corresponding to a few minutes before and after occultation by Jupiter. Noise temperature calculation was the sum of four major sources-- synchrotron radiation noise, planet thermal radiation noise, cosmic noise, and receiver noise. Synchrotron radiation noise is the result of radiation made by electrons spiraling around Jupiter's magnetic field. The effective noise temperature due to synchrotron radiation and planet thermal radiation depends on the spacecraft antenna pattern and its orientation with respect to the planet. The design approach to the antenna pattern was to make it symmetrical about the spin axis of the spacecraft but to limit its beamwidth in any plane containing the spin axis. Thus, the receiver noise temperature was found for the antenna described above to be 70°K or less. Therefore, the con- tribution of cosmic noise is relatively small. The final noise temperature is found to be from 2298 °K to 2580 °K. The measurement of link performance for this study is based upon the margin which incorporates excess capability based on available equipment. The parameters for the probe are listed in Table 7. Processing gain is based upon the comparison of the theoretical signal and the measured signal. The resulting required C/N before leaving 0 the probe is 29.1 dB-Hz. Other factors affecting link capacity are time varying and depend upon the geometrical relationship of the spacecraft and the probe with respect to each other and to Jupiter, its ionosphere, and its atmosphere. The details are too numerous to be included in this paper; however, one example of the link margin as 34 a function of time for four values of the periapsis distance is illustrated in Figure 11. Design parameters which can be varied are the probe antenna pattern (beamwidth), the spacecraft antenna beamwidth, and the angle (called squint angle) between the spacecraft antenna beamcenter and the spin axis. The periapsis distance refers to the curved trajectory path caused by the probe leaving the orbiter. Time from the probe entry to the spacecraft periapsis distance was chosen so that the margin at the beginning of the atmosphere descent was greater than 1.0 dB. Furthermore, it was chosen so that the longest time would be achieved from entry until the margin reached 1.0 dB. The granularity in the probe lead time used in the computation did not allow the initial entry margin to be the same for each periapsis distance, but the overall effect differs only slightly for changes of one or two minutes. 23 M1.l. 1tary Commun1cat1ons . • and Ant1]amm1ng . . . Cons1 · d erat1ons . Military communication satellite link analysis follows the same calculations described above; however, jamming and interception of a signal need to be considered--especially for secure communications required by the military. Code Division Muliple Access (CDMA) is a spread spectrum technique where the RF bandwidth and/or the total time epoch of the signal is "spread" beyond that required for a RF signal in order to provide special functions or to cope with unintended parties. Special functions are ranging, multiple-access, selective addressing and minimum power-density signaling. Many digital spread spectrum systems process a digital source of data rate R(bps), entering an encoder through a convolutional coder. The output of the 35 PERIAPSIS DISTANCE R. = 1.4 J 18.6 = PROBE LEAD TIME, mm co "'0 8 z 7 C) a:::: <( 6 ~ 5 z~ ___I 4 3 2 l 0 -l -2 -20 - 10 0 10 20 30 40 TIME FROM ENTRY, min Figure 11. Link Margin as a Function of Periapsis Distance24 36 TABLE 7 JUPITER PROBE COMMUNICATION SYSTEM PARAMETERS Probe Transmitter Power 60.0 w Line and Polarization Loss 0.8 dB Transmission Frequency 600.0 MHz Data Rate (50bps) 17.0 dB Processing Gain 1.5 dB Eb/N0 for Pe=lo-5 10.6 dB Required Carrier-to-Noise Density 29.1 dB-Hz 37 modem is spread over the entire spread bandwidth. One common spread spectrum system is frequency hopping where the single-hop bandwidth moves pseudorandomly over the total bandwidth. The antijamming (AJ) enters the link calculation as J/S, defined as the jammer power to signal power ratio corresponding to a specified bit error pro b a b 1"1" 1ty. 25 The expression of J/S is J/S = W/Rb = Processing gain (PG) (18) Eb/Jo reqd required Eb/J0 where the required ~/J0 is defined as the energy-per-bit to the jamming noise density required to maintain the link with a specified bit-error probability (P ). "W" represents the transmission e bandwidth. The AJ margin is: (19) = W/Rb (dB) (dB) , (20) EIRP J/EIRPT = PG (dB) + EIRPT (dBW) - EIRPJ (dBW) - PG (dB) + J/S (dB) • (21) Link analysis of a hypothetical military communication satellite (Table 8) is shown to illustrate calculating the uplink margin and the AJ margin based upon the required Eb/N • Table 8 is written in 0 tally form of the decibel values with negative signs representing the losses. The terminal EIRP represents the resulting transmitter power and antenna gain at the ground station. Path loss covers the distance to geosychronous orbit (21,915 nmi) and the frequency is in gigahertz. In addition, the "other losses" value is the summation of pointing 38 TABLE 8 HYPOTHETICAL MILITARY COMMUNICATION UPLINK CALCULATION Frequency 8000.00 MHz Data Rate 2.00 Mbps Range 21,914.90 nmi Required Eb/N0 for Pe=lo-4 PSK 10.00 dB Transmitter Power, lOOW 20.00 dBW Transmitter Circuit Loss -2.00 dB Transmitter Antenna Gain (peak) 51.57 dB dish diameter, 20 ft half-power beamwidth, 0.45° Terminal EIRP 69.57 =dB Path loss -202.67 dB Fade Allowance -4.00 dB Other Losses -6.00 dB Summation Of Losses -212.67 dB =- Receiver Antenna Gain,peak 35.09 dB dish diameter, 3 ft half-power beamwidth, 2.99° Edge-of-coverage Loss -2.00 dB Nominal Antenna Gain 33.09 dB Receiver Antenna Temperature 300.00 Receiver Temperature 3806.36 Noise Figure 11.5 dB System Temperature 4106.36 System Temperature (dB) 36.13 System G/T -3.04 Boltzmann's Constant 228.60 Carrier-to-Noise density ratio 82.46 dB-Hz 39 Data Rate,20 Mbps 63.01 dB-Hz Theoretically Available Eb/No 19.45 dB (C/N0 - R) Implementation Loss -1.50 dB Required Eb/N0 -10.00 dB Margin 7.95 dB ====-=-- '""""' ANTIJAMMER CALCULATIONS 1. Transmission Bandwidth, SO MH.z 77.00 dB-Hz 2. Data Rate, 2 Mbps -63.00 dB-Hz 3. Processing Gain 14.00 dB 4. Required EB/N0 (at detector) 10.00 dB s. EB/N0 at receiver (4. - 3.) - 4.00 dB 6. Assumed EB/N0 at receiver - 2.00 dB AJ Margin (6. - 5.) 2.00 dB 40 loss and polarization loss. The receiver antenna gain is the value for the edge-of-coverage area of the beam pattern so that the link performance represents the worst-case calculation for the margin. Summation of the doubled underscored values yields the resulting carrier-to-noise density ratio for the uplink calculation. In order to find the theoretically available Eb/N , equation (14) is used 0 to yield 19.45 dB. Accounting for implementation loss produces a practical value for the available Eb/N • In conclusion, the 0 difference between the required and available Eb/N is the margin 0 for the uplink. Processing gain for AJ calculations is the result of the transmission bandwidth and data rate described in equation (18). When considering AJ calculations, it is interesting to note that a jammer is not always along the transmission path of the communication link; thereby, the signal from the jammer may enter one of the sidelobes of the satellite antenna beam (see Figure 3). The AJ margin 1s calculated from equation (21). Space Shuttle Design Model Up to this point, the communication system model assumed that a limited number of losses were important in the communication link. However, there are additional losses (see Figure 7) that are important to the design of a communication link not accounted for in the link budget: AM/PM conversion, intermodulation products, and phase noise error. As an example, considerations of many sources must be taken into account concerning the loss of a signal for the Space Shuttle Orbiter and implementing a design to overcome the loss. Thermal 41 noise, drifts in transmitting and receiving oscillations, AM/PM conversions, and vibration effects produce phase noise components which degrade communication performance. In the uplink from ground to the Space Shuttle, the phase noise spectrum can be integrated from 9 Hz to 1 MHz and then doubled for the two-sided bandwidth to determine the phase variation. The rms phase error due to phase noise source was found to be 8.7°. From Figure 12, the probability of error as a function of Eb/N for different 0 phases shows that the degradation due to phase noise is less than 0.2 -4 dB for a P 10 • For the Shuttle-to-a-tracking-satellite e = (TDRS) link, the phase noise error is estimated not to exceed 10° so 4 that the Eb/N degradation at P 10- will be less than 0.3 o e = dB for the up and downlink. Hence, the phase noise error was found to have a 0.2 to 0.3 dB impact on the Eb/N • This analysis aided the 0 designers to minimize the effect of the bit error rate performance by adjusting the tracking loop bandwidth. Complete details of this analysis may be found in Appendix B. V. Surrnnary A general model for a communication system was presented including the losses or signal degradations that affect the performance of a communication link. The signal generated by a user enters a link to the earth station. At the earth station, a signal is processed and transmitted at an RF frequency to satellite where it is processed and retransmitted to the earth station. The earth station then processes 42 Figure 12. Bit-Error Probability vs. Ener~y per-Bit to Noise Density Ratio 6 43 the signal to the user. The performance of a communication link can be measured in several ways based upon the carrier-to-noise density ratio. Link margins are used to understand what safety factors exist if unexpected fading occurs, indicating to a designer whether increased power or a low noise receiver is needed. In addition, the available data rate for a digital communication link can be determined based upon an acceptable bit-error probability chosen for the system. The overall C/N , 0 derived from the uplink and downlink values, can also determine the overall data rate. Examples were presented for a variety of applications of link analysis. Two examples of link analysis for a direct broadcasting satellite used different approaches for determining link performance. The HDTV system was mostly concerned with the calculated C/N • 0 However, the DBS link budget compared the overall C/N value with a 0 predetermined threshold C/N in order to find the link margin. In 0 both cases, a 5 dB rain attenuation lowered the downlink carrier- to-noise ratio. A crosslink between two orbiting satellites deleted the necessity of using atmospheric or fading losses in the link calculation. Furthermore, the link calculation was set up in order to determine the antenna gain needed to reach another satellite. One of the more interesting examples examined the possibility of a ship-to-shore link using existing equipment. The link margin calculations were based upon the required EIRP value needed for the link and the available EIRP at the terminal. It was shown that the margin values were small so that the terminals would not handle any rain attenuation or fading. Next, a communication link between a 44 Jupiter probe and a small orbiter was studied for the unusual atmospheric attenuations. It was found that most of the attenuations were the result of absorption by ammonia and the high radiation levels that surround Jupiter. Finally, a hypothetical military satellite link was examined for antijamming performance. The communication satellites in orbit today are complex devices serving a multitude of users. Appendix C lists a brief description of international, domestic, and military communication satellites in use today. APPENDIX A SIGNAL LOSSES The follow remarks correspond to the numbers on the diagram of Figure 7. 1. Bandlimiting Loss The bandlimiting loss refers to the loss of signal energy due to the use of filters. 2. Local Oscillator Phase Noise Phase noise is the result of changes in supply voltages, oscillator temperatures, magnetic fields, vibration, output load impedance, and power line fluctuations. The phase-noise spectral density can be measured by using a phased-locked oscillator. Essentially, the oscillator to be tested is connected to a phase detector where a reference oscillator is used to track low frequency phase-noise fluctuations. The phase-locked loop effectively removes all frequencies less than some very low frequency f • The remaining phase noise term 1 enters a low pass filter of bandwidth fh where fh is above the frequency range of the modulation. A simplified power spectral density of the phase noise is shown in equation (22) with a white noise 27 component. It is assumed that the phase jitter is the sum of "flicker" noise components (one-sided). 45 46 ~a_ + No (22a) £1£2 G No n< 1 £1 AM/PM is a phase noise phenomenon associated with nonlinear devices, especially among TWTs used as amplifiers. Associated with AM/PM conversions are multicarrier intermodulation products which come from the nonlinear effects created by cross products of two or more input signals. A typical phase characteristic of a TWT is shown in Figure 13 where A(t) is the input envelope. A model of a TWT with phase variation is shown in Figure 14. For small input power 28 levels, 8 (A) : K/2 A2(t) (23) for a sufficiently small value of A. A single sinusoidal input with a small amount of AM is x(t) = A(l+m cos Wrot) coSW 0 t (24) where A(l + m cosw t) is the input envelope. 8(t) is given by m 8(t) : KA2(1 + 2 m COSWmt) m << 1 {25) 2 with peak deviation from the mean phase - KA 2m. ep To compute the intermodulation products resulting from the AM/PM nonlinearity, the input is a summation of sinusoids represented as 47 30° CD 1- u.. :c (/') 20° LLJ (/') <( :c a.. 10° 00------~----~----~----~----~----~ 0 0. 1 0. 2 0. 3 0. 4 0. 5 0. 6 2 RELATIVE INPUT POWER LEVEL P. I P "'A 1n o Figure 13. Typical AM/PM Conversion Phase Shift29 AMPLITUDE NONLINEARITY X( t) cp ) - PHASE - MODULATE T1 F(z) I\ ·I ~~~~~ Iw( t1 • F( A) cos ( w t + ~ + 8 ) 0 .....__ ENVELOPE ~ 8(A) z = A( t ) cos ( w t + ¢ + e) DETECTOR 0 -- - ~ Figure 14. Model of AM/PM Nonlinearities in a TWT Amplifier30 +> (XI 49 (26) : A(t)cos[W0 t+~(t)] The squared envelope is expressed as A2(t) = 2 =IAi +I1 AiAjcos(~i - ~j) and the phase function ~ is (28) The AM/PM output is the summation of sinusoids with each phase modulated by 0(A) z(t) =IAicos[w0 t+~i(t) + 0[A(t)]] (29) =1Aicos[w0 t + ~i(t)}cos0(A) - 1 Aisin[w0 t + ~i(t)}sin0(A) For small values of AM/PM distortion, the phase modulation is 0 << 1 and the output from (27),(28), and (29) becomes L Aisin[w0 t + ~il (30) = A(t) cos[w0 t + ~(t)J + d(t). Since 0(A)- KA2(t), d(t) = - KA2(t) {A(t)sin[w0 t + ~(t)J} (31) = -KA3(t)sin[w0 t + ~(t)J Spilker shows that, z(t) ; A(t) + KA3cos3[w 0 t + ~(t)] (32) ~ A(t) + K(3/4)A3cos[w0 t + ~(t)J + term at 3[w0 t + ~1 ' . 50 In other words, the distorted intermodulation products occur at the same frequencies as third-order intermodulation products for an instantanous cube-law amplitude nonlinearity. 4. Limiter Loss and Enhancement Limiter loss or enhancement refers to the effect of a hard limiter on the signal-plus-noise. A hard limiter is a power amplifier that modifies the input signal by amplifing the input signal to a predetermined level, clipping the signal when required. If two noise-free sinusoidals of equal strength enter a hardlimiter, the bandpass limiter output is a square wave that is modulated at a different frequency. 6. Antenna Efficiency Loss Antenna efficiency loss relates to the signal losses due to reradiation and scattering. 7. Modulation Loss Modulation loss refers to that part of transmitter power utilized as a carrier power rather than information-bearing power. 8. Radome Loss Radome loss is an attenuation and dissipation effect on the desired signal energy through the protective radome. 9. Pointing Loss Pointing loss is the loss of signal due to the antenna pointing off boresight. 10. Polarization Loss Polarization loss is a loss of signal due to mismatch in polarization between the transmitting and receiving antennas. 51 11. Space Loss Free-space transmission loss, commonly referred to as space loss, covers the loss across the transmission path as a function of frequency and range. As explained earlier, the equation for space loss in decibels is L 20 log[4nR/A] (33) s = = 92.796 + 20log f + 20 log R where f = frequency in gigahertz R = range in nautical miles. 12. Atmospheric Loss One of the principle causes of signal loss through the atmosphere is rain attenuation. Variations along the transmission path in the form of water vapor, heavy rain clouds, or fog cause the signal strength to vary. The estimation of rain attenuation is based on the statistical calculations of attenuation values from meteorological data at differing frequencies, locations, and path geometries. The statistical variations of rainfall intensity and attenuation along a path depend on the number, type, and intensity of rainstorms that occur on the designated signal path each year. No theoretical basis exists for the calculation of the desired rainfall statistics; therefore, they must be obtained empirically. Method of computing rain attenuation is described in "Prediction of Attenuation by Rain" by Robert K. Crane (see Bibiliography). Figure 8 shows an example of the relative magnitude of absorption due to water vapor, a liquid water cloud, and rain that is expected to be exceeded during less than 2% of the year on the zenith path. 52 13. Signals From Other Systems and Crosstalk When a filter precedes the AM/PM distorting element, it is possible to generate intelligible crosstalk where the frequency modulation on one channel is added to another. Signals from other sources occur from another satellite or a ground station non-intentionally transmitting near the same frequency band. 14. Simultaneous Transmitter Multicarrier Intermodulation Noise If a receiver antenna simultaneously receives and transmits multicarrier signals, unwanted intermodulation effects may contribute 31 to noise at this point. 15. Galactic, Star, and Terrestrial Noise There is a system noise temperature, T , expressed by s Ts = !a + (L - 1) T0 + Tr degree K (34) L L where T (NF - 1) T • r = o NF represents the noise figure, T is the antenna temperature due to a the source outside the antenna ( i.e. rain, atmosphere, galactic), and L is the loss in the antenna, feed, and waveguide components (0 to 3 dB). T is the ambient temperature; T represents the noise temperature o r of the receiver. 16. Feeder Loss Feeder loss is produced by the line between the antenna and the receiver, causing a dissipation effect. 17. Receiver Noise Receiver noise is an ideal receiver output noise as N k T B (35) o = r where, 53 k =Boltzmann's constant; T =receiver temperature (° K); r B = bandwidth. The receiver temperature can be expressed in terms of the receiver's noise figure as T = (NF - 1) 290 o K (36) r 18. Implementation Loss Implementation loss accounts for the fact that practical implementations do not match those of ideal and theoretical models. APPENDIX B SPACE SHUTTLE DESIGN MODEL In the Orbiter, a CW carrier tracking loop is employed by the S-hand (1.5 to 5.2 GHz) receiver. An example below will show the design philosophy used to characterize performance due to the phase noise components. Figure 15 shows the CW phase-locked loop and Figure 16 illustrates the diagram of a Costas loop. The phase error math model for either loop is shown by Figure 17. In Figure 17, 8(t) represents the input phase noise process, which serves to model channel Doppler shift and channel turbulence (AM/PM conversion). ~ 1 (t) represents the phase noise fluctuations before reception (at the transmitter). These two effects are combined to form an equivalent phase noise process. The phase detector subtracts the local VCO phase estimator-0(t) from the incoming phase noise 8(t) + ~ 1 (t) to produce the loop phase error ~(t). The equivalent phase detector characteristic is represented by C g(.) where C is the e e input signal amplitude. The output of the phase detector C g(~) is e affected by white Gaussian noise n (t), with a single-sided spectral g density N W/Hz, and independent of phase error ~. Furthermore, 0 there is a loop filter with transfer function F(p), where p denotes the Heaviside operator (p=d/dt). The output of the filter is the input to the VCO represented by K/p where K is the open loop gain. The combined effects of all the receiver phase noise contributors is modeled by ~ 2 (t). Equations of operation for the loops shown in Figures 15 and 54 E ( t) s ( t ,¢) ~® lll-1 z(t) F(p) 1----.--~ F o ( p)- -~:~ 1 + e(t) w--+ r(t,¢) f, vco Figure 15. CW Phase-Locked Loop 32 \J1 \J1 (t) zc Q-Channel r c ( t' "j) e(t) s(t) ... r ( t, (j) 8 c (t) e z (t) 1-Channel s TO DATA DETECTOR 32 Figure 16. Costas Loop Ul 0'\ GAUSS I AN ~WISE TRX.;S ISTOR OSCILLATOR I~STABILITY n PHASE r-- DETECTOR ___ I g LOOP FILTER 1~ 1 I . _(J_~.i(+ ¢ +·g(.) I ; ,!. 4+' + -1 F{p) I I I I ______j L e ,------, '------~: C!J!. EJ- : . I I+ I ~~ - ~OCAL- O~c7LL~\T~;' LOCAL REFEP-ENCE INSTABILITY figure 17. Equivalent Baseband Phase Error Model of Shuttle Phase-Locked Receiver32 VI '-I 58 16 are stochastic differential equations written in operator form with time suppressed, 0 = $2 + KF(p)[Aeg(~) + ng] (37) 33 (38) In equation (1), 8 represents the frequency of the local oscillator, ~$ $ - $ represents the transmitter/receiver phase = 1 2 noise instability, and ~ represents the phase error rate. For use with digital communications, a bit error probability error model is needed. A probability density function for an unstressed second-order loop is approximated by: 2 p(~) = exp [ a¢cos2l rer < 1r (39) 2 27TI 0 (a~) 2 where a~ is defined by the mean squared value of the phase error. 2 2 2 = ae + a~$ + a. (40) where 2 00 2 ae =l/27T fll-~(iw)l sa(w) dw (41) -oo 2 00 2 a~$ =l/27T fll-~(iw)l s~w(w) dw (42) -00 2 a = (43) 59 where BL is the single-sided loop bandwidth, I(x) is the modifed 0 Bessel function of the first kind of order zero. When the system data rate exceeds the loop bandwidth, it is assumed that the phase error does not change over one symbol duration. Therefore, conditional bit error probability with detection by a matched filter is (44) where 00 2 erfc x = 2/ITT J e-t dt X and . 35 Eb = energy per d ata b1t. The unconditional bit error probability is given by: lT Pe =J p(~) Pe(~) d~ (45) -lT lT 2 Pe = ___1---:::- J exp[cr~cos~] Pe(~)d~ (46) 2 -lT 2lTI 0 (cr~) Figure 12 graphically illustrates the probability of error for binary phase-shift-keying. These results are applied to the Shuttle operation to evaluate the effects of phase noise on the Payload/Shuttle and Tracking, Data, and Relay Satellite (TORS) Shuttle link performance. A double-sided loop bandwidth of a Deep Space Network (DSN) transponder used aboard the Shuttle is 18 Hz. In the forward link, the phase noise spectrum can be integrated from 9 Hz to 1 MHz and then doubled for the two-sided bandwidth to determine the phase variation. 63 - I /1 --~ I /. l / I ~·--1 I \ I I PRIMARY I I POWER t wattsl I I I I I ,J A I IV I I 100 I 1111 I I .v I I I I t't -+-_cAPACITY! I I ltel ckts x 10) i I I ,_,/1 I l eirp I~BWI I ,.....,/ I I I I I I I I 1966 1968 1970 1972 1974 1976 1978 YEAR 37 Figure 19. Evolution of INTELSAT Space System 64 establish a domestic satellite communication system. Telephone, television, data, and facsimile are provided throughout Canada with a performance equal to or excelling that of terrestrial systems. The earth segment covers 50 stations: two for heavy routed traffic, six for network television, eighteen for thin route traffic, two for northern telecommunications, twenty-six for remote television service, 38 and one trac k1ng,. te 1 emetry, and comman d stat1on. . Operation frequency bands are comprised of horizontally polarized 3.202/4.178 GHz downlink and 5.927/6.403 GHz uplink (vertically polarized). The radiated pattern is elliptically shaped to cover the entire Canadian nation. EIRP per transponder is 33 dBW obtained by a 5 watts TWT in each transponder; the transmitting antenna gain is 27 dB. A wideband receiver using tunnel diode amplifiers at 6 GHz is common to the twelve receivers with a G/T ratio of -7dB/°K. Each transponder is capable of providing 960 voice circuits, or one color TV and two 5 kHz audio circuits. The transmission bit rate is 61.248 Mbps. Two ground stations consist of 29.87 m diameter parabolic antennas with a gain of 63 dB in transmission and 59 dB in reception. The antenna and parametic front yield a G/T ratio of 37 dB/°K. In addition, the EIRP per carrier is 84 dBW. Six network television stations located near major cities utilize a 10.5 m diameter parabolic antenna with gains of 52.5 dB (transmission) and 50.5 dB (reception). For the parabolic antennas, G/T is 28 dB/°K and the EIRP per carrier is 83 dBW. U.S. Domestic Satellite System The WESTAR satellites are used for teletype, TWX, hotline 65 point-to-point, Central Telephone Bureau, telegrams and mailgrams. In addition, point-to-point voice, data, fascimile, and point-to-point or point-to-multipoint video is available. The RCA domestic satellite system includes TV distribution in Alaska, toll messages and bush telephone in Alaska, private-line video, voice and data to government agencies, commercial TV and radio, and CATV program distribution to receive-only stations. COMSAT General Corporation has three COMSTAR satellites leased to AT&T which are integrated into the national telephone network. Mobile Systems MARISAT systems used in maritime communications have made reliable, high quality voice, data, facsimile, and teletype services available to and from ships, interconnecting with domestic and international networks. The MARISAT system is also unique because it constitutes a multipurpose operational communication platform for UHF and SHF. Three transponders are used in the following manner. The first operates at 300/250 MHz in three separate channels on one 480 KHz wide and two 24kHz wide bandwidths. The second transponder is a 1.64/4.19 GHz repeater for ship-to-shore commercial traffic; the third transponder is for a 6.42/4.53 GHz wideband repeater for shore-to-ship commercial traffic. Satellite Business System (SBS) Digital Communication SBS is designed to service a variety of communication needs for a large community. The satellite acts as a central point for high 66 density traffic switching that minimizes the complexity of the ground station. Furthermore, the RF channel performance is geared for a bit 4 error probability of 10- and covers 99.5% time availablity. The capacity of the satellite, expressed in the number of voice circuits, is 12,000 voice capacity at 43 Mbps per tr~nsponder. The RF terminals include a parabolic antenna and feed assembly, a high-powered amplifier, low-noise amplifier, and up/down converters. DSCS III Satellite Current projections of growth for the 1980's time frame indicate a rapid increase of military users of X-band (frequencies from 5 GHz to 12 GHz) communications. Present usage of X-band communications are long haul, point-to-point communication. With an increase of users, the trend requires more efficient and faster means of communications including shipboard terminals as well as small tactical terminals. The DSCS III satellite uses Super High Frequency (SHF) communications consisting of multiple beam antennas and a six-channel transponder. The antennas on DSCS III include a 61-beam receive antenna with selective coverage and anti-jam capability and two 19-beam transmitting antennas with selective coverage. DSCS III satellite uses an electronic beam control to allow for arbitrary shaped (beamshaped) coverage and jammer discrimination. The channelization provided by the six-channel transponder allows multiple users on the ground and has an EIRP varying from 72 to 97 dBw. 39 When the transponder is connected to the earth coverage horn, the overall subsystem gain is a minimum of 145 dB and a maximum of 169 dB. FOOTNOTES lJ.J. Spilker, Jr., Digital Communication by Satellite, (Englewood Cliff:Prentice Hall,Inc.,l977),pp.l79. 2Harry L. Van Trees,"The Communication System",Satellite Communications,(New York:IEEE Press, 1979)p.73. 3 Spilker, p.l79. 4Merrill I. Skolnik, Introduction to Radar Systems, (New York: McGraw-Hill Book Co.,l980)pp.226-7. Sskolnik,p.226. 6J.S. Hollis, T.J. Lyon, L.C. Clayton, Microwave Antenna Measurements,(Atlanta: Scientific Atlanta,l970)p.6-6. 7van Trees,p.71. 8van Trees,p.72. 9van Trees,p.72. lOvan Trees,p.73. llspilker,p.l76. 12Bernard Sklar,"What the System Link Budget Tells the System Engineer or How I Learned to Count in Decibels.",(Class notes)p.2. 13van Trees,p.74. 14Robert K. Crane "Prediction of Attenuation by Rain",IEEE Transactions on Communications,COM-25,(1980),1718. lSK. Shamugham, Principles of Digital and Analog Communications, (New York:John Wiley, 1978)p.416. 16P.L.Bargellini, "A Synopsis of Commercial Satellite Communication System", Harry Van Trees, ED.,Satellite Communications, (New York:IEEE),p.33. 67 68 17J.Flasherty, J. Ramasastry, R.H.O'Conner,"A High Definition Television Service in the 12 GHz Band", CBS Television Network,p. 3. 18Paul Harris,"Comsat Anticipates a Dish on Every Roof; Hatches u.s. Broadcast-Satellite Plans",Microwaves,20,(1981),19. 19David M. Snider and David B. Coomber,"Satellite-to-Satellite Data Transfer and Control",AIAA 7th Communication Satellite Systems Conference Proceedings,Apr. 24-27,1978,p.458. 20J.Kaiser,"An Experimental Ship-Shore Satellite Communications Demonstration",COMSAT Technical Review,4,(1974),124. 21Preentr Communication Desi n Elements for Outer Planets Atmopheric Entry Probe, Hughes Aircraft, NASA Contract #NAS-2-9008, 1976)p.2-17. 22Hughes,p.2-4. 23Hughes.p.2-17. 24sklar,p.25. 25sklar,p.39. 26william C. Lindsey,"Phase Noise Effects on Space Shuttle Communication Link Performance",IEEE Transactions on Communications, COM-26,(1978),1534. 27sklar,p.2. 28spilker, p. 254. 29spilker,p.255. 30spilker, p. 261. 3lsklar,p.3. 32Lindsey,p.l533. 33Lindsey,p.l536. 34Lindsey,p.l534. 35Lindsey,p.l536. 36Lindsey,p.l540. 37Barnla,p.2-19. 69 38Bargellini,p.35. 39I.S.Haas and A.T.Finney,"The DSCS III Satellite A Defense Communication System for the BO's",AIAA 7th Communciation Satellite Systems Conference Proceedings,April 24-27,1978,p.351. BIBILIOGRAPHY Bargellini, P.L., "A Synopsis of Commercial Satellite Communication Systems", Harry Van Trees,ed. Satellite Communication,New York:IEEE Press,pp.29-43. Barnla, J.D.,and F.R. Zitzmann,"Digital Communications Satellite Systems of SBS",tEEE Aerospace and Electronic System Convention, 1977,pp.7-2A-7-21. Crane,Robert K.,"Prediction of Attenuation by Rain",IEEE Transactions on Communications,COM-28(1980),pp.l717-1730. Flasherty, J.,J.Ramasastry, R.A.O'Conner, A High Definition Television Service in the 12 GHZ. Band, CBS Network,p.l-9. Harris,Paul,"Comsat Anticipates a Dish On Every Roof; Hatches U.S. Broadcasting Satellite Plans",Microwaves,20,(198l),pp.l5-19. Haas,I.S. and A.T. Finney,"The DSCS III Satellite-A Defense Communication System for the 80's"AIAA 7th Communication Satellite Systems Conference Proceedings,l978,pp.351-358. Hollis,J.S., T.J. Lyon, L. C. Clayton,Microwave Antenna Measurement, Atlanta: Scientific Atlanta,l970. Kaiser,J. "An Experimental Ship-Shore Satellite Communication Demonstration"COMSAT Technical Review,4,(1978),pp.l19-138. Lindsey,William C. "Phase Noise Effects on Space Shuttle Communication Link Performance",IEEE Transactions on Communications, COM-26, (1978)pp.l532-1541. Preentry Communication Design Elements for Outer Planets Atmosphere Entry Probe,Hughes Aircraft Co.,l974. Pritchard, Wilbur L. "Satellite Communication -An Overview of the Problems and Programs"Proceedings of IEEE,65,(197l),pp.294-307. 70 71 Ristenbatt, Marlin P., James L. Daws, "Performance Criteria for Spread Spectrum Communications",IEEE Transactions on Communications,COM- 25,(1977)pp.756-761. Shamugham, K. Principles of Digital and Analog Communications,New York:John Wiley,l978. Sklar, Bernard,'~at the System Link Budget Tells the System Engineer or How I Learned to Count in Decibels."Class notes. pp.l.l4. Skolnik, Merrill I., Introduction to Radar Systems,New York: McGraw-Hill Book Co.,l980. Snider, David M. and Avid B. Coomber,"Satellite Data Transfer and Control", AIAA 7th Communication Satellite Systems Conference Proceedings, 1978,pp.457-470. Spilker,Jr. J.J. Digital Communication by Satellite,Englewood Cliff:Prentice Hall,l977.