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Frequency Modulation

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Frequency Modulation Unguided Media and Matched Filter After this lecture, you will be able to – describe the physical and transmission characteristics of various unguided media Example ? B.1 Unguided media Guided to unguided – Transmission • the signal is guided to an antenna via a guided medium • antenna radiates electromagnetic energy into the medium – Reception • antenna picks up electromagnetic waves from the surrounding medium. – Example • a voice signal from a telephone network is guided via a twisted pair to a base station of mobile telephone network • the antennas of the base station radiates electromagnetic energy into the air • the antenna of the mobile phone handset picks up electromagnetic waves B.2 Directional and Omnidirectional Directional – the transmitting antennaHow puts to out focus a focused an electromagnetic beam electromagnetic wave ? – the transmitting and receiving antennas must be aligned – Example • Satellite communication systems • For a satellite located at 35784km above the ground, a 1° beam covers 1962km2 B.3 Directional and Omnidirectional Omnidirectional – the transmitted signal spreads out in all directions and can be received by many antennas. – In general, the higher the frequency of a signal, the more it is possible to focus it into a directional beam – Example • mobile communication systems • radio broadcasting B.4 Operating freqeuncies Microwave – Frequencies in the range of about 30 MHz to 40 GHz are referred to as microwave frequencies – 2 GHz to 40 GHz • wavelength in air is 0.75cm to 15cm ¾wavelength = velocity / frequency • highly directional beams are possible • suitable for point-to-point transmission – 30 MHz to 1 GHz • suitable for omnidirectional applications B.5 Operating freqeuncies B.6 Terrestrial Microwave Physical description – limited to line-of-sight transmission. This means that microwaves must be transmitted in a straight line and that no obstructions can exists, such as buildings or mountains, between microwave stations. – To avoid possible obstructions, microwave antennas often are positioned on the tops of buildings, towers, or mountains. B.7 Terrestrial Microwave B.8 Terrestrial Microwave – With no intervening obstacles, the maximum distance between antennas is d = 7.14 kh • d is the distance between antennas in kilometers, • h is the antenna height in meters • k is an adjustment factor to account for the fact that microwaves are bent or refracted with the curvature of the earth • k ~ 4/3 • Example ¾two antennas at a height of 100m may be as far as 82km apart B.9 Terrestrial Microwave Applications – Long-distance telecommunication service • requires fewer amplifiers or repeaters than coaxial cable • requires line-of-sight transmission • Example ¾telephone system ¾TV distribution – Short point-to-point links • Data link between local area network • closed-circuit TV • bypass application B.10 Terrestrial Microwave B.11 Terrestrial Microwave B.12 Terrestrial Microwave Transmission characteristics – The higher the frequency used, the higher the potential bandwidth and therefore the higher the potential data rate Band (GHz) Bandwidth (MHz) Data rate (Mbps) 2 7 12 6 30 90 11 40 90 18 220 274 B.13 Terrestrial Microwave Attenuation 2 4πd L =10log dB λ • d is the distance • λ is the wavelength – attenuation varies as the square of the distance • for twisted pair and coaxial cable, loss varies Why ? logarithmically with distance – repeaters or amplifiers may be placed farther apart for microwave systems - 10 to 100 km is typical B.14 Terrestrial Microwave – Attenuation increases with rainfall, especially above 10 GHz Interference – the assignment of frequency bands is strictly regulated – OFTA (Office of telecommunications authority) • www.ofta.gov.hk B.15 Satellite Microwave Physical description – a satellite is a microwave relay station • link two or more ground-based microwave transmitter/receivers (known as earth stations or ground stations) – The satellite receives transmissionsWhy on differentone frequency band (uplink), amplifies or repeats the frequenciessignal, and transmits are it on another frequency (downlink). used? • A orbiting satellite operate on a number of frequency bands, called transponder channels B.16 Geostaionary Satellites – It is launched into an orbit above the equator at 35786 km. – This orbit distance means that the satellite is orbiting the earth as fast as the earth is rotating. • It appears to earth stations that the satellite is stationary, thus making communications more reliable and predictable. • Earth stations is less expensive because they can use fixed antennas. B.17 Low earth orbit (LEO) and medium earth orbit (MEO) satellites – For small mobile personal communications terminals, a network with significantly reduced transmission and processing delay is required. Such a service could be provided by low earth orbit (LEO) and medium earth orbit (MEO) satellite systems. • Delay is 250 -500ms for geostationary satellites – These systems can provide direct personal-terminal-to- personal-terminal connectivity. – Hybrid systems are also possible, requiring radios that can operate with multiple standards and switch seamlessly between systems. B.18 Applications – Television distribution • Direct broadcast satellite ¾video signals are transmitted directly to the home user – long-distance telephone transmission • point-to-point trunks between telephone exchange offices in public telephone networks • suffers from transmission delay B.19 Applications – private business networks • very small aperture terminal (VSAT) systems ¾ subscriber stations equipped with low cost VSAT antennas share a satellite transmission capacity for transmission to a hub station ¾the hub station can exchange messages with each of the subscribers B.20 Applications B.21 Applications – Global mobile communication systems – Example • Iridium ¾This system is designed to provide handheld personal communications between locations around the world at any time and without any knowledge of the location of other personal units before connection. ¾use 77 LEO satellites which will prevent the annoying delays during conservation. ¾The LEO satellites allow the use of low-power, handheld telephones. Why we need so ¾Orbit altitude: 755km many satellites B.22 Satellite microwave Frequency allocation – Optimum frequency range for satellite transmission is 1 - 10GHz • Below 1 GHz, there is significant noise from nature sources • About 10 GHz, the signal is severely attenuated by atmosphere B.23 Satellite microwave Fixed satellite service Typical frequency bands for uplink/downlink usual terminology 6/4 GHz C band 8/7 GHz X band 14/12 GHz Ku band 30/20 GHz Ka band B.24 Satellite microwave Mobile satellite service Typical frequency bands for uplink/downlink usual terminology 1.6/1.5 GHz L band 30/20 GHz Ka band Broadcasting satellite service Typical frequency bands for uplink/downlink usual terminology 12 GHz Ku band B.25 Broadcast Radio Physical description – omnidirectional Applications – AM broadcasting • operating frequencies ¾MF (medium frequency): 300 kHz - 3 MHz ¾HF (high frequency): 3 MHz - 30 MHz B.26 Broadcast Radio – HF is the most economic means of low information rate transmission over long distances (e.g. > 300km) – A HF wave emitted from an antenna is characterized by a groundwave and a skywave components. • The groundwave follows the surface of the earth and can provide useful communication over salt water up to 1000km and over land for some 40km to 160km. • The skywave transmission depends on ionospheric refraction. Transmitted radio waves hitting the ionosphere are bent or refracted. When they are bent sufficiently, the waves are returned to earth at a distant location. Skywave links can be from 160km to 12800km. B.27 Broadcast Radio groundwave B.28 Broadcast Radio Applications – FM broadcasting • operating frequencies ¾VHF (very high frequency): 30 MHz - 300 MHz – TV broadcasting • operating frequencies: ¾VHF ¾UHF (ultra high frequency): 300 MHz - 3000MHz B.29 Infrared – Does not penetrate walls • no security or interference problems – no frequency allocation issue • no licensing is required B.30.
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