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Chapter#9
This document provides an overview of satellite communication systems. It defines key terms like earth stations, uplinks, downlinks, and transponders. It describes how communications satellites can be categorized based on coverage area, service type, and usage. It also covers satellite orbit types including geostationary, medium earth, and low earth orbits. The document discusses factors that affect satellite link performance and describes frequency division multiple access and time division multiple access techniques.
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Chapter#9
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- 2. What is Satellite CommunicationSystem? In a satellite communications system, two or more stations on or near the earth communicate via one or more satellites that serve as relay stations in space.
- 3. Satellite-Related Terms EarthStations – antenna systems on or near earth Uplink – transmission from an earth station to a satellite Downlink – transmission from a satellite to an earth station Transponder – electronics in the satellite that convert uplink signals to downlink signals
- 4. Ways to Categorize CommunicationsSatellites Coverage area Global, regional, national Service type Fixed service satellite (FSS) Broadcast service satellite (BSS) Mobile service satellite (MSS) General usage Commercial, military, amateur, experimental
- 5. Classification of SatelliteOrbits Circular or elliptical orbit Circular with center at earth’s center Elliptical with one foci at earth’s center Orbit around earth in different planes Equatorial orbit above earth’s equator Polar orbit passes over both poles Other orbits referred to as inclined orbits Altitude of satellites Geostationary orbit (GEO) Medium earth orbit (MEO) Low earth orbit (LEO)
- 6. Geometry Terms Elevationangle - the angle from the horizontal to the point on the center of the main beam of the antenna when the antenna is pointed directly at the satellite Minimum elevation angle ? See next slide Coverage angle - the measure of the portion of the earth's surface visible to the satellite
- 7. Minimum Elevation Angle Reasons affecting minimum elevation angle of earth station’s antenna (>0o ) Buildings, trees, and other terrestrial objects block the line of sight Atmospheric attenuation is greater at low elevation angles Electrical noise generated by the earth's heat near its surface adversely affects reception
- 8. GEO Orbit Advantagesof the the GEO orbit No problem with frequency changes Tracking of the satellite is simplified High coverage area Disadvantages of the GEO orbit Weak signal after traveling over 35,000 km Polar regions are poorly served Signal sending delay is large
- 9. LEO Satellite Characteristics Circular/slightly elliptical orbit under 2000 km Orbit period ranges from 1.5 to 2 hours Diameter of coverage is about 8000 km Round-trip signal propagation delay less than 20 ms Maximum satellite visible time up to 20 min System must cope with large Doppler shifts Atmospheric drag results in orbital deterioration
- 10. LEO Categories LittleLEOs Frequencies below 1 GHz 5MHz of bandwidth Data rates up to 10 kbps Aimed at paging, tracking, and low-rate messaging Big LEOs Frequencies above 1 GHz Support data rates up to a few megabits per sec Offer same services as little LEOs in addition to voice and positioning services
- 11. MEO Satellite Characteristics Circular orbit at an altitude in the range of 5000 to 12,000 km Orbit period of 6 hours Diameter of coverage is 10,000 to 15,000 km Round trip signal propagation delay less than 50 ms Maximum satellite visible time is a few hours
- 12. Frequency Bands Availablefor Satellite Communications
- 13. Factors Dependency ofSatellite Link Performance Distance between earth station antenna and satellite antenna For downlink, terrestrial distance between earth station antenna and “aim point” of satellite Displayed as a satellite footprint (Figure 9.6) Atmospheric attenuation Affected by oxygen, water, angle of elevation, and higher frequencies
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- 16. Capacity Allocation Strategies Frequency division multiple access (FDMA) Time division multiple access (TDMA) Code division multiple access (CDMA)
- 17. Frequency-Division Multiplexing Alternativeuses of channels in point-to- point configuration 1200 voice-frequency (VF) voice channels One 50-Mbps data stream 16 channels of 1.544 Mbps each 400 channels of 64 kbps each 600 channels of 40 kbps each One analog video signal Six to nine digital video signals
- 18. Frequency-Division Multiple Access Factorswhich limit the number of subchannels provided within a satellite channel via FDMA Thermal noise Intermodulation noise Crosstalk
- 19. Forms of FDMA Fixed-assignment multiple access (FAMA) The assignment of capacity is distributed in a fixed manner among multiple stations Demand may fluctuate Results in the significant underuse of capacity Demand-assignment multiple access (DAMA) Capacity assignment is changed as needed to respond optimally to demand changes among the multiple stations
- 20. FAMA-FDMA FAMA –logical links between stations are preassigned FAMA – multiple stations access the satellite by using different frequency bands Uses considerable bandwidth
- 21. DAMA-FDMA Single channelper carrier (SCPC) – bandwidth divided into individual VF channels Attractive for remote areas with few user stations near each site But the SCPC suffers from inefficiency of fixed assignment DAMA – set of sub channels in a channel is treated as a pool of available links For full-duplex between two earth stations, a pair of sub channels is dynamically assigned on demand Demand assignment performed in a distributed fashion by earth station using (Common Signaling Channel) CSC
- 22. Reasons for IncreasingUse of TDM Techniques Cost of digital components continues to drop Advantages of digital components Use of error correction Increased efficiency of TDM Lack of intermodulation noise
- 23. FAMA-TDMA Operation Transmissionin the form of repetitive sequence of frames Each frame is divided into a number of time slots Each slot is dedicated to a particular transmitter Earth stations take turns using uplink channel Sends data in assigned time slot Satellite repeats incoming transmissions Broadcast to all stations Stations must know which slot to use for transmission and which to use for reception
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- 26. Waveguide frequency bands BandBandFrequency rangeFrequency range R band 1.70 to 2.60 GHz D band 2.20 to 3.30 GHz S band 2.60 to 3.95 GHz E band 3.30 to 4.90 GHz G band 3.95 to 5.85 GHz F band 4.90 to 7.05 GHz C band 5.85 to 8.20 GHz H band 7.05 to 10.10 GHz X band 8.2 to 12.4 GHz Ku band 12.4 to 18.0 GHz K band 15.0 to 26.5 GHz Ka band 26.5 to 40.0 GHz Q band 33 to 50 GHz U band 40 to 60 GHz V band 50 to 75 GHz W band 75 to 110 GHz F band 90 to 140 GHz D band 110 to 170 GHz Y band 325 to 500 GHz
Editor's Notes
- #3 A broadcast relay station, satellite station, relay transmitter, broadcast translator (U.S.), rebroadcaster (Canada) or repeater (two-way radio) is a broadcast transmitter which repeats or transponds the signal of another radio station or television station, usually to an area not covered by the signal of the originating station
- #11 What is a Paging Network? Paging networks are used as communication systems for sending a page to a subscriber of that system. A page is a message that is simulcast over the entire paging service area to all base stations at a time. The messages sent are either numeric, such as a telephone number, alphanumeric or even voice. Originally the messages were only used to alert the user that someone was trying to get in touch with them, but now pagers can be used to receive stock quotes or sports scores or even retrieve your email
- #14 Atmospheric Attenuation: A reduction in the intensity of electromagnetic radiation in the earth’s atmosphere as a result of the absorption and scattering of the radiation.
- #19 INTELSAT (International Telecommunications Satellite Organization) is a consortium of national satellite providers established under international treaty. INTELSAT purchases spacecraft based on the global demand for telephone and TV services; it operates as a wholesaler, providing space segment services to end users who operate their own earth stations. COMSAT is the U.S. member. INTELSAT is currently in the process of privatization.
- #21 In telecommunications a link is a communications channel that connects two or more communicating devices. This link may be an actual physical link or it may be a logical link that uses one or more actual physical links.
- #22 VF: Voice Frequency Channel