內(nèi)容無線網(wǎng)絡(luò)satellite

1、Chap 9 Satellite CommunicationsOverviewBasics of SatellitesTypes of SatellitesCapacity AllocationSatellite-related termsEarth stations -Antenna systems on or near earthUplink -transmission from an earth station to a satelliteDownlink -transmission from a satellite to an earth stationTransponder -ele

2、ctronics in the satellite that convert uplink signals to downlink signalsHow do Satellites WorkTwo Stations on Earth want to communicate through radio broadcast but are too far away to use conventional means.The two stations can use a satellite as a relay station for their communicationOne Earth Sta

3、tion sends a transmission to the satellite. This is called a Uplink.The satellite Transponder converts the signal and sends it down to the second earth station. This is called a Downlink.Basics: Advantages of SatellitesThe advantages of satellite communication over terrestrial communication are:The

4、coverage area of a satellite greatly exceeds that of a terrestrial system.Transmission cost of a satellite is independent of the distance from the center of the coverage area.Satellite to Satellite communication is very precise.Higher Bandwidths are available for use.Basics: Disadvantages of Satelli

5、tesThe disadvantages of satellite communication are:Launching satellites into orbit is costly.Satellite bandwidth is gradually ing used up.There is a larger propagation delay in satellite communication than in terrestrial communication. Basics: Factors in satellite communicationElevation Angle: The

6、angle of the horizontal of the earth surface to the center line of the satellite transmission beam.This affects the satellites coverage area. Ideally, you want a elevation angle of 0 degrees, so the transmission beam reaches the horizon visible to the satellite in all directions.However, because of

7、environmental factors like objects blocking the transmission, atmospheric attenuation, and the earth electrical background noise, there is a minimum elevation angle of earth stations.Basics: Factors in satellite communication (cont.)Coverage Angle: A measure of the portion of the earth surface visib

8、le to a satellite taking the minimum elevation angle into account.whereR = 6370 km (earths radius)h = satellite orbit height = coverage angle = minimum elevation angleBasics: Factors in satellite communication (cont.)The distance from the satellite to the furthest point of coverage isThe round-trip

9、transmission delay is in the rangeBasics: Factors in satellite communication (cont.)Other impairments to satellite communication:The distance between an earth station and a satellite (free space loss).Satellite Footprint: The satellite transmissions strength is strongest in the center of the transmi

10、ssion, and decreases farther from the center as free space loss increases.Atmospheric Attenuation caused by air and water can impair the transmission. It is particularly bad during rain and fog. Basics: How Satellites are usedService TypesFixed Service Satellites (FSS)Example: Point to Point Communi

11、cationBroadcast Service Satellites (BSS)Example: Satellite Television/RadioAlso called Direct Broadcast Service (DBS).Mobile Service Satellites (MSS)Example: Satellite PhonesTypes of SatellitesSatellite OrbitsGEOLEOMEOHEOFrequency BandsGeostationary Earth Orbit (GEO)These satellites are in orbit 35,

12、863 km above the earths surface along the equator.Objects in Geostationary orbit revolve around the earth at the same speed as the earth rotates. This means GEO satellites remain in the same position relative to the surface of earth.No problem with frequency changes (no Doppler effects)Tracking the

13、satellite is simplifiedGEO (cont.)AdvantagesA GEO satellites distance from earth gives it a large coverage area, almost a fourth (1/4) of the earths surface.GEO satellites have a 24 hour view of a particular area.These factors make it ideal for satellite broadcast and other multipoint applications.G

14、EO (cont.)DisadvantagesWeak signal due to long distance transmission (35000km)Polar regions are poorly servedSignal delay is substantial, approximately 240msLow Earth Orbit (LEO)LEO satellites are much closer to the earth than GEO satellites, ranging from 500 to 1,500 km above the surface.LEO satell

15、ites dont stay in fixed position relative to the surface, and are only visible for 15 to 20 minutes each pass.A network of LEO satellites is necessary for LEO satellites to be usefulLEO (cont.)AdvantagesBetter signal strengthLess time delayBandwidth efficientDisadvantagesNeed a network, which can be

16、 costlyNeed to cope with Doppler shifts caused by high speed relative movement.Atmospheric drag results in orbital deterioration.Requires handoversLEO CategoriesLittle LEOsFrequencies below 1GHz5MHz of bandwidthData rates up to 10kbpsAimed at paging, tracking, and low-rate messagingBig LEOsFrequenci

17、es above 1GHzSupport data rate up to a few megabits per secOffer same services as little LEOs in addition to voice and positioning servicesMedium Earth Orbit (MEO)A MEO satellite is in orbit somewhere between 8,000 km and 18,000 km above the earths surface. MEO satellites are similar to LEO satellit

18、es in functionality.MEO satellites are visible for much longer periods of time than LEO satellites, usually between 2 to 8 hours.MEO satellites have a larger coverage area than LEO satellites. MEO (cont.)AdvantageA MEO satellites longer duration of visibility and wider footprint means fewer satellit

19、es are needed in a MEO network than a LEO network.DisadvantageA MEO satellites distance gives it a longer time delay and weaker signal than a LEO satellite, though not as bad as a GEO satellite. Other OrbitsMolniya Orbit SatellitesUsed by Russia for decades.Molniya Orbit is an elliptical orbit. The

20、satellite remains in a nearly fixed position relative to earth for eight hours.A series of three Molniya satellites can act like a GEO satellite.Useful in near polar regions.Frequency BandsDifferent kinds of satellites use different frequency bands.LBand: 1 to 2 GHz, used by MSSS-Band: 2 to 4 GHz, u

21、sed by MSS, NASA, deep space researchC-Band: 4 to 8 GHz, used by FSSX-Band: 8 to 12.5 GHz, used by FSS and in terrestrial imaging, ex: military and meteorological satellitesKu-Band: 12.5 to 18 GHz: used by FSS and BSS (DBS)K-Band: 18 to 26.5 GHz: used by FSS and BSSKa-Band: 26.5 to 40 GHz: used by F

22、SS Satellite network configurationCapacity AllocationFDMAFAMA-FDMADAMA-FDMATDMA CDMAFDMAAn exampleAlternative uses of 36MHz channel1200 voice-frequency (VF) voice channelsOne 50Mbps data stream400 channels of 64kbps eachOne analog video signal, or 6-9 digital video signalsFDMA (cont.)The number of s

23、ub-channels is limited by three factors:Thermal noise (too weak a signal will be effected by background noise).Intermodulation noise (too strong a signal will cause noise).Crosstalk (cause by excessive frequency reusing). Forms of FDMA (cont.)FDMA can be performed in two ways:Fixed-assignment multip

24、le access (FAMA): The sub-channel assignments are of a fixed allotment. Ideal for broadcast satellite communication.Demand-assignment multiple access (DAMA): The sub-channel allotment changes based on demand. Ideal for point to point communication.FAMA-FDMADAMA-FDMASingle channel per carrier (SCPC)-

25、Bandwidth divided into individual VF channelsAttractive for remote areas with few user stations near each siteSuffers from insufficiency of fixed assignmentDAMA-set of subchannels in a channel is treated as a pool of available linksFor full-duplex between two earth-stations, a pair of subchannels is dynamically assigned on demandDemand assignment performed in a distributed fashion by earth station using CSCTDMATDMA (Time Division Multiple Access) breaks a transmission into multiple time slots, each one dedicated to a different transmitter