无线通信与网络chap9

SatelliteCommunicationsChapter9Satellite-RelatedTermsEarthStations–antennasystemsonornearearthUplink–transmissionfromanearthstationtoasatelliteDownlink–transmissionfromasatellitetoanearthstationTransponder–electronicsinthesatellitethatconvertuplinksignalstodownlinksignalsWaystoCategorizeCommunicationsSatellitesCoverageareaGlobal,regional,nationalServicetypeFixedservicesatellite(FSS)Broadcastservicesatellite(BSS)Mobileservicesatellite(MSS)GeneralusageCommercial,military,amateur,experimentalClassificationofSatelliteOrbitsCircularorellipticalorbitCircularwithcenteratearth’scenterEllipticalwithonefociatearth’scenterOrbitaroundearthindifferentplanesEquatorialorbitaboveearth’sequatorPolarorbitpassesoverbothpolesOtherorbitsreferredtoasinclinedorbitsAltitudeofsatellitesGeostationaryorbit(GEO)Mediumearthorbit(MEO)Lowearthorbit(LEO)GeometryTermsElevationangle-theanglefromthehorizontaltothepointonthecenterofthemainbeamoftheantennawhentheantennaispointeddirectlyatthesatelliteMinimumelevationangleCoverageangle-themeasureoftheportionoftheearth'ssurfacevisibletothesatelliteMinimumElevationAngleReasonsaffectingminimumelevationangleofearthstation’santenna(0o)Buildings,trees,andotherterrestrialobjectsblockthelineofsightAtmosphericattenuationisgreateratlowelevationanglesElectricalnoisegeneratedbytheearth'sheatnearitssurfaceadverselyaffectsreceptionGEOOrbitAdvantagesofthetheGEOorbitNoproblemwithfrequencychangesTrackingofthesatelliteissimplifiedHighcoverageareaDisadvantagesoftheGEOorbitWeaksignalaftertravelingover35,000kmPolarregionsarepoorlyservedSignalsendingdelayissubstantialLEOSatelliteCharacteristicsCircular/slightlyellipticalorbitunder2000kmOrbitperiodrangesfrom1.5to2hoursDiameterofcoverageisabout8000kmRound-tripsignalpropagationdelaylessthan20msMaximumsatellitevisibletimeupto20minSystemmustcopewithlargeDopplershiftsAtmosphericdragresultsinorbitaldeteriorationLEOCategoriesLittleLEOsFrequenciesbelow1GHz5MHzofbandwidthDataratesupto10kbpsAimedatpaging,tracking,andlow-ratemessagingBigLEOsFrequenciesabove1GHzSupportdataratesuptoafewmegabitspersecOffersameservicesaslittleLEOsinadditiontovoiceandpositioningservicesMEOSatelliteCharacteristicsCircularorbitatanaltitudeintherangeof5000to12,000kmOrbitperiodof6hoursDiameterofcoverageis10,000to15,000kmRoundtripsignalpropagationdelaylessthan50msMaximumsatellitevisibletimeisafewhoursFrequencyBandsAvailableforSatelliteCommunicationsSatelliteLinkPerformanceFactorsDistancebetweenearthstationantennaandsatelliteantennaFordownlink,terrestrialdistancebetweenearthstationantennaand“aimpoint”ofsatelliteDisplayedasasatellitefootprint(Figure9.6)AtmosphericattenuationAffectedbyoxygen,water,angleofelevation,andhigherfrequenciesSatelliteFootprintSatelliteNetworkConfigurationsCapacityAllocationStrategiesFrequencydivisionmultipleaccess(FDMA)Timedivisionmultipleaccess(TDMA)Codedivisionmultipleaccess(CDMA)Frequency-DivisionMultiplexingAlternativeusesofchannelsinpoint-to-pointconfiguration1200voice-frequency(VF)voicechannelsOne50-Mbpsdatastream16channelsof1.544Mbpseach400channelsof64kbpseach600channelsof40kbpseachOneanalogvideosignalSixtoninedigitalvideosignalsFrequency-DivisionMultipleAccessFactorswhichlimitthenumberofsubchannelsprovidedwithinasatellitechannelviaFDMAThermalnoiseIntermodulationnoiseCrosstalkFormsofFDMAFixed-assignmentmultipleaccess(FAMA)TheassignmentofcapacityisdistributedinafixedmanneramongmultiplestationsDemandmayfluctuateResultsinthesignificantunderuseofcapacityDemand-assignmentmultipleaccess(DAMA)CapacityassignmentischangedasneededtorespondoptimallytodemandchangesamongthemultiplestationsFAMA-FDMAFAMA–logicallinksbetweenstationsarepreassignedFAMA–multiplestationsaccessthesatellitebyusingdifferentfrequencybandsUsesconsiderablebandwidthDAMA-FDMASinglechannelpercarrier(SCPC)–bandwidthdividedintoindividualVFchannelsAttractiveforremoteareaswithfewuserstationsneareachsiteSuffersfrominefficiencyoffixedassignmentDAMA–setofsubchannelsinachannelistreatedasapoolofavailablelinksForfull-duplexbetweentwoearthstations,apairofsubchannelsisdynamicallyassignedondemandDemandassignmentperformedinadistributedfashionbyearthstationusingCSCReasonsforIncreasingUseofTDMTechniquesCostofdigitalcomponentscontinuestodropAdvantagesofdigitalcomponentsUseoferrorcorrectionIncreasedefficiencyofTDMLackofintermodulationnoiseFAMA-TDMAOperationTransmissionintheformofrepetitivesequenceofframesEachframeisdividedintoanumberoftimeslotsEachslotisdedicatedtoaparticulartransmitterEarthstationstaketurnsusinguplinkchannelSendsdatainassignedtimeslotSatelliterepeatsincomingtransmissionsBroadcasttoallstationsStationsmustknowwhichslottousefortransmissionandwhichtouseforreceptionFAMA-TDMAUplinkFAMA-TDMADownlink