Satellite-Communication---5--Modulation-and-Multip

2011/4/[e-mail]In1960sand1970s,thesignalswerealmostexclusivelyanalogIn1980s,opticalfiberscameintowidespreaduse,long-distancetelephonelinksusingopticalfibersaredigitalSection5:ModulationandMultiplexing1.FrequencyModulation2.AnalogFMTransmissionbySatellite3.DigitalTransmission4.DigitalModulationandDemodulation5.DigitalTransmissionofAnalogSignals6.TimeDivisionMultiplexing7.Summary1.FrequencyModulationFrequencymodulationUsedwidelyinanalogradiocommunicationsystemsVideosignaldistributionforthecableTVindustryisnowtheonlysignificantremaininganalogFMsatelliteserviceFordigitalsignals–frequencyshiftkeying(FSK),rarelyusedinsatellitelinks,PSKispreferredFMusedinanalogsatellitesystems-ProvideanimprovementinthebasebandS/NratiorelativetotheC/NratiointheIFpartofthereceiver1.FrequencyModulationUsually,wehaveabasebandS/Nratiogivenby(C/N)0:overallC/NratiointheearthstationreceiverattheinputtotheFMdemodulatorDisadvantage–needwidebandtoachievetheimprovementfactor,useupalotofpreciousRFbandwidth0/(/)inprovementSNCNFMdB1.FrequencyModulationInanFMsignalInformationiscarriedbychangesinthecarrierfrequency–frequencydeviation.AmplitudeofFMwaveisconstant–powerdoesn‟tchangewithmodulationFrequencyvariesdirectlyinproportiontothevoltageofthebasebandmodulationsignalInstantaneousfrequencyfiisgivenby•fc:carrierfrequency•f:frequencydeviation•m(t):modulatingvoltage•kf:modulatorconstant,modulatorsensitivity()iccfffffkmt2011/4/1021.FrequencyModulationHowtorecovertheinformationsignalfromanFMwaveNeedafrequencytovoltageconverterwithaconstantK=1/kfvolts/HzTherecoveredbasebandsignalisFeaturesvaluablefortelemetrylinks•AnFMradiolinkhaveunitygainbetweenbasebandandthereceiverbaseband•CanalsotransmitaDClevel–voltageisrepresentedbyfofcarrier()1/()fvtfkmt1.FrequencyModulationWaveformEquationforFMAfteratimet,thephaseangle,(t),oftheFMwavemodulatedbyaninformationsignalm(t)0:startingphaseangleatt=0DifficulttoevaluatethespectrumoftheFMwaveifwedidn‟tknowananalyticalexpressionform(t)00()22()tcftfkmtdt00()cos(2())tFMcfVtAtkmtdt1.FrequencyModulationFMisanonlinearmodulationtechniqueThespectrumofFMwaveformismuchmorecomplexthanthatofAMwaveformBandwidthofFMsignalsMosttextsexpandtheFMwaveformequationusingasinusoidaltesttoneasthebasebandmodulatingsignal–leadstoaninfinitenumberofharmonictermsoftheformJn()cos(cnm)t,misthefrequencyofthetesttone.ThemagnitudesofthetermsaregivenbyBesselfunctioncoefficientsJn().1.FrequencyModulationCarson‟srule:AmuchmoresimpleandusefulwaytodeterminethebandwidthrequiredforanFMsignalEmpiricalWorkswellfpk:peakfrequencydeviationfmax:highestfrequencypresentinthemodulatingsignalmax2()pkBffHz1.FrequencyModulationTwokeypointsofFMbandwidthFrequencydeviationofthecarrierisdirectlyproportionaltothemodulatingsignalvoltageBandwidthrequiredtotransmitanFMsignalisfoundfromCarson‟srule1.FrequencyModulationC/NratioattheoutputofIFamplifierC=A2/2:carrierpowerisconstantforFMwaveN0B:noisepoweratthefilteroutputBasebandS/NratioforFMsignalsS:basebandsignalpower–signalvoltage~frequencydeviationNout:basebandnoisepower–integratingN0overthebasebandbandwidth2200/(/2)/()/(2)CNANBANB221()()2peakrmsSKfKf2122233002122/3foutfKKNNfdfNffAA2011/4/1031.FrequencyModulationConsiderthebasebandextendsalmosttoDC,f10B:IFbandwidthofreceiverfpk:peakfrequencydeviationattransmitterfmax:maximumfrequencyofbasebandsignal1.8dB:numericalratio3/210max10maxS/Nimprovementfactor(/)/10log(/)20log(/)1.8outpeakSNCNBfffdB1.FrequencyModulationToincreasebasebandS/N,S/NimprovementfactorcanbemadelargebydeviationrationS/Nimprovementfactorlager,D–larger,RFbandwidthwidermax/peakDffmaxmax2()2(1)RFpeakBfffD1.FrequencyModulationPre-emphasisandde-emphasisReducethenoisepowerImprovethebasebandS/NNoisepowerspectraldensityisproportionaltosquareofbasebandfrequency,higherbasebandfrequency–largernoisepower2122233002122/3foutfKKNNfdfNffAA1.FrequencyModulationFormostvoiceandvideosignals,theenergyinthebasebandspectrumatthehigherfrequenciesislowUsePre-emphasisinthebasebandofthetransmittertoamplifythehighfrequenciesinthesignalUseDe-emphasisaftertheFMdemodulatortoreducethehighfrequenciesinthesignal1.FrequencyModulationDe-emphasisCharacteristicsofaLPFCutallthehighfrequencycontentofthesignalWithGde(f)=b/fb•fb:basebandfrequency•b:constantPre-emphasesComplementarycircuitWithGpe(f)=fb/b1.FrequencyModulationSignalisunaffectedbytwoprocessesNoisepowerissignificantlyreducedU:unweightedP:increasedtermbyde-emphasisDependsonthenaturalofthesignal,basebandbandwidthandthecircuitsused10max10max(/)/10log(/)20log(/)1.8UpeakSNCNBfffPdB2011/4/1042.AnalogFMTransmissionbySatelliteForthefirst20yearsofSATCOMMostsignals–analogformTransmittedusingFMVoicechannelsweremultiplexedbyfrequencydivisionmultiplexing(FDM)•FDMlargelydisappearedasdigitaltransmissionoveropticalfibersreplacedanalogtransmissionovermicrowaveandcoaxialcableli