通信原理(英文版)3

13.1IntroductionAnalogmodulation:modulationofacarrierbyasourcebasebandanalogsignalCarrier:adeterministicperiodicwaveform－cosinusoidalwhere,A---amplitude0---angularfrequencyofcarrier0---initialphaseDefinition:Modulatingsignalm(t)－signalfromthesourceModulatedsignals(t)－signalafterbeingmodulatedModulator－thedeviceformodulationFigure3.1.1ModulatorModulatorModulatedsignals(t)Modulatingsignalm(t)Chapter3Analogmodulationsystem)cos()(00tAtc2PurposeofmodulationFrequencyspectrummovement---foraccommodatingtherequirementofchanneltransmissionorforcombiningseveralsignalsformulti-channeltransmissionImprovementofanti-jammingabilityClassificationofanalogmodulation:Linearmodulation:AM,SSB,DSB,VSB,…Nonlinearmodulation(angularmodulation):FM,PM3(a)FrequencyspectraldensityofinputsignalS(f)f0f-f00M(f)f0(b)Frequencyspectraldensityofoutputsignal3.2Linearmodulation3.2.0BasicconceptAssume:thecarrieris:c(t)=Acos0t=Acos2f0tmodulatingsignalisaenergysignalm(t),itsspectrumisM(f)carrier:c(t)multiplicationresult:s(t)filteroutput:s(t)“”isusedtoexpressFouriertransform:where,()()mtMf)(cos)(0fStAtm)]()([2)(00ffMffMAfSH(f)s(t)Modulatedsignals(t)Modulatingsignalm(t)Acos0t43.2.1AMBasicprinciplesLet:m(t)=[1+m(t)]，|m(t)|1，m(t)|max=m－modulationindex,thenwehavetheAMsignals(t)=[1+m(t)]Acos0t，where,[1+m(t)]0,i.e.,theenvelopeofs(t)isnon-negative+1==m(t)101+m(t)101+m(t)5FrequencydensityContaindiscretecarriercomponentWhenm(t)iscosinusoidal,andm＝100％,sumofthetwosidebandpower＝halfofcarrierpowercarrierpoweruppersidebandpowerlowersidebandpowerFigure3.2.3Waveformandspectrumofmodulatedsignalm(t)s(t)M(f)C(f)c(t)A-Atfm-fmf0-f02fmS(f)2fm-f0f0ffftt6ReceptionofAMsignal:envelopedetectonPrinciple:Characteristics:assumetheinputvoltageiswhereistheinputnoisevoltageofthedetectorTheenvelopeofy(t):ForlargeS/N:ttnttnAtmtysc00sin)(cos)}()]('1{[)(ttnttnsc00sin)(cos)()()}()]('1{[)(22tntnAtmtVscy)()]('1[)(tnAtmtVcyRectifierLow-passfilterFigure3.2.4Envelopedetector7Afterdetection(D.C.componenthasbeenfiltered):Signaltonoisepowerratioofoutputsignal:∵signaltonoisepowerratiobeforedetectionequals∴RatioofS/Nbeforeandafterdetection:Sincem(t)1，obviouslytheaboveratior0/riislessthan1,i.e.,theS/Ndecreasedafterdetection.)()(')(tnAtmtvc)](/)('[2220tnAtmErc)(/)('121222tnAtmEri222222220)]('1[)('2)(/)('121)(/)('tmtmEtnAtmtnAtmErrci83.2.2DSBmodulationPrinciple:whenthemodulationsignalm(t)hasnoD.C.component,DSBsignalisobtained.Frequencyspectrum:thetwosidebandscontainidenticalinformation.M(f)f0f00-f0fS(f)(a)FrequencyspectraldensityofmodulatingsignalFigure3.2.5Spectrumofdouble-sidebandmodulationsignalUpper-sideband(b)FrequencyspectraldensityofmodulatedsignalUpper-sidebandLower-sideband9Demodulation:needlocalcarrierLetreceivedDSBsignalbelocalcarrierofreceiverbeProductoftheabovetwovoltagesisAfterlow-passfiltered,weobtainTheoutputsignalequalsm(t)/2,onlyifthelocalcarrierhasnofrequencyandphaseerror.Advantage&disadvantage:DSBsignalcansavetransmittingpower,butthereceivingcircuitwillberathercomplicated.ttm0cos)(])cos[(0t]})2cos[()){cos((21])cos[(cos)()(000tttmtttmtr)cos()(21ttmFigure3.2.6BlockdiagramofDSBsignaldemodulatorBasebandsignalm(t)Receivedsignals(t)cos0tr(t)H(f)103.2.3SSBmodulationPrinciples:TwosidebandscontainidenticalinformationOnlyonesidabandshouldbetransmittedToolowfrequencycomponentsinm(t)isnotallowedDemodulation:localcarrierisneededSince,ifz(t)=x(t)y(t)，then,wehaveZ()=X()Y()DuringdemodulationofSSBsignal,thecarriercos0tismultipliedwiththereceivedsignal;itisequivalenttoconvolutionofcarrierandsignalspectrainfrequencydomain.Figure3.2.7Frequencyspectrumofsingle-sidebandsignal(b)Upper-sidebandfiltercharacteristicandsignalfrequencyspectrumf00f-f00ff0-f0HH(f)HH(f)S(f)Upper-sidebandUpper-sideband(c)Lower-sidebandfiltercharacteristicandsignalfrequencyspectrum-f0f0fHL(f)S(f)Lower-sideband(a)SignalfrequencyspectrumbeforefilteringUpper-sidebandS(f)Upper-sidebandLower-sideband11Touseuppersidebandasexampleshowsdemodulatedsignalafterlow-passfiltering.AdvantagesofSSB:smallertransmittingpowerandbandwidththanDSBsignal.Figure3.2.7Frequencyspectrumofsingle-sidebandsignal(b)Upper-sidebandfiltercharacteristicandsignalfrequencyspectrumf00f-f00ff0-f0HH(f)HH(f)S(f)Upper-sidebandUpper-sideband(c)Lower-sidebandfiltercharacteristicandsignalfrequencyspectrum-f0f0fHL(f)S(f)Lower-sideband(a)SignalfrequencyspectrumbeforefilteringUpper-sidebandS(f)Upper-sidebandLower-sideband123.2.4VSBmodulationAdvantagesofVSB:Duringdemodulation,nolocalcarrierisnecessary;modulatingsignalmaycontainverylowfrequencyandD.C.components.Principles:VSBisstilllinearmodulation.ThefrequencyspectrumofthemodulatedsignalisAssumethetransferfunctionofthefilterofmodulatorisH(f)，thenthefrequencyspectrumofthemodulatedsignalafterfilteringis)]()([2)(00ffMffMAfSs(t)Acos0tH(f)Demodulatedsignals(t)Modulatingsignalm(t))()]()([2)(00fHffMffMAfS13Let’sfindtheconditionwhichshouldbesatisfi