无线系统的微波与射频设计

ModulationTechniques,MicrowaveandRFDesignofWirelessSystems,Chapter9,Dr.ZhangYonghong,Comparingwithtransmittingbasebandsignaldirectly,totransmitdatabymodulatingahigherfrequencycarrierwavehastheeffecton:,controllingtheradiatedfrequencyspectrum.moreefficientuseoftheallocatedRFbandwidth.flexibilityinaccommodatingdifferentbasebandsignalformats.,AmplitudeFrequencyPhase,AMFMPM,Analogmodulation(varycontinuously)Digitalmodulation(changeindiscretesteps),moreefficientuseoftheradiospectrum.usuallyrequireslesspower.CDMAoverafadingcommunicationschannel.morecompatiblewiththeuseoferrorcorrectingcodes.,Incontrasttoanalogmodulation,digitalmodulationhas:,9.1AnalogModulation9.2BinaryDigitalModulation9.3ErrorProbabilitiesforBinaryModulation9.4EffectofRayleighFadingonBitErrorRats9.5M-aryDigitalModulation,9.1AnalogModulation,Basicanalogmodulation:AM(SSB,DSB),FM,PM,PPM(PulsePositionModulation)isusedinUWB(ultrawideband)system.,1.Signal-SidebandModulation2.DSB-SCModulation3.DSB-LCModulation4.EnvelopeDetectionofDSB-LCModulation5.FrequencyModulation(FM),1.Signal-SidebandModulation,m(t):bandlimitedmodulatingwaveformfM:themaximumfrequencyofm(t),n0/2:two-sidedpowerspectraldensityofGaussianwhitenoise.Contributedbythetransmitterchannelandnoisegeneratedbytheinputstagesofthereceiver.,ThedemodulatorLOisidenticalinfrequencyandphasewiththemodulatorLO-synchronousorcoherentdemodulator.,Inputsignalpower:,Outputsignalpower:,Inputnarrowbandnoisepower:,Outputnoisepower:,TheoutputSNR:,Conclusion:SSBdemodulatordoesnotdegradetheinputSNR.,2.DSB-SCModulation,Double-sidebandsuppressedcarrier(DSB-SC),Conclusion:theDSB-SCdemodulatorimprovestheinputsignal-to-noiseratiobyafactoroftwo.,FSSB=2FDSB,But,Conclusion:ThecoherentSSBandDSB-SCdemodulatorshavethesameSNRperformance.,3.DSB-LCModulation,Advantage:thecarriersignalcanbeusedasareferencesignaltophase-lookthelocaloscillatortosynchronizationwiththeincomingsignal.,Double-SidebandLarge-Carrier(DSB-LC),Disadvantage:thecarrierpowerincreasesthetotalinputpowerbutdoesnotcontainanymodulationinformation.,m:modulationindex,IFm1,DSB-LCDSB-SC,m=1impliesareductioninSNRof4.8dB.,4.EnvelopeDetectionofDSB-LCModulation,AdvantageofDSB-LC:canbedetectedbyusinganenvelopedetector,withoutaLOandmixer,resultsinamuchsimplereceivercircuit,suchasbroadcastAMradio.,noncoherentdemodulation,SeriousdistortionforsmallSi/NiSNRIf|m|1overmodulation,itwillnotcorrectlyrecoverthemodulatingwaveform.,5.FrequencyModulation(FM),Application:Broadcastradio,televisionsound,two-wayvoiceradio,AMPScellulartelephonesystem.,AnFMwaveform:,Where,modulatingsignal,Modulationindex:,:Maximumfrequencydeviation.,Spectrum:sidebandsarespacedatfmoneithersideofthecarrieratIF.amplitudes,givenbyAJn(),decreaseforlargen.,Carsonsrule,B,IFbandwidth,Throughdifferentiatorandenvelopedetection,theoutputvoltage:,Fortheequaltransmitpower,Improvingfactor:,Conclusion:FMallowsanimprovementinSNRattheexpenseofincreasedbandwidth,whileAMdoesnot.,S/N,B,=4SNRFM=72SNRDSB-LC(m=1)BFM=5BDSB,9.2BinaryDigitalModulation,Amplitudeshiftkeying(ASK)Frequencyshiftkeying(FSK)Phaseshiftkeying(PSK),1.BinarySignals2.AmplitudeShiftKeying3.FrequencyShiftKeying(FSK)4.PSK5.CarrierSynchronization,1.BinarySignals,Return-to-Zerocode,Non-Return-to-Zerocode,PolarNRZcodeDC=0,2.AmplitudeShiftKeying,wherem(t)=0or1,(a)Modulator,IdenticaltotheDSB-SCmodulator,Synchronousdemodulation:,Afterlow-passfiltering,(b)Synchronousdemodulation,Notice:LOhaspreciselythesamephaseandfrequencyastheincomingsignal,ordistortionmaybeintroduced.,Envelopedetectionnoncoherent,noLO,(c)Envelopedetection,3.FrequencyShiftKeying(FSK),-frequencydeviation,IF=1,theupbranchoutput:1/2Thedownbranchoutput:0IF=2,theupbranchoutput:0Thedownbranchoutput:1/2reversephaseItrequirestwocoherentLOoperatingat1,and2.,(PLLdetector,thecontrolvoltageofVCOinPLL),Envelopedetector,4.PSK,Thephaseofthecarrierwaveis“0”or“180”.,m(t)=1or-1,Duetothesharptransitionscausedbyphasereversal,thespectrumofthePSKwaveformisrelativelywideinbandwidth,resultingthatPSKisimpracticalformultichannelwirelesssystems.,PSKmodulator,ASK:Non-constantenvelopemodulationCoherentdemodulationNoncoherentdemodulation(envelopedetection),FSK:ConstantenvelopemodulationCoherentdemodulationNoncoherentdemodulation(afterconversion),PSK:ConstantenvelopemodulationCoherentdemodulation,5.CarrierSynchronization,Theeffectofaphaseerroristhattheoutputsignalisreducedinamplitudebycos,whileanerrorinfrequencyintroducesafactorofcost.Thebiterrorratesofenvelopedetectionarenotasgoodasthoseobtainedwithcoherentdetection.,Twowaystorealizesynchronization:transmitapilotcarrier,usedtophase-locktheLO.useacarrier-recoverycircuit.Useaphase-lockedlooporbyfrequencymultiplieranddivider.,Infact,employingdigitalsignalprocessing(DSP)circuitstoperformallfunctionofsignalconditioning,carrierrecoveryandsynchronizationdemodulation,andsignalformatting.,9.3ErrorProbabilitiesforBinaryModulation,Thepresenceofnoiseinacommunicationchannelintroducesthepossibilitythaterrorswillbemadeduringthedetectionprocess.,1.PCMSignalandDetectors2.SynchronousASK3.SynchronousPSK4.SynchronousFSK5.BitrateandBandwidthEfficient6.ComparisonofASKFSKandPSKSystems,1.PCMSignalandDetectors,Pulsecodedmodulation(PCM),where,Define:bitenergy,Theoutputnoisepower,Thevarianceofthegaussianprobabilitydistributionfunction,2.SynchronousASK,Idealifs(t)=s2(t)=0,s0(T)=0ifs(t)=s1(t)=V,s0(T)=VTPracticalifs0(T)+n0(T)VT/2,m(t)=1,Let,3.SynchronousPSK,ASK:VT/2,Forthesameprobabilityoferror,PSKrequiresonlyone-fourththepowerofanASKsystem.SinceanASKsignalisoffhalfthetime,intermsofaveragetransmitpower,thePSKresultisbetterbyafactoroftwo(3dB).,DuetosymmetryofthePSKsignalandthedemodulator.,4.SynchronousFSK,Threshold:0thesignallevelsaresimilartothePSKcase.,Forthenoisevoltages:,n1andn2areuncorrelated.,Conclusion:ThetotalnoisepoweroftheFSKdemodulatorisdoubledrelativetothesynchronousASKorPSKdemodulator.,Conclusion:synchronousFSKrequires3dBmoresignalpowerthanequivalentPSKsystemforthesameprobabilityoferror.synchronousFSKrequires3dBlesspowerthananASKsystemonapeakpowerbasis.FSKandASKhaveequalerrorrateswhencomparedintermsofaveragetransmitpower.,5.BitRateandBandwidthEfficient,DefineRb:,dimensionofEb:WS,dimensionofn0:W/Hz,Eb/n0:dimensionless,thebitrateofthebinarymessagesignaldimensionofRb:bps(bitpersecond),Thesignalpower:,Itmeanstheerrorratewillincreasewithanincreaseinbitrate,forafixednoisepowerspectrumdensity,andisindependentofthereceiverbandwidth.,IFbandwidthfRFbandwidth,Rb:dependonthetypeofmodulation,fmayrangefromonetoseveraltimesthebitrateRb.,bandwidthefficiency(bps/Hz)of1bps/Hz.,Forabinarymodulationmethod,transmittingonebiteachbitperiod.,meansthat,Thisisforthebaseband,notfortheIForRF.,6.ComparisonofASKFSKandPSKSystems,CoherentPSK:Thelowesterrorrate,9.6dBfor10-5,highpriceforLOandwidesignalbandwidth(2Rb4Rb).Bestinfadingenvironments.Usedinspaceandsatellitecommunications.,CoherentFSK:Requires34dBmorepowerthanPSK.12.6dBfor10-5.,NoncoherentFSK:13.4dBfor10-5,widespreadhistoricalapplicationinawidevarietyofsystems,suchasdatemodems,teletype,fax.,CoherentASK:Transmitterissimple.15.6dBfor10-5,verypoorinafadingenvironment,lowdatarateslimitedtoshort-range,low-lost,usedintelemetryandRFID.,NoncoherentASK:Transmitterandreceiveraresimple,16.5dBfor10-5,verypoorinafadingenvironment,lowdatarates,short-range,low-cost,usedintelemetryandRFID.,EXAMPLE9.3,page319.,Eb/n0forPe=10-5,9.4EffectofRayleighFadingonBitErrorRats,PDFofaRayleighfadingis,1.EffectofRayleighFadingonCoherentPSK,Where,istheaveragereceivedbitenergy-to-noise,powerspectraldensityratioofthefadedreceivedsignal.,2.EffectofRayleighFadingonNoncoherentFSK,ForenvelopedetectionofFSK,3.ComparisonofFadedandNonfadedErrorRates,Conclusion:Fadinghastheeffectofdramaticincreasingtherequiredbitenergy-to-noiseratio.Error-correctingcodescanbeusedveryeffectivelytoimprovetheerrorrateforchannelfadingoccurringinshortbursts.,Nonfadedcase:,Eyediagram,EXAMPLE9.4,p.323.,9.5M-aryDigitalModulation,Binarymodulationmethods(ASK,FSKandPSK)transmitonebitpersignalinginterval,withabandwidthefficiencyof1bps/Hz.,Thedatacanbedividedintogroups.Eachgrouphavenbinarycodes.OnesymbolinM=2nistransmittedineachsignalinginterval.Thusabandwidthefficiencyofnbps/Hzisachieved.,1.QuadraturePhaseShiftKeying2.ProbabilityofErrorforQPSK3.M-aryPhaseShiftKeying4.QuadratureAmplitudeModulation(QAM)5.ChannelCapacity,1.QuadraturePhaseShiftKeying,n=2,M=4,+,=,ThecarrierofQPSK:,Note:EachQPSKphasestatecanbeusedtorepresenttwobitsofdata.ThebandwidthoftheQPSKspectrumisnarrowerthanthespectrumofaBPSKsignal,becauseoftheaveragetransitionbetweenphasestatesis900.TheoutputoftheQPSKmodulatorisadoublesidebandsuppressedcarriersignal.TheQPSKoutputisaconstantenvelopesignal.Thechannelshouldbeconstantgroupdelay.,BlockdiagramofaQPSKmodulator,GrayCoding,Advantage:Whenanerroroccurs,itismostlikelythatonlyoneofthebitswillbeinerror,ratherthanbothbits.,2.ProbabilityofErrorforQPSK,Theoverallprobabilityoferrorforasymbolis:,AsymbolerrorismostlikelytocauseonlyasinglebiterrorforGraycoding.Sinceeachsymbolcontainstwobits,thebiterrorrateforQPSKwillbeone-halfthesymbolerrorrate:,forBPSK,LedtotheextensiveuseofQPSKmodulationinawidevarietyofapplications,suchasCDMA-P