无线系统的微波与射频设计-课件

ModulationTechniques

MicrowaveandRFDesignofWirelessSystemsChapter9Comparingwithtransmittingbasebandsignaldirectly,totransmitdatabymodulatingahigherfrequencycarrierwavehastheeffecton:①controllingtheradiatedfrequencyspectrum.②moreefficientuseoftheallocatedRFbandwidth.③flexibilityinaccommodatingdifferentbasebandsignalformats.AmplitudeFrequencyPhaseAMFMPMAnalogmodulation(varycontinuously)Digitalmodulation(changeindiscretesteps)①moreefficientuseoftheradiospectrum.②usuallyrequireslesspower.CDMA③overafadingcommunicationschannel.④morecompatiblewiththeuseoferrorcorrectingcodes.Incontrasttoanalogmodulation,digitalmodulationhas:§9.1AnalogModulation§9.2BinaryDigitalModulation§9.3ErrorProbabilitiesforBinaryModulation§9.4EffectofRayleighFadingonBitErrorRats§9.5M-aryDigitalModulation§9.1AnalogModulationBasicanalogmodulation:AM(SSB,DSB),FM,PMPPM(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-SCModulationDouble-sidebandsuppressedcarrier(DSB-SC)

Conclusion:

theDSB-SCdemodulatorimprovestheinputsignal-to-noiseratiobyafactoroftwo.FSSB=2FDSBButConclusion:

ThecoherentSSBandDSB-SCdemodulatorshavethesameSNRperformance.3.DSB-LCModulationAdvantage:thecarriersignalcanbeusedasareferencesignaltophase-lookthelocaloscillatortosynchronizationwiththeincomingsignal.Double-SidebandLarge-Carrier(DSB-LC)Disadvantage:thecarrierpowerincreasesthetotalinputpowerbutdoesnotcontainanymodulationinformation.

m:modulationindexIFm>>1,DSB-LC

DSB-SCm=1impliesareductioninSNRof4.8dB.4.EnvelopeDetectionof

DSB-LCModulation

AdvantageofDSB-LC:canbedetectedbyusinganenvelopedetector,withoutaLOandmixer,resultsinamuchsimplereceivercircuit,suchasbroadcastAMradio.noncoherentdemodulationSeriousdistortionforsmallSi/NiSNRIf|m|>1overmodulation,itwillnotcorrectlyrecoverthemodulatingwaveform.5.FrequencyModulation(FM)Application:Broadcastradio,televisionsound,two-wayvoiceradio,AMPScellulartelephonesystem.AnFMwaveform:

Where,modulatingsignal

Modulationindex:

Δω:Maximumfrequencydeviation.Spectrum:①sidebandsarespacedatfmoneithersideofthecarrieratωIF.②amplitudes,givenbyAJn(

),decreaseforlargen.Carson’srule

↑,B↑IFbandwidthThroughdifferentiatorandenvelopedetection,theoutputvoltage:

FortheequaltransmitpowerImprovingfactor:Conclusion:FMallowsanimprovementinSNRattheexpenseofincreasedbandwidth,whileAMdoesnot.

↑,S/N↑,B↑

=4SNRFM=72SNRDSB-LC(m=1)BFM=5BDSB§9.2BinaryDigitalModulationAmplitudeshiftkeying(ASK)Frequencyshiftkeying(FSK)Phaseshiftkeying(PSK)1.BinarySignals2.AmplitudeShiftKeying3.FrequencyShiftKeying(FSK)4.PSK5.CarrierSynchronization1.BinarySignalsReturn-to-ZerocodeNon-Return-to-ZerocodePolarNRZcodeDC=02.AmplitudeShiftKeyingwherem(t)=0or1(a)ModulatorIdenticaltotheDSB-SCmodulatorSynchronousdemodulation:Afterlow-passfiltering

(b)SynchronousdemodulationNotice:LOhaspreciselythesamephaseandfrequencyastheincomingsignal,ordistortionmaybeintroduced.Envelopedetectionnoncoherent,noLO(c)Envelopedetection

3.FrequencyShiftKeying(FSK)

Δω--frequencydeviation

IFω=ω1,theupbranchoutput:1/2Thedownbranchoutput:0IFω=ω2,theupbranchoutput:0Thedownbranchoutput:1/2reversephaseItrequirestwocoherentLOoperatingatω1,andω2.(PLLdetector,thecontrolvoltageofVCOinPLL)Envelopedetector

4.PSKThephaseofthecarrierwaveis“0

”or“180

”.m(t)=1or-1

Duetothesharptransitionscausedbyphasereversal,thespectrumofthePSKwaveformisrelativelywideinbandwidth,resultingthatPSKisimpracticalformultichannelwirelesssystems.PSKmodulatorASK:Non-constantenvelopemodulationCoherentdemodulationNoncoherentdemodulation(envelopedetection)FSK:

ConstantenvelopemodulationCoherentdemodulationNoncoherentdemodulation(afterconversion)PSK:ConstantenvelopemodulationCoherentdemodulation5.CarrierSynchronization

Theeffectofaphaseerror

isthattheoutputsignalisreducedinamplitudebycos,

whileanerrorΔωinfrequencyintroducesafactorofcos

t.Thebiterrorratesofenvelopedetectionarenotasgoodasthoseobtainedwithcoherentdetection.Twowaystorealizesynchronization:①transmitapilotcarrier,usedtophase-locktheLO.②useacarrier-recoverycircuit.Useaphase-lockedlooporbyfrequencymultiplieranddivider.Infact,employingdigitalsignalprocessing(DSP)circuitstoperformallfunctionofsignalconditioning,carrierrecoveryandsynchronizationdemodulation,andsignalformatting.§9.3ErrorProbabilitiesforBinaryModulation

Thepresenceofnoiseinacommunicationchannelintroducesthepossibilitythaterrorswillbemadeduringthedetectionprocess.1.PCMSignalandDetectors2.SynchronousASK3.SynchronousPSK4.SynchronousFSK5.BitrateandBandwidthEfficient6.ComparisonofASKFSKandPSKSystems1.PCMSignalandDetectorsPulsecodedmodulation(PCM)whereDefine:bitenergyTheoutputnoisepowerThevarianceofthegaussianprobabilitydistributionfunction2.SynchronousASKIdealifs(t)=s2(t)=0,s0(T)=0ifs(t)=s1(t)=V,s0(T)=VTPracticalifs0(T)+n0(T)<VT/2,m(t)=0ok!ifs0(T)+n0(T)>VT/2,m(t)=1×Let

3.SynchronousPSK

If

Thresholdlevel:0ASK:>VT/2Forthesameprobabilityoferror,PSKrequiresonlyone-fourththepowerofanASKsystem.SinceanASKsignalisoffhalfthetime,intermsofaveragetransmitpower,thePSKresultisbetterbyafactoroftwo(3dB).DuetosymmetryofthePSKsignalandthedemodulator.4.SynchronousFSKThreshold:0→thesignallevelsaresimilartothePSKcase.Forthenoisevoltages:

n1andn2areuncorrelated.

Conclusion:

ThetotalnoisepoweroftheFSKdemodulatorisdoubledrelativetothesynchronousASKorPSKdemodulator.Conclusion:①synchronousFSKrequires3dBmoresignalpowerthanequivalentPSKsystemforthesameprobabilityoferror.②synchronousFSKrequires3dBlesspowerthananASKsystemonapeakpowerbasis.③FSKandASKhaveequalerrorrateswhencomparedintermsofaveragetransmitpower.5.BitRateandBandwidthEfficientDefineRb:

dimensionofEb:W

Sdimensionofn0:W/HzEb/n0:dimensionlessthebitrateofthebinarymessagesignaldimensionofRb:bps(bitpersecond)Thesignalpower:

Itmeanstheerrorratewillincreasewithanincreaseinbitrate,forafixednoisepowerspectrumdensity,andisindependentofthereceiverbandwidth.IFbandwidthΔf<RFbandwidthRb:dependonthetypeofmodulation

ΔfmayrangefromonetoseveraltimesthebitrateRb.S/Nbeforedemodulation=S/Nafterdemodulation.Referencetoequation(9.2),(9.3),(9.4)and(9.64,(9.65).bandwidthefficiency(bps/Hz)of1bps/Hz.Forabinarymodulationmethod,transmittingonebiteachbitperiod.meansthatThisisforthebaseband,notfortheIForRF.6.ComparisonofASKFSK

andPSKSystemsCoherentPSK:Thelowesterrorrate,9.6dBfor10-5,highpriceforLOandwidesignalbandwidth(2Rb~4Rb).Bestinfadingenvironments.Usedinspaceandsatellitecommunications.CoherentFSK:

Requires3~4dBmorepowerthanPSK.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.

ASKFSKPSK4

Eb/n0

2

Eb/n0Eb/n015.6dB12.6dB9.6dBEb/n0forPe=10-5

§9.4EffectofRayleighFading

onBitErrorRatsPDFofaRayleighfadingiswhere

isthermsvalueofthedistributionofr(t).1.EffectofRayleighFading

onCoherentPSKWhereistheaveragereceivedbitenergy-to-noisepowerspectraldensityratioofthefadedreceivedsignal.2.EffectofRayleighFading

onNoncoherent

FSKForenvelopedetectionofFSK3.ComparisonofFadedand

NonfadedErrorRatesConclusion:①Fadinghastheeffectofdramaticincreasingtherequiredbitenergy-to-noiseratio.②Error-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.ChannelCapacity1.QuadraturePhaseShiftKeyingn=2,M=4+=ThecarrierofQPSK:

Note:

EachQPSKphasestatecanbeusedtorepresenttwobitsofdata.ThebandwidthoftheQPSKspectrumisnarrowerthanthespectrumofaBPSKsignal,becauseoftheaveragetransitionbetweenphasestatesis900.TheoutputoftheQPSKmodulatorisadoublesidebandsuppressedcarriersignal.TheQPSKoutputisaconstantenvelopesignal.Thechannelshouldbeconstantgroupdelay.BlockdiagramofaQPSKmodulatorGrayCodingAdvantage:Whenanerroroccurs,itismostlikelythatonlyoneofthebitswillbeinerror,ratherthanbothbits.S04501,1S113500,1S2-13500,0S3-4501,02.ProbabilityofErrorforQPSKTheoverallprobabilityoferrorforasymbolis:

AsymbolerrorismostlikelytocauseonlyasinglebiterrorforGraycoding.Sinceeachsymbolcontainstwobits,thebiterrorrateforQPSKwillbeone-halfthesymbolerrorrate:forBPSK

LedtotheextensiveuseofQPSKmodulationinawidevarietyofapplications,such