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SignalEncodingTechniquesChapter6ReasonsforChoosingEncodingTechniquesDigitaldata,digitalsignalEquipmentlesscomplexandexpensivethandigital-to-analogmodulationequipmentAnalogdata,digitalsignalPermitsuseofmoderndigitaltransmissionandswitchingequipmentReasonsforChoosingEncodingTechniquesDigitaldata,analogsignalSometransmissionmediawillonlypropagateanalogsignalsE.g.,opticalfiberandunguidedmediaAnalogdata,analogsignalAnalogdatainelectricalformcanbetransmittedeasilyandcheaplyDonewithvoicetransmissionovervoice-gradelinesSignalEncodingCriteriaWhatdetermineshowsuccessfulareceiverwillbeininterpretinganincomingsignal?Signal-to-noiseratioDatarateBandwidthAnincreaseindatarateincreasesbiterrorrateAnincreaseinSNRdecreasesbiterrorrateAnincreaseinbandwidthallowsanincreaseindatarateFactorsUsedtoCompareEncodingSchemesSignalspectrumWithlackofhigh-frequencycomponents,lessbandwidthrequiredWithnodccomponent,accouplingviatransformerpossibleTransferfunctionofachannelisworsenearbandedgesClockingEaseofdeterminingbeginningandendofeachbitpositionFactorsUsedtoCompareEncodingSchemesSignalinterferenceandnoiseimmunityPerformanceinthepresenceofnoiseCostandcomplexityThehigherthesignalratetoachieveagivendatarate,thegreaterthecostBasicEncodingTechniquesDigitaldatatoanalogsignalAmplitude-shiftkeying(ASK)AmplitudedifferenceofcarrierfrequencyFrequency-shiftkeying(FSK)FrequencydifferencenearcarrierfrequencyPhase-shiftkeying(PSK)PhaseofcarriersignalshiftedBasicEncodingTechniquesAmplitude-ShiftKeyingOnebinarydigitrepresentedbypresenceofcarrier,atconstantamplitudeOtherbinarydigitrepresentedbyabsenceofcarrierwherethecarriersignalisAcos(2πfct)tstfAc2cos01binary0binaryAmplitude-ShiftKeyingSusceptibletosuddengainchangesInefficientmodulationtechniqueOnvoice-gradelines,usedupto1200bpsUsedtotransmitdigitaldataoveropticalfiberBinaryFrequency-ShiftKeying(BFSK)Twobinarydigitsrepresentedbytwodifferentfrequenciesnearthecarrierfrequencywheref1andf2areoffsetfromcarrierfrequencyfcbyequalbutoppositeamountststfA12costfA22cos1binary0binaryBinaryFrequency-ShiftKeying(BFSK)LesssusceptibletoerrorthanASKOnvoice-gradelines,usedupto1200bpsUsedforhigh-frequency(3to30MHz)radiotransmissionCanbeusedathigherfrequenciesonLANsthatusecoaxialcableMultipleFrequency-ShiftKeying(MFSK)MorethantwofrequenciesareusedMorebandwidthefficientbutmoresusceptibletoerrorfi=fc+(2i–1–M)fdfc=thecarrierfrequencyfd=thedifferencefrequencyM=numberofdifferentsignalelements=2LL=numberofbitspersignalelementtfAtsii2cosMi1MultipleFrequency-ShiftKeying(MFSK)Tomatchdatarateofinputbitstream,eachoutputsignalelementisheldfor:Ts=LTsecondswhereTisthebitperiod(datarate=1/T)So,onesignalelementencodesLbitsMultipleFrequency-ShiftKeying(MFSK)Totalbandwidthrequired2MfdMinimumfrequencyseparationrequired2fd=1/TsTherefore,modulatorrequiresabandwidthofWd=2L/LT=M/TsMultipleFrequency-ShiftKeying(MFSK)Phase-ShiftKeying(PSK)Two-levelPSK(BPSK)UsestwophasestorepresentbinarydigitststfAc2costfAc2cos1binary0binarytfAc2costfAc2cos1binary0binaryPhase-ShiftKeying(PSK)DifferentialPSK(DPSK)PhaseshiftwithreferencetopreviousbitBinary0–signalburstofsamephaseasprevioussignalburstBinary1–signalburstofoppositephasetoprevioussignalburstPhase-ShiftKeying(PSK)Four-levelPSK(QPSK)Eachelementrepresentsmorethanonebitts42costfAc11432costfAc432costfAc42costfAc010010Phase-ShiftKeying(PSK)MultilevelPSKUsingmultiplephaseangleswitheachanglehavingmorethanoneamplitude,multiplesignalselementscanbeachievedD=modulationrate,baudR=datarate,bpsM=numberofdifferentsignalelements=2LL=numberofbitspersignalelementMRLRD2logPerformanceBandwidthofmodulatedsignal(BT)ASK,PSKBT=(1+r)RFSKBT=2DF+(1+r)RR=bitrate0r1;relatedtohowsignalisfilteredDF=f2-fc=fc-f1PerformanceBandwidthofmodulatedsignal(BT)MPSKMFSKL=numberofbitsencodedpersignalelementM=numberofdifferentsignalelementsRMrRLrBT2log11RMMrBT2log1QuadratureAmplitudeModulationQAMisacombinationofASKandPSKTwodifferentsignalssentsimultaneouslyonthesamecarrierfrequencytftdtftdtscc2sin2cos21QuadratureAmplitudeModulationReasonsforAnalogModulationModulationofdigitalsignalsWhenonlyanalogtransmissionfacilitiesareavailable,digitaltoanalogconversionrequiredModulationofanalogsignalsAhigherfrequencymaybeneededforeffectivetransmissionModulationpermitsfrequencydivisionmultiplexingBasicEncodingTechniquesAnalogdatatoanalogsignalAmplitudemodulation(AM)AnglemodulationFrequencymodulation(FM)Phasemodulation(PM)AmplitudeModulationtftxntsca2cos1AmplitudeModulationcos2fct=carrierx(t)=inputsignalna=modulationindexRatioofamplitudeofinputsignaltocarriera.k.adoublesidebandtransmittedcarrier(DSBTC)SpectrumofAMsignalAmplitudeModulationTransmittedpowerPt=totaltransmittedpowerins(t)Pc=transmittedpowerincarrier212actnPPSingleSideband(SSB)VariantofAMissinglesideband(SSB)SendsonlyonesidebandE
本文标题:无线通信与网络chap6.
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