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1、目錄Lesson11DigitalCommunication1Lesson217SourceCoding17Lesson326DigitalModulation26Lesson426OptimumReceivers26Lesson526SpreadSpectrumSignalsforDigitalCommunications26Lesson626MultiuserCommunications26Lesson726IntroductonofDigitalCommunicationoverFadingChannels2663Lesson1DigitalCommunicati
2、onInthisbook,wepresentthebasicprinciplesthatunderlinetheanalysisanddesignofdigitalcommunicationsystem.Thesubjectofdigitalcommunicationsinvolvesthetransmissionofinformationindigitalformfromasourcethatgeneratestheinformationtooneormoredestinations.Ofparticularimportanceintheanalysisanddesi
3、gnofcommunicationsystemsarethecharacteristicsofthephysicalchannelsthroughwhichtheinformationistransmitted.Thecharacteristicsofthechannelgenerallyaffectthedesignofthebasicbuildingblocksofthecommunicationsystems.Below,wedescribetheelementsofacommunicationsystemandtheirfunctions.1-1Elements
4、ofaDigitalCommunicationSystemFigure1-1illustratesthefunctionaldiagramandthebasicelementsofadigitalcommunicationsystem.Thesourceoutputmaybeeitherananalogsignal,suchasaudioorvideosignal,oradigitalsignal,suchastheoutputofateletypemachine,thatisdiscreteintimeandhasafinitenumberofoutputcharac
5、ters.Inadigitalcommunication,themessagesproducedbythesourceareconvertedintoasequenceofbinarydigits.Ideally,weshouldliketorepresentthesourceoutput(message)byasfewbinarydigitsaspossible.Inotherwords,weseekanefficientrepresentationofthesourceoutputthatresultsinlittleornoredundancy.Theproces
6、sofefficientlyconvertingtheoutputofeitherananalogordigitalsourceintoasequenceofbinarydigitsiscalledsourceencodingordatacompression.Thesequenceofbinarydigitsfromthesourceencoder,whichwecalltheinformationsequence,ispassedtothechannelencoder.Thepurposeofthechannelencoderistointroduce,inacon
7、trolledmanner,someredundancyinthebinaryinformationsequencethatcanbeusedatthereceivertoovercometheeffectsofnoiseandinterferenceencounteredinthetransmissionofthesignalthroughthechannel.Thus,theadded63redundancyservestoincreasethereliabilityofthereceiveddataandimprovesthefid
