毕业设计(论文)外文资料翻译系别：电子信息系专业：通信工程班级：姓名：学号：外文出处：网络资源附件：1.原文；2.译文2013年03月DigitalCommunications1INTRODUCTIONInthisbook,wepresentthebasicprinciplesthatunderlietheanalysisanddesignofdigitalcommunicationsystems.Thesubjectofdigitalcommunicationsinvolvesthetransmissionofinformationindigitalformfromasourcethatgeneratestheinformationtooneormoredestinations.Ofparticularimportanceintheanalysisanddesignofcommunicationsystemsarethecharacteristicsofthephysicalchannelsthroughwhichtheinformationistransmitted.Thecharacteristicsofthechannelgenerallyaffectthedesignofthebasicbuildingblocksofthecommunicationsystem.Below,wedescribetheelementsofacommunicationsystemandtheirfunctions.1.1ELEMENTSOFADIGITALCOMMUNICATIONSYSTEMFigure1.1illustratesthefunctionaldiagramandthebasicelementsofadigitalcommunicationsystem.Thesourceoutputmaybeeitherananalogsignal,suchasaudioorvideosignal,oradigitalsignal,suchastheoutputofateletypemachine,thatisdiscreteintimeandhasafinitenumberofoutputcharacters.Inadigitalcommunicationsystem,themessagesproducedbythesourceareconvertedintoasequenceofbinarydigits.Ideally,weshouldliketorepresentthesourceoutput(message)byasfewbinarydigitsaspossible.Inotherwords,weseekanefficientrepresentationofthesourceoutputthatresultsinlittleornoredundancy.Theprocessofefficientlyconvertingtheoutputofeitherananalogordigitalsourceintoasequenceofbinarydigitsiscalledsourceencodingordatacompression.Thesequenceofbinarydigitsfromthesourceencoder,whichwecalltheinformationsequence,ispassedlothechannelencoder.Thepurposeofthechannelencoderistointroduce,inacontrolledmanner,someredundancyinthebinaryinformationsequencethatcanbeusedatthereceivertoovercometheeffectsofnoiseandinterferenceencounteredinthetransmissionofthesignalthroughthechannel.Thus,theaddedredundancyservestoincreasethereliabilityofthereceiveddataandimprovesthefidelityofthereceivedsignal.Ineffect,redundancyintheinformationsequenceaidsthereceiverindecodingthedesiredinformationsequence.Forexample,a(trivial)formofencoding