Chapter_1._Introduction_Notes

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1、BlockDiagramofDigitalCommunicationSystem信息InformationInformationSourceDestination来源Channel目的地通道…expandingthemodel调制器sourcechannelsourcemodulatorencoderencoderDigitale.g.voice编码器channelBW,A/D,signal‘boosting’carrierredundancytocrosschannel+noise+sourcechanneldestinationdemodulator

2、decoderdecoder解码器RemovecarrierdataErrorchecks,D/AConverttobinaryFigure1SourceCoding目标•Theobjectiveofsourceencodingistoreducethenumberofbitstransmittedforeachmessagebytakingadvantageofanyredundancyinthemessage.Forexampleifonecan准确传达conveyamessageaccuratelybytransmitting4bits,theni

3、tisinefficienttotransmitthatsamemessageusing7bits.Inordertosend7bitsinthesameamountoftimewouldrequirefastertransmissionspeedandhigherbandwidth.•Insourcecoding,thesourceencodermapsthedigitalmessagesignalgeneratedatthesourceoutputintoanothersignalindigitalform.Themappingisone-to-on

4、e(i.e.foreachsourcemessagethereisonlyoneuniquesourceencoderoutput),andtheobjectiveisto消除eliminateorreduceredundancysoastoprovideanefficientrepresentationofthesourceoutput.冗余•Sincethesourceencodermappingisone-to-one,thesourcedecodersimplyperformstheinversemappingandtherebydelivers

5、tothedestinationareproductionoftheoriginaldigitalsourceoutput.Theprimarybenefitthusgainedfromtheapplicationofsourcecodingisa带宽压缩技术reducedbandwidthrequirement.(Allcompressiontechniquesusedintransmittingdatausesomeformofsourceencodingmethod.)编码ChannelCoding•Theobjectiveofchannelcod

6、ingistoreducetheeffectofnoiseonthetransmittedsignalasitpassesthroughthechannel.•Inchannelcoding,thechannelencodermapstheincomingdigitalsignalintoachannelinput.Thechanneldecodermapsthechanneloutputintoanoutputdigitalsignalinsuchawaythattheeffectofchannelnoiseisminimised.Thecombine

7、droleofthechannel可靠encoderanddecoderistoprovideforreliablecommunicationoveranoisychannel.Thisprovisionissatisfiedbyintroducingredundancytothesignalrepresentingthemessageinaprescribedfashioninthechannelencoder(beforetransmittingitintothe利用channel),andexploitingthisredundancyinthec

8、hanneldecodertoreconstructtheoriginalcha