数据与计算机通信第9章

WilliamStallings

DataandComputerCommunications

7thEditionChapter9SpreadSpectrum主要内容9.1TheConceptofSpreadSpectrum9.2Frequency-HoppingSpreadSpectrum9.3DirectSequenceSpreadSpectrum9.4Code-DivisionMultipleAccess9.1SpreadSpectrumAnalogordigitaldataAnalogsignalSpreaddataoverwidebandwidthMakesjammingandinterceptionharderFrequencyhopingSignalbroadcastoverseeminglyrandomseriesoffrequenciesDirectSequenceEachbitisrepresentedbymultiplebitsintransmittedsignalChippingcodeSpreadSpectrumConceptInputfedinto(流入,fed是Feed的过去时)channelencoderProducesnarrowbandwidthanalogsignalaroundcentralfrequencySignalmodulatedusingsequenceofdigitsSpreadingcode/sequenceTypicallygeneratedbypseudonoise/pseudorandom(伪随机)

numbergeneratorIncreasesbandwidthsignificantlySpreadsspectrumReceiverusessamesequencetodemodulatesignalDemodulatedsignalfedintochanneldecoderGeneralModelofSpreadSpectrumSystemGains(结果)ImmunityfromvariousnoiseandmultipathdistortionIncludingjammingCanhide/encryptsignalsOnlyreceiverwhoknowsspreadingcodecanretrievesignalSeveraluserscansharesamehigherbandwidthwithlittleinterferenceCellulartelephonesCodedivisionmultiplexing(CDM)Codedivisionmultipleaccess(CDMA)PseudorandomNumbersGeneratedbyalgorithmusinginitialseedDeterministicalgorithm(确定算法)NotactuallyrandomIfalgorithmgood,resultspassreasonabletestsofrandomnessNeedtoknowalgorithmandseedtopredict(预知)sequence9.2FrequencyHoppingSpreadSpectrum(FHSS)SignalbroadcastoverseeminglyrandomseriesoffrequenciesReceiverhopsbetweenfrequenciesinsyncwithtransmitterEavesdroppers(窃听者)hearunintelligible(无法理解的)blips(短促的尖音信号,哗哗的声音)JammingononefrequencyaffectsonlyafewbitsBasicOperationTypically2kcarriersfrequenciesforming2kchannelsChannelspacingcorrespondswithbandwidthofinputEachchannelusedforfixedinterval300msinIEEE802.11SomenumberofbitstransmittedusingsomeencodingschemeMaybefractionsofbit(seelater)SequencedictatedbyspreadingcodeFrequencyHoppingExampleFrequencyHoppingSpreadSpectrumSystem(Transmitter)FrequencyHoppingSpreadSpectrumSystem(Receiver)SlowandFastFHSSFrequencyshiftedeveryTcsecondsDurationofsignalelementisTssecondsSlowFHSShasTc

TsFastFHSShasTc<TsGenerallyfastFHSSgivesimprovedperformanceinnoise(orjamming)SlowFrequencyHopSpreadSpectrumUsingMFSK(M=4,k=2)FastFrequencyHopSpreadSpectrumUsingMFSK(M=4,k=2)FHSSPerformanceConsiderationsTypicallylargenumberoffrequenciesusedImprovedresistancetojamming9.3DirectSequenceSpreadSpectrum(DSSS)直接序列EachbitrepresentedbymultiplebitsusingspreadingcodeSpreadingcodespreadssignalacrosswiderfrequencybandInproportion(比例)tonumberofbitsused10bitspreadingcodespreadssignalacross10timesbandwidthof1bitcodeOnemethod:CombineinputwithspreadingcodeusingXORInputbit1invertsspreadingcodebitInputzerobitdoesn’talterspreadingcodebitDatarateequaltooriginalspreadingcodePerformancesimilartoFHSSDirectSequenceSpreadSpectrumExampleDirectSequenceSpreadSpectrumTransmitterDirectSequenceSpreadSpectrumTransmitterDirectSequenceSpreadSpectrumUsingBPSKExampleApproximate

(近似)

Spectrumof

DSSSSignal9.4CodeDivisionMultipleAccess(CDMA)MultiplexingTechniqueusedwithspreadspectrumStartwithdatasignalrateDCalledbitdatarateBreakeachbitintokchips(片段,片码)accordingtofixedpatternspecifictoeachuserUser’scodeNewchannelhaschipdataratekDchipspersecondE.g.k=6,threeusers(A,B,C)communicatingwithbasereceiverRCodeforA=<1,-1,-1,1,-1,1>CodeforB=<1,1,-1,-1,1,1>CodeforC=<1,1,-1,1,1,-1>CDMAExampleCDMAExplanationConsiderAcommunicatingwithbaseBaseknowsA’scodeAssumecommunicationalreadysynchronizedAwantstosenda1Sendchippattern<1,-1,-1,1,-1,1>A’scodeAwantstosend0Sendchip[pattern<-1,1,1,-1,1,-1>ComplementofA’scodeDecoderignoresothersourceswhenusingA’scodetodecodeOrthogonalcodesCDMAforDSSSnuserseachusingdifferent