无线通信与网络chap7.

SpreadSpectrumChapter7SpreadSpectrumInputisfedintoachannelencoderProducesanalogsignalwithnarrowbandwidthSignalisfurthermodulatedusingsequenceofdigitsSpreadingcodeorspreadingsequenceGeneratedbypseudonoise,orpseudo-randomnumbergeneratorEffectofmodulationistoincreasebandwidthofsignaltobetransmittedSpreadSpectrumOnreceivingend,digitsequenceisusedtodemodulatethespreadspectrumsignalSignalisfedintoachanneldecodertorecoverdataSpreadSpectrumSpreadSpectrumWhatcanbegainedfromapparentwasteofspectrum?ImmunityfromvariouskindsofnoiseandmultipathdistortionCanbeusedforhidingandencryptingsignalsSeveraluserscanindependentlyusethesamehigherbandwidthwithverylittleinterferenceFrequencyHopingSpreadSpectrum(FHSS)SignalisbroadcastoverseeminglyrandomseriesofradiofrequenciesAnumberofchannelsallocatedfortheFHsignalWidthofeachchannelcorrespondstobandwidthofinputsignalSignalhopsfromfrequencytofrequencyatfixedintervalsTransmitteroperatesinonechannelatatimeBitsaretransmittedusingsomeencodingschemeAteachsuccessiveinterval,anewcarrierfrequencyisselectedFrequencyHopingSpreadSpectrumChannelsequencedictatedbyspreadingcodeReceiver,hoppingbetweenfrequenciesinsynchronizationwithtransmitter,picksupmessageAdvantagesEavesdroppershearonlyunintelligibleblipsAttemptstojamsignalononefrequencysucceedonlyatknockingoutafewbitsFrequencyHopingSpreadSpectrumFHSSUsingMFSKMFSKsignalistranslatedtoanewfrequencyeveryTcsecondsbymodulatingtheMFSKsignalwiththeFHSScarriersignalFordatarateofR:durationofabit:T=1/Rsecondsdurationofsignalelement:Ts=LTsecondsTcTs-slow-frequency-hopspreadspectrumTcTs-fast-frequency-hopspreadspectrumFHSSPerformanceConsiderationsLargenumberoffrequenciesusedResultsinasystemthatisquiteresistanttojammingJammermustjamallfrequenciesWithfixedpower,thisreducesthejammingpowerinanyonefrequencybandDirectSequenceSpreadSpectrum(DSSS)EachbitinoriginalsignalisrepresentedbymultiplebitsinthetransmittedsignalSpreadingcodespreadssignalacrossawiderfrequencybandSpreadisindirectproportiontonumberofbitsusedOnetechniquecombinesdigitalinformationstreamwiththespreadingcodebitstreamusingexclusive-OR(Figure7.6)DirectSequenceSpreadSpectrum(DSSS)DSSSUsingBPSKMultiplyBPSKsignal,sd(t)=Ad(t)cos(2fct)byc(t)[takesvalues+1,-1]togets(t)=Ad(t)c(t)cos(2fct)A=amplitudeofsignalfc=carrierfrequencyd(t)=discretefunction[+1,-1]Atreceiver,incomingsignalmultipliedbyc(t)Since,c(t)xc(t)=1,incomingsignalisrecoveredDSSSUsingBPSKCode-DivisionMultipleAccess(CDMA)BasicPrinciplesofCDMAD=rateofdatasignalBreakeachbitintokchipsChipsareauser-specificfixedpatternChipdatarateofnewchannel=kDCDMAExampleIfk=6andcodeisasequenceof1sand-1sFora‘1’bit,Asendscodeaschippatternc1,c2,c3,c4,c5,c6Fora‘0’bit,Asendscomplementofcode-c1,-c2,-c3,-c4,-c5,-c6Receiverknowssender’scodeandperformselectronicdecodefunctiond1,d2,d3,d4,d5,d6=receivedchippatternc1,c2,c3,c4,c5,c6=sender’scode665544332211cdcdcdcdcdcddSuCDMAExampleUserAcode=1,–1,–1,1,–1,1Tosenda1bit=1,–1,–1,1,–1,1Tosenda0bit=–1,1,1,–1,1,–1UserBcode=1,1,–1,–1,1,1Tosenda1bit=1,1,–1,–1,1,1ReceiverreceivingwithA’scode(A’scode)x(receivedchippattern)UserA‘1’bit:6-1UserA‘0’bit:-6-0UserB‘1’bit:0-unwantedsignalignoredCDMAforDirectSequenceSpreadSpectrumCategoriesofSpreadingSequencesSpreadingSequenceCategoriesPNsequencesOrthogonalcodesForFHSSsystemsPNsequencesmostcommonForDSSSsystemsnotemployingCDMAPNsequencesmostcommonForDSSSCDMAsystemsPNsequencesOrthogonalcodesPNSequencesPNgeneratorproducesperiodicsequencethatappearstoberandomPNSequencesGeneratedbyanalgorithmusinginitialseedSequenceisn’tstatisticallyrandombutwillpassmanytestofrandomnessSequencesreferredtoaspseudorandomnumbersorpseudonoisesequencesUnlessalgorithmandseedareknown,thesequenceisimpracticaltopredictImportantPNPropertiesRandomnessUniformdistributionBalancepropertyRunpropertyIndependenceCorrelationpropertyUnpredictabilityLinearFeedbackShiftRegisterImplementationPropertiesofM-SequencesProperty1:Has2n-1onesand2n-1-1zerosProperty2:ForawindowoflengthnslidalongoutputforN(=2n-1)shifts,eachn-tupleappearsonce,exceptfortheallzerossequenceProperty3:Sequencecontainsonerunofones,lengthnOnerunofzeros,lengthn-1Onerunofonesandonerunofzeros,lengthn-2Tworunsofonesandtworunsofzeros,lengthn-32n-3runsofonesand2n-3runsofzeros,length1PropertiesofM-SequencesProperty4:Theperiodicautocorrelationofa±1m-sequenceisotherwise...2N,N,0,11τNRDefinitionsCorrelationTheconceptofdetermininghowmuchsimilarityonesetofdatahaswithanotherRangebetween–1and11Thesecondsequencematchesthefirstsequence0Thereisnorelationatallbetweenthetwosequences-1ThetwosequencesaremirrorimagesCrosscorrelationThecomparisonbetweentwosequencesfromdifferentsourcesratherthanashiftedcopyofasequencewithitselfAdvantagesofCrossCorrelationThecrosscorrelationbetweenanm-sequenceandnoiseislowThispropertyisusefultothereceiverinfilteringoutnoiseThecrosscorrelationbetweentwodifferentm-sequencesislowThispropertyisusefulforCDMAapplicat