无线通信基础(双语)-教学课件-13

ReviewofLastClass3.4.2SpreadingCodes3.4.1Spread-SpectrumPrinciples

IncreasedtolerancetointerferenceLowprobabilityofdetectionorinterceptionIncreasedtolerancetomultipathIncreasedrangingcapabilityCansharethesamefrequencybandwithotherusers3.4.1Spread-SpectrumPrinciplesWhySpreadSpectrum?3.4.2SpreadingCodesAutocorrelationTheconceptofdetermininghowmuchsimilarityofashiftedcopyofasequencewithitselfCrosscorrelation

ThecomparisonbetweentwosequencesfromdifferentsourcesratherthanashiftedcopyofasequencewithitselfIntroductionProperty1:

Has2r-1onesand2r-1-1zerosProperty2:

GoodShiftcorrelationpropertyProperty3:3.4.2SpreadingCodesm-Sequences

IncreasedtolerancetointerferenceLowprobabilityofdetectionorinterceptionIncreasedtolerancetomultipathIncreasedrangingcapabilityCansharethesamefrequencybandwithotherusers3.4.2SpreadingCodesIncreasedtolerancetomultipath3.4.2SpreadingCodesPath11111000010Path21111000010TsIncreasedrangingcapability3.4.2SpreadingCodesPN1111000010DataSymbolCansharethesamefrequencybandwithotherusers

3.4.2SpreadingCodesGoldsequencesconstructedbytheXORoftwom-sequenceswiththesameclockingCodeshavewell-definedcrosscorrelationpropertiesOnlysimplecircuitryneededtogeneratelargenumberofuniquecodes3.4.2SpreadingCodesGoldSequences3.4.2SpreadingCodesGoldSequences3.4.2SpreadingCodesWalshcodesChapter3----Lecture5SpreadSpectrumCommunication(2)3.4SpreadSpectrum3.4.2SpreadingCodes3.4.1Spread-SpectrumPrinciples3.4.3Direct-SequenceSpreadSpectrum3.4.4Frequency-HoppedSpreadSpectrum

3.4.5CodeDivisionMultipleAccess

3、Direct-SequenceSpreadSpectrumIntroductionProcessGainandJammingMarginCodeSynchronizationRakeReceiver3.4.3Direct-SequenceModulationMessagesignalmultipliedbyalargeBW“spreadingsignal”Pseudo-noisecodesequencewitha“chiprate”severalordersofmagnitudelargerthanthedatarateofthemessageEachbitinoriginalsignalrepresentedbymultiplebitsintransmittedsignalSSsignalshavenoise-likepropertiesPNsequenceappearsrandombutcanbereproducedinadeterministicmannerIntroduction3.4.3Direct-SequenceModulationIntroductionSpreadDespreadIntroduction3.4.3Direct-SequenceModulationInaspreadspectrumsystem,theprocessgain(or‘processinggain')istheratioofthespreadbandwidthtotheunspreadbandwidth.Itisusuallyexpressedindecibels(dB).Theprocessgainistheratiobywhichunwantedsignalsorinterferencecanbesuppressedrelativetothedesiredsignalwhenbothsharethesamefrequencychannel.ProcessGainandJammingMargin3.4.3Direct-SequenceModulationIfa1KHzsignalisspreadto100KHz,theprocessgainwouldbe?Example3.3Theprocessgainexpressedasanumericalratiowouldbe100,000/1,000=100Orindecibels,10log10(100)=20dB3.4.3Direct-SequenceModulationProcessGainandJammingMarginJammingmarginTellsthemagnitudeofadditionalinterferenceandnoisethatcanbeinjectedtothechannelwithouthazardingsystemoperation.3.4.3Direct-SequenceModulationCodeSynchronization3.4.3Direct-SequenceModulationSynchronizationuncertaintyTimeuncertaintyDistancebetweenTx-Rx(propagationdelay)DifferentphasebetweenTx-Rx(PNsequence)Twosteps:Acquisition(coarsealignment)Tracking(finealignment)3.4.3Direct-SequenceModulation

AcquisitionPhaseReceivedsignalandthelocallygeneratedPNsequencearecorrelatedwithacoarsetimesteptoproduceameasureofsimilaritybetweenthetwo.CodeSynchronization3.4.3Direct-SequenceModulation-1-111-11-1

1-11-1-1-11-1

11-11-1-1

-1

-1-111-11-1-1

-1-1-111-11-17Acquisition-1-111-11-1-1-111-11-1CodeSynchronization3.4.3Direct-SequenceModulationDZk(1-PF)ZZkPZPFZ(1-PF)Z(1-PD)Z(1-PF)ZPFZZkPFZCodeSynchronization3.4.3Direct-SequenceModulationSearchtimeisreducedattheexpenseofamorecomplexandcostlyimplementationCodeSynchronization3.4.3Direct-SequenceModulation

TrackingPhaseCodeSynchronizationRakeReceiver3.4.3Direct-SequenceModulationPath11111000010Path211110000104、Frequency-HoppedSpreadSpectrumIntroductionIncreasedtolerancetointerference3.4.4Frequency-HoppedSpreadSpectrumIntroduction3.4.4Frequency-HoppedSpreadSpectrum

Datasentbyhoppingtransmittercarriertoseeminglyrandomchannelswhichareknownonlytodesiredreceiver，Receiver“de-hops”receivedsignaltoobtaintransmittedmessagesignalOneachchannel,databurstsaresentusingconventionalnarrowbandmodulationSpectrumBWoverwhichhoppingoccursiscalledthetotalhoppingBW=WssProcessinggain=Wss/B

Introduction3.4.4Frequency-HoppedSpreadSpectrumFastfrequencyhoppingMorethanonefrequencyhopduringeachtransmittedsymbolSlowfrequencyhoppingOneormoresymbolssentintimeintervalbetweenhopsIntroduction3.4.4Frequency-HoppedSpreadSpectrumfLfHIncreasedtolerancetointerference3.4.4Frequency-HoppedSpreadSpectrumd2d1d3SignalInterferenceIncreasedtolerancetointerference5、CodeDivisionMultipleAccess

Introduction

CodeDivisionMultipleAccess

FrequencyHoppedMultipleAccessComparisonofDSMAandFHMAThemeExamples3.4.5CodeDivisionMultipleAccessIntroductionTwotechniquesFrequencyHoppedMultipleAccess(FHMA)DirectSequenceMultipleAccess(DSMA)AlsocalledCodeDivisionMultipleAccess–CDMA3.4.5CodeDivisionMultipleAccessCodeDivisionMultipleAccessDownLinkUpLink3.4.5CodeDivisionMultipleAccessCodeDivisionMultipleAccessUpLink:3.4.5CodeDivisionMultipleAccessCodeDivisionMultipleAccessUse1:Use2:11-1-1-11-1-111-1-1-11Despreadingcode111-1-1-11-1

020-2-2006

1-1-1111-1

20-2002-28Despreadingcode2

-111-1-1

-116-8UpLink:3.4.5CodeDivisionMultipleAccessCodeDivisionMultipleAccessDownLink:3.4.5CodeDivisionMultipleAccessFeaturesofDS-CDMASystems

Signalsshareacommonwidebandchannelatthesametime

SignalsaredistinguishedfromeachotherbyusingdifferentPNcodes

Receiverprocessesonlythedesiredsignal.Allotherusers’signalsappearasinterference(multipleaccessinterference)tothedesiredsignal

DS-CDMAsystemisinterference-limited.However,TDMAandFDMAarebandwidth-limited3.4.5CodeDivisionMultipleAccessAdvantagesofDS-CDMASystems

Highspectrumefficiency

Lowmobilestationtransmitpower

Soft-handover

PathDiversity(RAKEFingers)

Security

Anti-jammingcapability3.4.5CodeDivisionMultipleAccess

Difficulttoacquireandmaintainthesynchronization

Powercontrolisneededtocombatthenear-farproblemDisadvantagesofDS-CDMASystems3.4.5CodeDivisionMultipleAccessNearFarProblemandPowerControlAtareceiver,thesignalsmaycomefromvarious(multiplesources)

EachsourcemayhavedifferentdistancestothebasestationBMMMMpr(M)3.4.5CodeDivisionMultipleAccessNearFarProblemandPowerControl3.4.5CodeDivisionMultipleAccessFrequencyHoppedMultipleAccess

Awidebandradiochannelisused.Samewidebandspectrumisused

Thecarrierfrequencyofusersarevariedinapseudo-randomfashion.Eachuserisusinganarrowbandchannel(spectrum)ataspecificinstanceoftime.Therandomchangeinfrequencymakethechangeofusingthesamenarrowbandchannelverylo