Rake receiver.pdf

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1、www.WirelessCommunication.NLChapter:AnalogandDigitalTransmissionSection:CDMARakereceiverIf,inamobileradiochannelreflectedwavesarrivewithsmallrelativetimedelays,selfinterferenceoccurs.DirectSequence(DS)SpreadSpectrumisoftenclaimedtohaveparticularpropertiesthatmakesitlessvulnerabletomultipathrecep

2、tion.Inparticular,therakereceiverarchitectureallowsanoptimalcombiningofenergyreceivedoverpathswithdifferent.Itavoidswavecancellation(fades)ifdelayedpathsarrivewithphasedifferencesandappropriatelyweighssignalscominginwithdifferentsignal-to-noiseratios.Therakereceiverconsistsofmultiplecorrelators,

3、inwhichthereceivesignalismultipliedbytime-shiftedversionsofalocallygeneratedcodesequence.Theintentionistoseparatesignalssuchthateachfingeronlyseessignalscominginoverasingle(resolvable)path.Thespreadingcodeischosentohaveaverysmallautocorrelationvalueforanynonzerotimeoffset.Thisavoidscrosstalkbetw

4、eenfingers.Inpractice,thesituationislessideal.Itisnotthefullperiodicautocorrelationthatdeterminesthecrosstalkbetweensignalsindifferentfingers,butrathertwopartialcorrelations,withcontributionsfromtwoconsecutivebitsorsymbols.Ithasbeenattemptedtofindsequencesthathavesatisfactorypartialcorrelationva

5、lues,butthecrosstalkduetopartial(non-periodic)correlationsremainssubstantiallymoredifficulttoreducethantheeffectsofperiodiccorrelations.TherakereceiverisdesignedtooptimallydetectedaDS-CDMAsignaltransmittedoveradispersivemultipathchannel.Itisanextensionoftheconceptofthematchedfilter.Figure:Matche

6、dfilterreceiverforAWGNchannel.Inthematchedfilterreceiver,thesignaliscorrelatedwithalocallygeneratedcopyofthesignalwaveform.If,however,thesignalisdistortedbythechannel,thereceivershouldcorrelatetheincomingsignalbyacopyoftheexpectedreceivedsignal,ratherthanbyacopyoftransmittedwaveform.Thustherecei

7、vershouldestimatethedelayprofileofchannel,andadaptitslocallygeneratedcopyaccordingtothisestimate.Inamultipathchannel,delayedreflectionsinterferewiththedirectsignal.However,aDS-CDMAsignalsufferingfrommultipathdispersioncanbed