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1、.word可编辑.Chapter1010.1Achiprateof3.84Mcpscorrespondstoachipintervalof(1/3.84)μsec=0.26μsec.Asnotedattheendofchapter2,inintroducingtheRAKEreceiverconcept,delayspreadsgreaterthanthechipintervalallowindividualmultipathraystoberesolved.Adelayspreadof0.26μseccorrespondstoapathdiff
2、erenceof0.26μsecx3x108m/sec=78m.Atachiprateof1.2288Mcps,thechipintervalis(1/1.2288)μsec=0.81μsec,correspondingtoapathdifferenceof0.81x300=240m.10.2(a)SayabitrateofRbps,withabitwidthof1/Risinitiallyused.Nowdoublethebitrateto2Rbutcollectsuccessivepairsofbitsandtransmittheminpar
3、allel,withawidthof1/Reach.EachofthetwobitsteamstransmittedatRbpsisnowequivalenttotheoneinitialstreamatRbps.Chip-encodingeachofthesetwobitstreamsatthesamechiprateastheinitialRbpsbitstreamresultsinthesamespreadinggainbeingmadeavailable.Thetransmissionbandwidthisthesameinthetwoc
4、asesaswell.Butitisclearthatthetwobitstreamsmustbeseparatelydistinguishable.Thisisdonebychoosingorthogonalcodes,similartotheorthogonalcodesdiscussedatthebeginningofsection6.2inchapter6.(Callingtwoseparatecodesequencesciandcj,orthogonalitymeansci•cj=0,i≠j.)But,asnotedin(6.2),ps
5、eudorandomcodesusedinpracticearenotcompletelyorthogonal,and,justasinthecaseofdifferentusersusingpseudorandomorthogonalcodesdiscussedinpreviousdescriptionsofCDMAsystems,interferencewillresult.Sincetheinterferenceduetoincompleteorthogonalityis,inthiscase,duetointerferencebetwee
6、ncodesofthesameuser,thisiscalledself-interference.Althoughtheexampleusedhereisoneofdoublingthebitrate,itisclearthatthesameanalysisappliestomorethantwoparallelbitstreams,ortomulticodetransmissioningeneral.(See(b)following.)(b)Serial-to-parallelconversion:collectthreesuccessive
7、bits,transmittedatarateof3Randtransmittheminparallel,wideningeachbitto1/R,henceatarateofRbps.(c)Eachbitstreammustbeseparatelydecoded,henceaRAKEreceiverisrequiredforeach.Summingmultiplequasi-uncorrelatedbitstreamsandusingthesumtodrivethemodulatormustresultinawiderenvelopevaria
8、tionattheoutput..专业.专注..word可编辑.10.3TheobjectwithaRAKEreceiveristore
