移动无线通信第11章

1、Chapter10Achiprateof3.84Mcpscorrespondstoachipintervalof(1/3.84)卩sec=0.26卩seccorrespondstoapathdiffereneeof0.268m/sec=78secx3x10m.Atachiprateof1.2288Mcps,thechipintervalis(1/1.2288)卩sec=0.81卩seccorrespondingtoapathdifferenceof0.81x300=240m.(a)SayabitrateofRbps,withabitwidthof1/Risinitiallyused.Nowdo

2、ublethebitrateto2Rbutcollectsuccessivepairsofbitsandtransmittheminparallel,withawidthof1/Reach.EachofthetwobitsteamstransmittedaRtbpsisnowequivalenttotheoneinitialstreamatRbps.Chip-encodingeachofthesetwobitstreamsatthesamechiprateastheinitialRbpsbitstreamresultsinthesamespreadinggainbeingmadeavailab

3、le.Thetransmissionbandwidthisthesameinthetwocasesaswell.Butitisclearthatthetwobitstreamsmustbeseparatelydistinguishable.Thisisdonebychoosingorthogonalcodes,similartotheorthogonalcodesdiscussedatthebeginningofsectioninchapter6.(Callingtwoseparatecodesequencecsiandcj,orthogonalitymeanso?cj=0,i壬)jBut,a

4、snotedin(6.2),pseudorandomcodesusedinpracticearenotcompletelyorthogonal,and,justasinthecaseofdifferentusersusingpseudorandomorthogonalcodesdiscussedinpreviousdescriptionsofCDMAsystems,interferencewillresult.Sincetheinterferenceduetoincompleteorthogonalityis,inthiscase,duetointerferencebetweencodesof

5、thesameuser,thisiscalledself-interference.Althoughtheexampleusedhereisoneofdoublingthebitrate,itisclearthatthesameanalysisappliestomorethantwoparallelbitstreams,ortomulticodetransmissioningeneral.(Se(eb)following.)Serial-to-parallelconversion:collectthreesuccessivebits,transmittedatarateof3Randtrans

6、mittheminparallel,wideningeachbitto1R/,henceatarateofRbps.Eachbitstreammustbeseparatelydecoded,henceaRAKEreceiverisrequiredforeach.Summingmultiplequasi-uncorrelatedbitstreamsandusingthesumtodrivethemodulatormustresultinawiderenvelopevariationattheoutput.TheobjectwithaRAKEreceiveristoreceiveseparate,

7、distinguishablereplicasofasignal.Thismeans,asdiscussedinchapter2,usingadigitalsignalmuchnarrowerthanthedelayspread.ThisisaccomplishedinCDMAsystemsbythechip-encodingprocess.Referringbackto(2.48)and(2-49)inchapter2,thisresultsinasignalwhosetransmissionbandwidthisgreaterthanthecoherencebandwidth,or,equ

8、ivalently,aconditionunderwhichfrequency-selectivefadingresults.(a)Theuseofrate-1/2convolutionalencoding,withtailbitsrequired,plusanyadditionalerrorcorrectionrequired,resultsinatransmissionratemorethandoubletheactualdatarate.Alocal-areaenvironment,withmuchshorterpropagationdistances,provideshigherrec

9、eived-power,hencehigherreceived-energyconditions,thandoesawider-areaenvironment.Hencehigherbitratesarepossible.(SeeSec.6.4ofchapter6.)(c)Asnotedinchapters5and6,PSKprovidesbetterbit-errorperformanceininterferenceandnoisethandoesQPSK.Thebasestation,transmittinginthedownlinkdirection,canusehigher-power

10、transmissionthanamobileintheuplinkdirection.ItcanthereforeuseQPSKtransmission,allowinghigherbitratestobeusedinthedownlinkdirection.Peopleengagedinreal-timeinteractiveconversationcannottoleratedelayslongerthan50-100msecintransmissionoftheinformationpacketssentbetweenusers.(Thiswasnotedmanyyearsagowit

11、hsatellitetransmissionofvoice.)Asimilarproblemariseswhensuccessivepacketsencountervariabledelays.Receivedpacketsmustbealignedtohavethesamespacingaswhentransmitted.Thismeansdelayingthefirstpacketenoughtoalignallsuccessivepacketsfollowing.Iftherealignmentdelayistoohigh,thisagaincreatesanintolerablepro

12、blem.Inthestreamingclasscase,withouttwo-sideinteractiontakingplace(examples-streamingmusicorvideo),delayintransmissiongenerallyposesnoproblem.Butreconstructingtheoriginalbitstreamfromsuccessivepacketstransmitteddoesposeaproblemifsuccessivepacketsaredelayeddifferentially.E-mailisgenerallyconsideredno

13、n-real-time,however：Usersgenerallyacceptafewsecondsofdelayinaskingforrepliestomessagestransmitted.Differentcodesareusedtotransmitthesetwodifferentchannels.Thereceivingsystem,knowingthecodesused,canseparatethetwoout.(a)Thechiprateis3.84Mcps.Atatransmissionrateof120kbps,thespreadingfactoris3840/120=32

14、.Amobiletransmittinguplinkinapoorpropagationenvironmentoratalongerdistancefromthebasestation,resultinginreducedpowerreceivedatthebasestation,mustincreaseitsspreadingfactortocompensate.Notefromchapter6thatthereceivingsystemmusthaveaminimumreceivedbitenergytomaintainproperreception.Thisisaccomplishedi

15、nCDMAsystemsbyeitherspreadingthesignaloverawiderband,thusreducingtheinterference,orbyslowingupthebittransmissionrate,hencewideningthebitinterval.Withthechipratefixed,thebitratemustbesloweddown.Withthespreadingfactorincreasedto128,thebitrateisreducedbyafactoroffourto30kbps.Thecorrespondingamountofdat

16、aina10msecintervalisreducedcorrespondinglyfromlessthan600bitstolessthan150bits.(b)MSBSTOCo1-5hzBCHjRACHpreamblef-RACHpreamble,morepowerjAICHjfRACHf(a)Fig.8-22indicatesthevariousIS-95channels.NotethatthecontrolchannelsinIS-95B,althoughcarryingsomewhatdifferentlabels,arethesameasinIS-95：F-PICHinIS-95B

17、correspondstothepilotchannelinIS-95;F-SYNCHtosync;F-PCHtopaging;RACHtoaccesschannels.Thebasicdifferenceisintheintroductionofthesupplementarycodechannels,SCCH,inIS-95B,allowingthetransmissionbitratetobeincreased.(b)Thesupplementarycodechannels,SCCH,upto7foreachfundamentalchannel,FCH,aretransmittedasW

18、alshcodes,allinparallelwiththeFCH,henceincreasingthetransmissionbit.(Seetheserial-to-parallelmulticodediscussiononpp.312,313ofSec.10.2,aswellasinproblem10.2above.)(c)Ata115.2kbpstransmissionrate,8xthebasicrateof14.4kbps,a1200-byteor9600-bitpacketrequiresfive20-msecframesfortransmission：Aframeaccommo

19、dates8x31bytes=248bytesofdataatthisrate.Hence4completeframesplus0.8ofafifthframeareneededtotransmitthe1200-bytepacket.(Paddingbitsarethenusedtocompletethelastframe.)Sincethefive20msecframestake0.1sectobetransmitted,theeffectivedatarateis9600bits/0.1sec=96kbps.Considernowan800-byte=6400-bitpacket.Thi

20、swillrequire800/248=3.2,orfourframesfortransmission.Theeffectivedatarateisthen6400/0.08sec=80kbps.(a)Thechipspreadingrateremainsfixedat1.2288Mcpswhilethedatabitsper20msecintervalincrease.(SeeFig.10.3.)Hencethespreadinggain,chiprate/datarate,decreasesinverselyasthedatarate.Sincethehigherdataratesyste

21、msusesmallerspreadinggains,theywouldbeusedinlocal-areaorindoorcellularenvironmentsforwhichthepropagationlosses,hencepowerreceived,arelessthaninwide-areaenvironments.(b)Forthedatarateof307.2kbps,6120bitsarefedinevery20msec.TheCRCof16bitsandthe8tailbitsadd24bits,sothat6144bitsarefedintotheencoderevery

22、20msec,correspondingto307.2kbpsinput.Theactualinputratetothesystemis6120/20=306kbps.ComparingFigs.10.4and10.5withFig.8.22fortheIS-95trafficandcontrolchannels,itisapparentthatcdma2000retainsthebasicIS-95channels,plusanenhancedgrouptoprovideimprovedandhigherbitrateperformance.NotefromFig.10.4thatcdma2

23、000hasareversedirectionaccesschannelplusthereverse,variablebitrate,fundamentalchannel,comparabletotheaccessandvariablebitratetrafficchannelsshowninFig.8.22.Fig.10.5showsthatcdma2000has,intheforwarddirection,thepilot,sync,paging,andvariablebitratefundamentaltrafficchannelsappearinginFig.8.22.Itisthus

24、backward-compatiblewithIS-95.Theothercdma2000channels,inbothdirections,providetheenhancedcapabilitynotedaboveanddescribedinthetext.(Note:Thisproblemshouldread“Comparethevariouschanne-CDMA-ofWandcdma2000.”Thecdma2000channelsdescribedinthetextarephysicalchannels.Logicalchannelsforthatsystemarenotdiscu

25、ssedinthetext.)Focushereisonbasiccommunicationprocedures.Downlink(forward)direction:W-CDMA-BroadcastchannelBCH,transmittedoverPCCPCH,andsynchch.SCH,time-multiplexed,carryinformationnecessaryforamobiletobegincommunicating.cdma2000-F-PICHandF-SYNCHcarrythisinformation.Uplink(reverse)direction:W-CDMA-M

26、obiletransmitsRACHpreambleoverPRACH.cdma2000-MobiletransmitsR-ACHpreamble+registrationmessage.Powerisrampedupandpreamblesrepeatedinbothcasesifnoackreceived.Downlink(forward)direction:W-CDMA-BSstationacksRACHpreamblewithAICH.cdma2000-BSstationacksoverF-CACHTOCo1-5hzMSBSJF-PICH,F-SYNCHJfR-ACHpreamblef

27、(Repeat,increasedpower,ifnoack)JF-CACHJfR-CCCHfor,307.2kbps:2028bits/(4slotsx5/3msec/slot)rates614.4and1228.8kbpsthenmultiplesof307.2kbps921.6kbps:3072bits/(2slotsx5/3msec)Symbolratesforthesedataratesusing1/5(1/3)coderateandQPSKare5x2=10(or3x2=6)xdatarates.For1/3coderate,8-PSK,symbolrateis9xdatarate

28、;1/3coderate,16-QAMhassymbolrate12xdatarate.Interleavingreduceschaneeofbursterrors.(b)Seesolutionto10.4(a)above.Thefollowingfiguresshowtheprotocollayersinvokedoneachsystemalongthepathforsendingtheapplicationdata.Theapplicationdatatravelsalongtheredline.Headersforeachlayerareaddedorstrippedoffwhenthe

29、dataarepasseddowntothelowerlayer,orsentuptothehigherlayer.Forexample,inthescenario1,theIPheaderisaddedbytheIPlayerofM1toformanIP-PDU,anditisstrippedbytheIPlayerofthewebserver.ThentheTCP-PDUissentuptotheTCPlayer.Anotherexample:whentheIPlayerofSGSNreceivesaUDP-PDUfromtheUDPlayer,itaddsanIPheader(withS

30、GSN,GGSNIPaddressesasthesource,destinationaddresses)toformanIP-PDU.WhentheIPlayerofGGSNreceivesanIPpacketdestinedtoitselffromL2,itstripsofftheIPheaderandsendstheUDP-PDUtotheUDPlayer.1applicationscenario2ndapplicationscenario(a)Thetwosetsoflogicalchannels,takenfromTables8.1and10.8arecomparedbelow.Not

31、ethatthebroadcastandcommoncontrolchannelsareessentiallycomparable,withtheoneexceptionthatGSMhasnospecificprovisionformulticastcommunication(theabilityforamessagetobedeliveredtoamultiplesetofdestinations.)Differencesoccurinthededicated-channelset,withsomeoftheinformationcarriedbySDCCHandSACCHintheGSM

32、casecarriedbyPACCHandPTCCHinGPRS.GSMGPRSBroadcastchannelsFCCHFCCHSCHSCHBCCHPBCCHCommonControlch.PCHPPCHAGCHPAGCHRACHPRACHPNCHDedicatedControlch.SDCCHPACCHSACCHFACCHPTCCH(b)ThecomparisonisreadilymadebycomparingFig.10.16withtheaccesscontrolportionofFig.8.3.IntheGSMcase,thepagingchannelPCHwouldbeusedto

33、pageamobileinplaceofPPCHinGPRS.ThemobileinGSMwouldrespondwithanRACHmessage,inplaceofPRACHinGPRS.AcknowledgementofcorrectreceiptoftheRACHmessagewouldthenbemadeusingtheAGCHchannelinplaceofPAGCH.(Butnote,aspointedoutinthetext,thataccesscontrolinGPRSsystemscanfallbackontheGSMchannelsandmessages.)ThemobileinGSMwoulda