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1、CHAPTER6MultiusercapacityandopportunisticcommunicationInChapter4,westudiedseveralspecificmultipleaccesstechniques(TDMA/FDMA,CDMA,OFDM)designedtosharethechannelamongsev-eralusers.Anaturalquestionis:whatarethe“optimal”multipleaccessschemes?Toaddressthisquestion,onemustnowstepbackandtake
2、afun-damentallookatthemultiuserchannelsthemselves.Informationtheorycanbegeneralizedfromthepoint-to-pointscenario,consideredinChapter5,tothemultiuserones,providinglimitstomultiusercommunicationsandsuggestingoptimalmultipleaccessstrategies.Newtechniquesandconceptssuchassuccessivecancell
3、ation,superpositioncodingandmultiuserdiversityemerge.Thefirstpartofthechapterfocusesontheuplink(many-to-one)anddownlink(one-to-many)AWGNchannelwithoutfading.Fortheuplink,anoptimalmultipleaccessstrategyisforalluserstospreadtheirsignalacrosstheentirebandwidth,muchlikeintheCDMAsysteminCh
4、apter4.However,ratherthandecodingeveryusertreatingtheinterferencefromotherusersasnoise,asuccessiveinterferencecancellation(SIC)receiverisneededtoachievecapacity.Thatis,afteroneuserisdecoded,itssignalisstrippedawayfromtheaggregatereceivedsignalbeforethenextuserisdecoded.Asimilarstrateg
5、yisoptimalforthedownlink,withsignalsfortheuserssuperimposedontopofeachotherandSICdoneatthemobiles:eachuserdecodestheinformationintendedforalloftheweakerusersandstripsthemoffbeforedecodingitsown.Itisshownthatinsituationswhereusershaveverydisparatechannelstothebase-station,CDMAtogetherw
6、ithsuccessivecancellationcanoffersignificantgainsovertheconventionalmultipleaccesstechniquesdiscussedinChapter4.Inthesecondpartofthechapter,weshiftourfocustomultiuserfadingchannels.OneofthemaininsightslearntinChapter5isthat,forfastfadingchannels,theabilitytotrackthechannelatthetransmi
7、ttercanincreasepoint-to-pointcapacitybyopportunisticcommunication:transmittingathighrateswhenthechannelisgood,andatlowratesornotatallwhenthechannelispoor.Weextendthisinsighttothemultiusersetting,bothfortheuplink2282296.1UplinkAWGNchannelandforthedownlink.Theperformancegainofopportunis
8、ticco
