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1、CHAPTER3Point-to-pointcommunication:detection,diversity,andchanneluncertaintyInthischapterwelookatvariousbasicissuesthatariseincommunicationoverfadingchannels.Westartbyanalyzinguncodedtransmissioninanarrowbandfadingchannel.Westudybothcoherentandnon-coherentdetection.Inbothcasestheerrorprobabili
2、tyismuchhigherthaninanon-fadedAWGNchannel.Thereasonisthatthereisasignificantprobabilitythatthechannelisinadeepfade.Thismotivatesustoinvestigatevariousdiversitytechniquesthatimprovetheperformance.Thediversitytechniquesoperateovertime,frequencyorspace,butthebasicideaisthesame.Bysendingsignalsthat
3、carrythesameinformationthroughdifferentpaths,multipleindependentlyfadedreplicasofdatasymbolsareobtainedatthereceiverendandmorereliabledetectioncanbeachieved.Thesimplestdiversityschemesuserepetitioncoding.Moresophisticatedschemesexploitchanneldiversityand,atthesametime,efficientlyusethedegreesof
4、freedominthechannel.Comparedtorepetitioncoding,theyprovidecodinggainsinadditiontodiversitygains.Inspacediversity,welookatbothtransmitandreceivediversityschemes.Infrequencydiversity,welookatthreeapproaches:•single-carrierwithinter-symbolinterferenceequalization,•direct-sequencespread-spectrum,•o
5、rthogonalfrequencydivisionmultiplexing.Finally,westudytheimpactofchanneluncertaintyontheperformanceofdiversitycombiningschemes.Wewillseethat,insomecases,havingtoomanydiversitypathscanhaveanadverseeffectduetochanneluncertainty.Tofamiliarizeourselveswiththebasicissues,theemphasisofthischapterison
6、concretetechniquesforcommunicationoverfadingchannels.InChapter5wetakeamorefundamentalandsystematiclookanduseinformationtheorytoderivethebestperformanceonecanachieve.Atthatfundamentallevel,wewillseemanyoftheissuesdiscussedhererecur.Thederivationsinthischaptermakerepeateduseofafewkeyresultsinvect
7、ordetectionunderGaussiannoise.WedevelopandsummarizethebasicresultsinAppendixA,emphasizingtheunderlyinggeometry.Thereaderis4950Point-to-pointcommunicationencouragedtotakealookattheappendixbeforeproceedingwiththischapterandtoreferba