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1、1FundamentalsThischapterdescribesthefundamentalsoftoday’swirelesscommunications.Firstadetaileddescriptionoftheradiochannelanditsmodelingarepresented,followedbytheintroductionoftheprincipleofOFDMmulti-carriertransmission.Inaddition,ageneraloverviewofthespreadspectrumtechnique,especia
2、llyDS-CDMA,isgivenandexamplesofpotentialapplicationsforOFDMandDS-CDMAareanalyzed.ThisintroductionisessentialforabetterunderstandingoftheideabehindthecombinationofOFDMwiththespreadspectrumtechnique,whichisbrieflyintroducedinthelastpartofthischapter.1.1RadioChannelCharacteristicsUnder
3、standingthecharacteristicsofthecommunicationsmediumiscrucialfortheappropriateselectionoftransmissionsystemarchitecture,dimensioningofitscomponents,andoptimizingsystemparameters,especiallysincemobileradiochannelsareconsideredtobethemostdifficultchannels,sincetheysufferfrommanyimperfe
4、ctionslikemultipathfading,interference,Dopplershift,andshadowing.Thechoiceofsystemcomponentsistotallydifferentif,forinstance,multipathpropagationwithlongechoesdominatestheradiopropagation.Therefore,anaccuratechannelmodeldescribingthebehaviorofradiowavepropagationindifferentenvironme
5、ntssuchasmobile/fixedandindoor/outdoorisneeded.Thismayallowone,throughsimulations,toestimateandvalidatetheperformanceofagiventransmissionschemeinitsseveraldesignphases.1.1.1UnderstandingRadioChannelsInmobileradiochannels(seeFigure1-1),thetransmittedsignalsuffersfromdifferenteffects,
6、whicharecharacterizedasfollows:Multipathpropagationoccursasaconsequenceofreflections,scattering,anddiffractionofthetransmittedelectromagneticwaveatnaturalandman-madeobjects.Thus,atthereceiverantenna,amultitudeofwavesarrivesfrommanydifferentdirectionswithdifferentdelays,attenuations,
7、andphases.Thesuperpositionofthesewavesresultsinamplitudeandphasevariationsofthecompositereceivedsignal.Dopplerspreadiscausedbymovingobjectsinthemobileradiochannel.Changesinthephasesandamplitudesofthearrivingwavesoccurwhichleadtotime-variantmultipathpropagation.Evensmallmovementsonth
8、eorderofthewaveleng