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1、1FundamentalsThischapterdescribesthefundamentalsoftoday’swirelesscommunications.Firstadetaileddescriptionoftheradiochannelanditsmodelingarepresented,followedbytheintroductionoftheprincipleofOFDMmulti-carriertransmission.Inaddition,ageneraloverviewoft
2、hespreadspectrumtechnique,especiallyDS-CDMA,isgivenandexamplesofpotentialapplicationsforOFDMandDS-CDMAareanalyzed.ThisintroductionisessentialforabetterunderstandingoftheideabehindthecombinationofOFDMwiththespreadspectrumtechnique,whichisbrieflyintrod
3、ucedinthelastpartofthischapter.1.1RadioChannelCharacteristicsUnderstandingthecharacteristicsofthecommunicationsmediumiscrucialfortheappropriateselectionoftransmissionsystemarchitecture,dimensioningofitscomponents,andoptimizingsystemparameters,especia
4、llysincemobileradiochannelsareconsideredtobethemostdifficultchannels,sincetheysufferfrommanyimperfectionslikemultipathfading,interference,Dopplershift,andshadowing.Thechoiceofsystemcomponentsistotallydifferentif,forinstance,multipathpropagationwithlo
5、ngechoesdominatestheradiopropagation.Therefore,anaccuratechannelmodeldescribingthebehaviorofradiowavepropagationindifferentenvironmentssuchasmobile/fixedandindoor/outdoorisneeded.Thismayallowone,throughsimulations,toestimateandvalidatetheperformanceo
6、fagiventransmissionschemeinitsseveraldesignphases.1.1.1UnderstandingRadioChannelsInmobileradiochannels(seeFigure1-1),thetransmittedsignalsuffersfromdifferenteffects,whicharecharacterizedasfollows:Multipathpropagationoccursasaconsequenceofreflections,
7、scattering,anddiffractionofthetransmittedelectromagneticwaveatnaturalandman-madeobjects.Thus,atthereceiverantenna,amultitudeofwavesarrivesfrommanydifferentdirectionswithdifferentdelays,attenuations,andphases.Thesuperpositionofthesewavesresultsinampli
8、tudeandphasevariationsofthecompositereceivedsignal.Dopplerspreadiscausedbymovingobjectsinthemobileradiochannel.Changesinthephasesandamplitudesofthearrivingwavesoccurwhichleadtotime-variantmultipathpropagation.Evensmallmovementsontheorderofthewaveleng
