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1、mobileandcellularradio移动和细胞广播incomparisontotherelativestabilityandmodesttechnicaldevelopmentswhichareoccurringinlonghaulwidebandmicrowavecommunicationsystemsthereisrapiddevelopmentandexpandingdeploymentofnewmobilepersonalcommunicationsystem.Theserangfromwidecoverageareapagers,forsimpledatamessage
2、transmission,whichemploycommonstandardsandhenceachievecontiguouscoverageoverlargegeographicalareas,suchasallthemajorurbancentresandtransportroutesinEurope,AsiaorthecontinentalUSA.Thischapterdiscussesthespecialchannelcharacteristicsofmobilesystemsandexaminesthetypicalcellularclustersadoptedtoachie
3、vecontinuouscommunicationwiththemobileuser.Itthenhighlightstheimportantpropertiesofcurrent,andemerging,TDMAandcodedivisionmultipleaccess(CDMA),mobiledigitalcellularcommunicationsystems.PrivatemobileradioTerrestrialmobileradioworksbestataround250MHzaslowerfrequenciesthanthissufferfromnoiseandinter
4、ferencewhilehigherfrequenciesexperiencemultipathpropagationfrombuildings,etc,section15.2.Inpracticemodestfrequencybandsareallocatedbetween60MHzand2GHz.Privatemobileradio(PMR)isthesystemwhichisusedbytaxicompanies,countycouncils,healthauthorities,ambulanceservices,fireservices,theutilityindustries,
5、etc,formobilecommunications.PMRhasthreespectralatVHF,onejustbelowthe88to108MHzFMbroadcastbandandonejustabovethisbandwithanotherallocationatapproximately170MHz.TherearealsotwoallocationsatUHFaround450MHz.allthesespectralallocationsprovideatotalofjustover1000radiochannelswiththechannelsplacedat12KH
6、zchannelspacingsorcentrefrequencyoffsets.Withinthe12khzwidechannaltheanaloguemodulationinPMRtypicallyallows7khzofbandwidthforthesignaltransmission.whenfurtherallowanceismadeforthefrequencydriftintheoscillatorsofthesesystemsapeakdeviationofonly2to3khzisavailableforthespeechtraffic.Trafficisnormall
7、yimpressedonthesesystemsbyamplitudemodulationorfrequencymodulationandagainthereceiverisoftheubiquitoussuperheterodynedesign,Figure1.4.Adoubleconversionreceiverwithtwoseparatelocaloscillatorstagesisusuallyrequiredtoachi
