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1、mobileandcellularradio移动和细胞广播incomparisontotherelativestabilityandmodesttechnicaldevelopmentswhichareoccurringinlonghaulwidebandmicrowavecommunicationsystemsthereisrapiddevelopmentandexpandingdeploymentofnewmobilepersonalcommunicationsystem.Theserangfromwidecoverageareapagers,forsimpled
2、atamessagetransmission,whichemploycommonstandardsandhenceachievecontiguouscoverageoverlargegeographicalareas,suchasallthemajorurbancentresandtransportroutesinEurope,AsiaorthecontinentalUSA.Thischapterdiscussesthespecialchannelcharacteristicsofmobilesystemsandexaminesthetypicalcellularcl
3、ustersadoptedtoachievecontinuouscommunicationwiththemobileuser.Itthenhighlightstheimportantpropertiesofcurrent,andemerging,TDMAandcodedivisionmultipleaccess(CDMA),mobiledigitalcellularcommunicationsystems.PrivatemobileradioTerrestrialmobileradioworksbestataround250MHzaslowerfrequenciest
4、hanthissufferfromnoiseandinterferencewhilehigherfrequenciesexperiencemultipathpropagationfrombuildings,etc,section15.2.Inpracticemodestfrequencybandsareallocatedbetween60MHzand2GHz.Privatemobileradio(PMR)isthesystemwhichisusedbytaxicompanies,countycouncils,healthauthorities,ambulanceser
5、vices,fireservices,theutilityindustries,etc,formobilecommunications.PMRhasthreespectralatVHF,onejustbelowthe88to108MHzFMbroadcastbandandonejustabovethisbandwithanotherallocationatapproximately170MHz.TherearealsotwoallocationsatUHFaround450MHz.allthesespectralallocationsprovideatotalofju
6、stover1000radiochannelswiththechannelsplacedat12KHzchannelspacingsorcentrefrequencyoffsets.Withinthe12khzwidechannaltheanaloguemodulationinPMRtypicallyallows7khzofbandwidthforthesignaltransmission.whenfurtherallowanceismadeforthefrequencydriftintheoscillatorsofthesesystemsapeakdeviation
7、ofonly2to3khzisavailableforthespeechtraffic.Trafficisnormallyimpressedonthesesystemsbyamplitudemodulationorfrequencymodulationandagainthereceiverisoftheubiquitoussuperheterodynedesign,Figure1.4.Adoubleconversionreceiverwithtwoseparatelocaloscillatorstagesisusuallyrequiredtoachi
