无线通信与网络chap10

CellularWirelessNetworksChapter10CellularNetworkOrganizationUsemultiplelow-powertransmitters(100Worless)AreasdividedintocellsEachservedbyitsownantennaServedbybasestationconsistingoftransmitter,receiver,andcontrolunitBandoffrequenciesallocatedCellssetupsuchthatantennasofallneighborsareequidistant(hexagonalpattern)FrequencyReuseAdjacentcellsassigneddifferentfrequenciestoavoidinterferenceorcrosstalkObjectiveistoreusefrequencyinnearbycells10to50frequenciesassignedtoeachcellTransmissionpowercontrolledtolimitpoweratthatfrequencyescapingtoadjacentcellsTheissueistodeterminehowmanycellsmustintervenebetweentwocellsusingthesamefrequencyApproachestoCopewithIncreasingCapacityAddingnewchannelsFrequencyborrowing–frequenciesaretakenfromadjacentcellsbycongestedcellsCellsplitting–cellsinareasofhighusagecanbesplitintosmallercellsCellsectoring–cellsaredividedintoanumberofwedge-shapedsectors,eachwiththeirownsetofchannelsMicrocells–antennasmovetobuildings,hills,andlamppostsCellularSystemOverviewCellularSystemsTermsBaseStation(BS)–includesanantenna,acontroller,andanumberofreceiversMobiletelecommunicationsswitchingoffice(MTSO)–connectscallsbetweenmobileunitsTwotypesofchannelsavailablebetweenmobileunitandBSControlchannels–usedtoexchangeinformationhavingtodowithsettingupandmaintainingcallsTrafficchannels–carryvoiceordataconnectionbetweenusersStepsinanMTSOControlledCallbetweenMobileUsersMobileunitinitializationMobile-originatedcallPagingCallacceptedOngoingcallHandoffAdditionalFunctionsinanMTSOControlledCallCallblockingCallterminationCalldropCallsto/fromfixedandremotemobilesubscriberMobileRadioPropagationEffectsSignalstrengthMustbestrongenoughbetweenbasestationandmobileunittomaintainsignalqualityatthereceiverMustnotbesostrongastocreatetoomuchcochannelinterferencewithchannelsinanothercellusingthesamefrequencybandFadingSignalpropagationeffectsmaydisruptthesignalandcauseerrorsHandoffPerformanceMetricsCellblockingprobability–probabilityofanewcallbeingblockedCalldroppingprobability–probabilitythatacallisterminatedduetoahandoffCallcompletionprobability–probabilitythatanadmittedcallisnotdroppedbeforeitterminatesProbabilityofunsuccessfulhandoff–probabilitythatahandoffisexecutedwhilethereceptionconditionsareinadequateHandoffPerformanceMetricsHandoffblockingprobability–probabilitythatahandoffcannotbesuccessfullycompletedHandoffprobability–probabilitythatahandoffoccursbeforecallterminationRateofhandoff–numberofhandoffsperunittimeInterruptionduration–durationoftimeduringahandoffinwhichamobileisnotconnectedtoeitherbasestationHandoffdelay–distancethemobilemovesfromthepointatwhichthehandoffshouldoccurtothepointatwhichitdoesoccurHandoffStrategiesUsedtoDetermineInstantofHandoffRelativesignalstrengthRelativesignalstrengthwiththresholdRelativesignalstrengthwithhysteresisRelativesignalstrengthwithhysteresisandthresholdPredictiontechniquesPowerControlDesignissuesmakingitdesirabletoincludedynamicpowercontrolinacellularsystemReceivedpowermustbesufficientlyabovethebackgroundnoiseforeffectivecommunicationDesirabletominimizepowerinthetransmittedsignalfromthemobileReducecochannelinterference,alleviatehealthconcerns,savebatterypowerInSSsystemsusingCDMA,it’sdesirabletoequalizethereceivedpowerlevelfromallmobileunitsattheBSTypesofPowerControlOpen-looppowercontrolDependssolelyonmobileunitNofeedbackfromBSNotasaccurateasclosed-loop,butcanreactquickertofluctuationsinsignalstrengthClosed-looppowercontrolAdjustssignalstrengthinreversechannelbasedonmetricofperformanceBSmakespoweradjustmentdecisionandcommunicatestomobileoncontrolchannelTrafficEngineeringIdeally,availablechannelswouldequalnumberofsubscribersactiveatonetimeInpractice,notfeasibletohavecapacityhandleallpossibleloadForNsimultaneoususercapacityandLsubscribersLN–nonblockingsystemLN–blockingsystemBlockingSystemPerformanceQuestionsProbabilitythatcallrequestisblocked?Whatcapacityisneededtoachieveacertainupperboundonprobabilityofblocking?Whatistheaveragedelay?Whatcapacityisneededtoachieveacertainaveragedelay?TrafficIntensityLoadpresentedtoasystem:=meanrateofcallsattemptedperunittimeh=meanholdingtimepersuccessfulcallA=averagenumberofcallsarrivingduringaverageholdingperiod,fornormalizedhAFactorsthatDeterminetheNatureoftheTrafficModelMannerinwhichblockedcallsarehandledLostcallsdelayed(LCD)–blockedcallsputinaqueueawaitingafreechannelBlockedcallsrejectedanddroppedLostcallscleared(LCC)–userwaitsbeforeanotherattemptLostcallsheld(LCH)–userrepeatedlyattemptscallingNumberoftrafficsourcesWhethernumberofusersisassumedtobefiniteorinfiniteFirst-GenerationAnalogAdvancedMobilePhoneService(AMPS)InNorthAmerica,two25-MHzbandsallocatedtoAMPSOnefortransmissionfrombasetomobileunitOnefortransmissionfrommobileunittobaseEachbandsplitintwotoencouragecompetitionFrequencyreuseexploitedAMPSOperationSubscriberinitiatescallbykeyinginphonenumberandpressessendkeyMTSOverifiesnumberandauthorizesuserMTSOissuesmessagetouser’scellphoneindicatingsendandreceive