无线通信 Lecture 3 Cellular

Chapter3TheCellularConcept-SystemDesignFundamentals2I.IntroductionGoalsofaCellularSystemHighcapacityLargecoverageareaEfficientuseoflimitedspectrumLargecoveragearea-BellsysteminNewYorkCityhadearlymobileradioSingleTx,highpower,andtalltowerLowcostLargecoveragearea-BellsysteminNewYorkCityhad12simultaneouschannelsfor1000squaremilesSmall#usersPoorspectrumutilizationWhatarepossiblewayswecouldincreasethenumberofchannelsavailableinacellularsystem?3CellularconceptFrequencyreusepattern4Cellslabeledwiththesameletterusethesamegroupofchannels.CellCluster:groupofNcellsusingcompletesetofavailablechannelsManybasestations,lowerpower,andshortertowersSmallcoverageareascalled“cells”Eachcellallocateda%ofthetotalnumberofavailablechannelsNearby(adjacent)cellsassigneddifferentchannelgroupstopreventinterferencebetweenneighboringbasestationsandmobileusers5Samefrequencychannelsmaybereusedbycellsa“reasonable”distanceawayreusedmanytimesaslongasinterferencebetweensamechannel(co-channel)cellsisacceptablelevelAsfrequencyreuse↑→#possiblesimultaneoususers↑→#subscribers↑→butsystemcost↑(moretowers)Toincreasenumberofuserswithoutincreasingradiofrequencyallocation,reducecellsizes(morebasestations)↑→#possiblesimultaneoususers↑Thecellularconceptallowsallmobilestobemanufacturedtousethesamesetoffreqencies***Afixed#ofchannelsservesalarge#ofusersbyreusingchannelsinacoveragearea***6II.FrequencyReuse/PlanningDesignprocessofselecting&allocatingchannelgroupsofcellularbasestationsTwocompeting/conflictingobjectives:1)maximizefrequencyreuseinspecifiedarea2)minimizeinterferencebetweencells7Cellsbasestationantennasdesignedtocoverspecificcellareahexagonalcellshapeassumedforplanningsimplemodelforeasyanalysis→circlesleavegapsactualcell“footprint”isamorphous(nospecificshape)whereTxsuccessfullyservesmobileunitbasestationlocationcellcenter→omni-directionalantenna(360°coverage)notnecessarilyintheexactcenter(canbeuptoR/4fromtheideallocation)8cellcorners→sectoredordirectionalantennason3cornerswith120°coverage.verycommomNotethatwhatisdefinedasa“corner”issomewhatflexible→asectoredantennacovers120°ofahexagonalcell.Soonecandefineacellashavingthreeantennasinthecenterorantennasat3corners.9III.SystemCapacityS:total#ofduplexchannelsavailableforuseinagivenarea;determinedby:amountofallocatedspectrumchannelBW→modulationformatand/orstandardspecs.(e.g.AMPS)k:numberofchannelsforeachcell(kS)N:clustersize→#ofcellsformingclusterS=kN10M:#oftimesaclusterisreplicatedoverageographiccoverageareaSystemCapacity=Total#DuplexChannels=CC=MS=MkN(assumingexactlyMNcellswillcoverthearea)Ifclustersize(N)isreducedandthegeographicareaforeachcelliskeptconstant:Thegeographicareacoveredbyeachclusterissmaller,soMmust↑tocovertheentirecoveragearea(moreclustersneeded).Sremainsconstant.SoC↑ThesmallestpossiblevalueofNisdesirabletomaximizesystemcapacity.11ClustersizeNdetermines:distancebetweenco-channelcells(D)levelofco-channelinterferenceAmobileorbasestationcanonlytoleratesomuchinterferencefromothercellsusingthesamefrequencyandmaintainsufficientquality.largeN→largeD→lowinterference→butsmallMandlowC!Tradeoffinqualityandclustersize.Thelargerthecapacityforagivengeographicarea,thepoorerthequality.12Frequencyreusefactor=1/NeachfrequencyisreusedeveryNcellseachcellassignedk≒S/NNcells/clusterconnectwithoutgapsspecificvaluesarerequiredforhexagonalgeometryN=i2+ij+j2wherei,j≧1TypicalNvalues→3,4,7,12;(i,j)=(1,1),(2,0),(2,1),(2,2)13Tofindthenearestco-channelneighborsofaparticularcell(1)Moveicellsalonganychainofhexagons,then(2)turn60degreesandmovejcells.141516IV.ChannelAssignmentStrategiesGoalistominimizeinterference&maximizeuseofcapacitylowerinterferenceallowssmallerNtobeused→greaterfrequencyreuse→largerCTwomainstrategies:FixedorDynamicFixedeachcellallocatedapre-determinedsetofvoicechannelscallswithincellonlyservedbyunusedcellchannelsallchannelsused→blockedcall→noserviceseveralvariationsMSCallowscelltoborrowaVC(thatistosay,aFVC/RVCpair)fromanadjacentcelldonorcellmusthaveanavailableVCtogive17DynamicchannelsNOTallocatedpermanentlycallrequest→goestoservingbasestation→goestoMSCMSCallocateschannel“onthefly”allocationstrategyconsiders:likelihoodoffuturecallblockinginthecellreusedistance(interferencepotentialwithothercellsthatareusingthesamefrequency)channelfrequencyAllfrequenciesinamarketareavailabletobeused18Advantage:reducescallblocking(thatistosay,itincreasesthetrunkingcapacity),andincreasesvoicequalityDisadvantage:increasesstorage&computationalload@MSCrequiresreal-timedatafromentirenetworkrelatedto:channeloccupancytrafficdistributionRadioSignalStrengthIndications(RSSI's)fromallchannels19V.HandoffStrategiesHandoff:whenamobileunitmovesfromonecelltoanotherwhileacallisinprogress,theMSCmusttransfer(handoff)thecalltoanewchannelbelongingtoanewbasestationnewvoiceandcontrolchannelfrequenciesveryimportanttask→oftengivenhigherprioritythannewcallItisworsetodropanin-progresscallthantodenyanewone20Minimumuseablesignallevellowestacceptable