蜂窝系统切换技术中英文对照外文翻译文献.docx

蜂窝系统切换技术中英文对照外文翻译文献.docx

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蜂窝系统切换技术中英文对照外文翻译文献

中英文资料对照外文翻译

原文：

HandoffinCellularSystems

CellularSystem

DeploymentScenarios

Theradiopropagationenvironmentandrelatedhandoffchallengesaredifferentindifferentcellularstructures.Ahandoffalgorithmwithfixedparameterscannotperformwellindifferentsystemenvironments.Specificcharacteristicsofthecommunicationsystemsshouldbetakenintoaccountwhiledesigninghandoffalgorithms.Severalbasiccellularstructures（e.g.,macrocells,microcells,andoverlaysystems）andspecialarchitectures（e.g.,underlays,multichannelbandwidthsystems,andevolutionaryarchitectures）aredescribednext.Integratedcordlessandcellularsystems,integratedcellularsystems,andintegratedterrestrialandsatellitesystemsarealsodescribed.

Macrocells

Macrocellradiiareinseveralkilometers.Duetothelowcellcrossingrate,centralizedhandoffispossibledespitethelargenumberofMSstheMSChastomanage.Thesignalqualityintheuplinkanddownlinkisapproximatelythesame.ThetransitionregionbetweentheBSsislarge;handoffschemesshouldallowsomedelaytoavoidflip-flopping.However,thedelayshouldbeshortenoughtopreservethesignalqualitybecausetheinterferenceincreasesastheMSpenetratesthenewcell.Thiscellpenetrationiscalledcelldragging.Macrocellshaverelativelygentlepathlosscharacteristics.Theaveraginginterval（i.e.,thetimeperiodusedtoaveragethesignalstrengthvariations）shouldbelongenoughtogetridoffading

fluctuations.First-andsecond-generationcellularsystemsprovidewide-areacoverageevenincitiesusingmacrocells.Typically,aBStransceiverinamacrocelltransmitshighoutputpowerwiththeantennamountedseveralmetershighonatowertoilluminatealargearea.

Microcells

Somecapacityimprovementtechniques（e.g.,largerbandwidths,improvedmethodsforspeechcoding,channelcoding,andmodulation）willnotbesufficienttosatisfytherequiredservicedemand.Theuseofmicrocellsisconsideredthesinglemosteffectivemeansofincreasingthecapacityofcellularsystems.Microcellsincreasecapacity,butradioresourcemanagementbecomesmoredifficult.Microcellscanbeclassifiedasone-,two-,orthreedimensional,dependingonwhethertheyarealongaroadorahighway,coveringanareasuchasanumberofadjacentroads,orlocatedinmultilevelbuildings,respectively.Microcellscanbeclassifiedashotspots（serviceareaswithahighertrafficdensityorareasthatarecoveredpoorly）,downtownclusteredmicrocells（contiguousareasservingpedestriansandmobiles）,andin-building3-Dcells（servingofficebuildingsandpedestrians）.Typically,aBStransceiverinamicrocelltransmitslowoutputpowerwiththeantennamountedatlamppostlevel（approximately5maboveground）.TheMSalsotransmitslowpower,whichleadstolongerbatterylife.SinceBSantennashavelowerheightscomparedtothesurroundingbuildings,RFsignalspropagatemostlyalongthestreets.Theantennamaycover100–200mineachstreetdirection,servingafewcityblocks.Thispropagationenvironmenthaslowtimedispersion,whichallowshighdatarates.

Microcellsaremoresensitivetothetrafficandinterferencethanmacrocellsduetoshort-termvariations（e.g.,trafficandinterference

variations）,medium/long-termalterations（e.g.,newbuildings）,andincrementalgrowthoftheradionetwork（e.g.,newBSs）.Thenumberofhandoffspercellisincreasedbyanorderofmagnitude,andthetimeavailabletomakeahandoffisdecreased.Usinganumbrellacellisonewaytoreducethehandoffrate.Duetotheincreaseinthemicrocellboundarycrossingsandexpectedhightrafficloads,ahigherdegreeofdecentralizationofthehandoffprocessbecomesnecessary.

Microcellsencounterapropagationphenomenoncalledthecornereffect.Thecornereffectischaracterizedbyasuddenlargedrop（e.g.,20–30dB）insignalstrength（e.g.,at10–20mdistance）whenamobileturnsaroundacorner.

Thecornereffectisduetothelossofthelineofsight（LOS）componentfromtheservingBStotheMS.Thecornereffectdemandsafasterhandoffandcanchangethesignalqualityveryfast.Thecornereffectishardtopredict.Alongmeasurementaveragingintervalisnotdesirableduetothecornereffect.MovingobstaclescantemporarilyhinderthepathbetweenaBSandanMS,whichresemblesthecornereffect.ReferencestudiesthepropertiesofsymmetricalcellplansinaManhattan-typeenvironment.Cellplansaffectsignal-to-interferenceratio（SIR）performanceintheuplinkanddownlinksignificantly.Symmetricalcellplanshavefournearestco-channelBSslocatedatthesamedistance.Suchcellplanscanbeclassifiedintohalf-square（HS）,full-square（FS）,andrectangular（R）cellplans.Thesecellplansaredescribednext.

Half-SquareCellPlan—ThiscellplanplacesBSswithomnidirectionalantennasateachintersection,andeachBScovershalfablockinallfourdirections.Thiscellplanavoidsthestreetcornereffectandprovidesthehighestcapacity.ThiscellplanhasonlyLOShandoffs.Figure2showsanexampleofahalf-squarecellplaninamicrocellularsystem.

Full-SquareCellPlan—ThereisaBSwithanomnidirectionalantennalocatedateveryotherintersection,andeachBScoversablockinallfourdirections.ItispossibleforanMStoexperiencethestreetcornereffectforthiscellplan.TheFScellplancanhaveLOSorNLOShandoffs.Figure3showsanexampleofafullsquarecellplaninamicrocellularsystem.

RectangularCellPlan—EachBScoversafractionofeitherahorizontalorverticalstreetwiththeBSlocatedinthemiddleofthecell.Thiscellplancaneasilybeadaptedtomarketpenetration.FewerBSswithhightransmitpowercanbeusedinitially.Asuserdensityincreases,newBSscanbeaddedwithreducedtransmitpowerfromappropriateBSs.Thestreetcornereffectispossibleforthiscellplan.TheRcellplancanhaveLOSorNLOShandoffs.Figure4showsanexampleofarectangularcellplaninamicrocellularsystem.Macrocell/MicrocellOverlaysCongestionofcertainmicrocells,thelackofserviceofmicrocellsinsomeareas,andhighspeedofsomeusersaresomereasonsforhigherhandoffratesandsignalingloadformicrocells.Toalleviatesomeoftheseproblems,amixed-cellarchitecture（calledanoverlay/underlaysystem）consistingoflargesizemacrocells（calledumbrellacellsoroverlaycells）andsmall-sizemicrocells（calledunderlaycells）canbeused.Figure5illustratesanoverlaysystem.

Themacrocell/microcelloverlayarchitectureprovidesabalancebetweenmaximizingthenumberofusersperunitareaandminimizingthenetworkcontrolloadassociatedwithhandoff.Macrocellsprovidewide-areacoveragebeyondmicrocellserviceareasandensurebetterintercellhandoff.Microcellsprovidecapacityduetogreaterfrequencyreuseandcoverareaswithhightrafficdensity（calledhotspots）.Examplesofhotspotsincludeanairport,arailwaystation,oraparkinglot.Inlesscongestedareas（e.g.,areasbeyondacitycenteroroutsidethemainstreetsofacity）trafficdemandisnotveryhigh,andmacrocellscanprovideadequatecoverageinsuchareas.MacrocellsalsoservehighspeedMSsandtheareasnotcoveredbymicrocells（e.g.,due

tolackofchannelsortheMSbeingoutofthemicrocellrange）.Also,afterthemicrocellularsystemisusedtoitsfullestextent,theoverflowtrafficcanberoutedtomacrocells.Oneoftheimportantissuesfortheoverlay/underlaysystemisthedeterminationofoptimumdistributionofchannelsinthemacrocellsandmicrocells.

Referenceevaluatesfourapproachestosharingtheavailablespectrumbetweenthetwotiers.Approach1usesTDMAformicrocellandCDMAformacrocell.Approach2usesCDMAformicrocellandTDMAformacrocell.Approach3usesTDMAinbothtiers,whileapproach4usesorthogonalfrequencychannelsinbothtiers.Theoverlay/underlaysystemhasseveraladvantagesoverapuremicrocellsystem:

•TheBSsarerequiredonlyinhightrafficloadareas.Sinceitisnotnecessarytocoverthewholeserviceareawithmicrocells,infrastructurecostsaresaved.

•Thenumberofhandoffsinanoverlaysystemismuchlessthaninamicrocellsystembecausefast-movingvehiclescanbeconnectedtotheoverlaymacrocell.

•BothcallingfromanMSandlocationregistrationcaneasilybedonethroughthemicrocellsystem.Thereareseveralclassesofumbrellacells.Inoneclass,orthogonalchannelsaredistributedbetweenmicrocellsandmacrocells.Inanotherclass,microcellsusechannelsthataretemporarilyunusedbymacrocells.Inyetanotherclass,microcellsreusethechannelsalreadyassignedtomacrocellsanduseslightlyhighertransmitpowerlevelstocounteracttheinterferencefromthemacrocells.

Withintheoverlay/underlaysystemenvironment,fourtypesofhandoversneedtobemanaged[19]:

microcelltomicrocell,microcelltomacrocell,macrocelltomacrocell,andmacrocelltomicrocell.Referencedescribescombinedcellsplittingandoverlaying.Reuseofchannelsinthetwocellsisdonebyestablishinganoverlaidsmallcellservedbythesamecellsiteasthelargecell.Smallcellsreusethesplitcell’schannelsbecauseofthelargedistancebetweenthesplitcellandthesmallinnercell,whilethelargecellcannotreusethesechannels.Overlaidcellsareapproximately50percentmorespectrallyefficientthansegmenting（theprocessofdistributingthechannelsamongthesmall-andlargesizecellstoavoidinterference）.Apracticalapproachforimplementationofamicrocellsystemoverlaidwithanexistingmacrocellsystemisproposedin.Thisreferenceintroduceschannelsegregation（aself-organizeddynamicchannelassignment）andautomatictransmitpowercontrol