外文翻译宽带CDMA无线接入网的基本概念Word格式.docx

外文翻译宽带CDMA无线接入网的基本概念Word格式.docx

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BasicConceptsofWCDMARadioAccessNetwork

1.Background

Therehasbeenatremendousgrowthinwirelesscommunicationtechnologyoverthepastdecade.Thesignificantincreaseinsubscribersandtraffic,newbandwidthconsumingapplicationssuchasgaming,musicdownloadingandvideostreamingwillplacenewdemandsoncapacity.Theanswertothecapacitydemandistheprovisionofnewspectrumandthedevelopmentofanewtechnology-WidebandCDMAorhereinafterreferredtoasWCDMA.WCDMAwasdevelopedinordertocreateaglobalstandardforrealtimemultimediaservicesthatensuredinternationalroaming.WiththesupportofITU（InternationalTelecommunicationUnion）aspecificspectrumwasallocated-20Hzfor3Gtelecomsystems.Theworkwaslatertakenoverbythe3GPP（3rdGenerationPartnershipProject）,whichisnowtheWCDMAspecificationbodywithdelegatesfromallovertheworld.

2.WCDMAadevelopmentfromGSMandCDMA

NaturallytherearealotofdifferencesbetweenWCDMAandGSMsystems,huttherearemanysimilaritiesaswell.

TheGSMBaseStationSubsystem（BSS）andtheWCDMARadioAccessNetwork（RAN）arebothconnectedtotheGSMcorenetworkforprovidingaradioconnectiontothehandset.Hence,thetechnologiescansharethesamecorenetwork.Furthermore,bothGSMBSSandWCDMARANsystemsarebasedontheprinciplesofacellularradiosystem.TheGSMBaseStationController（ESC）correspondstotheWCDMARadioNetworkController（RNC）.TheGSMRadioBaseStation（RBS）correspondstotheWCDMARES,andtheA-interfaceofGSMwasthebasisofthedevelopmentoftheIu-interfaceofWCDMA,whichmainlydiffersintheinclusionofthenewservicesofferedbyWCDMA.Thesignificantdifferences,apartfromthelackofinterfacebetweentheGSMBSCsandGSMAbis-interfacetoprovidemulti-vendoroperability,aremoreofasystemicmatter.TheGSMsystemusesTDMA（TimeDivisionMultipleAccess）technologywithalotofradiofunctionalitybasedonmanagingthetimeslots.TheWCDMAsystemontheotherhandusesCDMA,whichmeansthatboththehardwareandthecontrolfunctionsaredifferent.ExamplesofWCDMA-specificfunctionsarefastpowercontrolandsofthandover.

CodeDivisionMultipleAccessandWCDMA

CodeDivisionMultipleAccess（CDMA）isamultipleaccesstechnologywheretheusersareseparatedbyuniquecodes,whichmeansthatalluserscanusethesamefrequencyandtransmitatthesametime.Withthefastdevelopmentinsignalprocessing,'

ithasbecomefeasibletousethetechnologyforwirelesscommunication,alsoreferredtoasWCDMAandCDMA2000.

InCDMAOneandCDMA2000,a1.25MHzwideradiosignalismultipliedbyaspreadingsignal（whichisapseudo-noisecodesequence）withahigherratethanthedatarateofthemessage.Theresultantsignalappearsasseeminglyrandom,butiftheintendedrecipienthastherightcode,thisprocessisreversedandtheoriginalsignalisextracted.Usinguniquecodesmeansthatthesamefrequencyisrepeatedinallcells.whichiscommonlyreferredtoasafrequencyre-useof1.

WCDMAisastepfurtherintheCDMAtechnology.Itusesa5MHzwideradiosignalandachiprateof3.84Mcps,whichisaboutthreetimeshigherthanthechiprateofCDMA2000.Themainbenefitsofawidebandcarrierwithahigherchiprateare:

●Supportforhigherbitrates

●Higherspectrumefficiencythankstoimprovedtrunkingefficiency（i.e.abetterstatisticalaveraging）

●HigherQoS

Further,experiencefromsecond-generationsystemslikeGSMandCDMAOnehasenabledimprovementstobeincorporatedinWCDMA.FocushasalsobeenputonensuringthatasmuchaspossibleofWCDMAoperators'

investmentsinGSMequipmentcanbereused.Examplesarethere-useandevolutionofthecorenetwork,thefocusonco-sitingandthesupportofGSMhandover.InordertouseGSMhandover,the'

subscribersneeddualmodehandsets.

3.RadioNetworkFunctionality

Foroptimaloperationofacompletewirelesssystemi.e.fromhandsettoradioaccessnetwork（RAN）severalfunctionsareneededtocontroltheradionetworkandthemanyhandsetsusingit.Allfunctionsdescribedinthissection,except'

forHandovertoGSM,'

areessentialandthereforenecessaryforaWCDMAsystem.

3.1Powercontrol

Thepowercontrolregulatesthetransmitpoweroftheterminalandbasestation,whichresultsinlessinterferenceandallowsmoreusersonthesamecarrier.Transmitpowerregulationthusprovidesmorecapacityinthenetwork.Withafrequencyre-useof1,itisveryimportanttohaveefficientpowercontrolinordertokeeptheinterferenceataminimum.Foreachsubscriberservicetheaimisthatthebasestationshallreceivethesamepowerlevelfromallhandsetsinthecellregardlessofdistancefromthebasestation.Ifthepowerlevelfromonehandsetishigherthanneeded,thequalitywillbeexcessive,takingadisproportionateshareoftheresourcesandgeneratingunnecessaryinterferencewiththeothersubscribersinthenetwork..Ontheotherhand,ifpowerlevelsaretoolowthiswillresultinpoorquality.Inordertokeepthereceivedpoweratasuitablelevel,WCDMAhasafastpowercontrolthatupdatespowerlevels1500timeseverysecond.Bydoingthattherapidchangeintheradiochannelishandled.Toensuregoodperformance,powercontrolisimplementedinboththeup-linkandthedown-link,whichmeansthatboththeoutputpowersofthehanpsetandthebasestationarefrequentlyupdated.

Powercontrolalsogivesrisetoaphenomenoncalled"

cellbreathing"

.Thisisthetrade-offbetweencoverageandcapacity,whichmeansthatthesizeofthecellvariesdependingonthetrafficload.Whenthenumberofsubscribersinthecellislow（lowload）,goodqualitycanbeachievedevenatalongdistancefromthebasestation.Ontheotherhand,whenthenumberofusersinthecellishigh,thelargenumberofsubscribersgeneratesahighinterferencelevelandsubscribershavetogetclosertothebasestationtoachievegoodquality.

3.2Softandsofterhandover

Withsoftandsofterhandoverfunctionalitythehandsetcancommunicatesimultaneouslywithtwoormorecellsintwoormorebasestations.Thisflexibilityinkeepingtheconnectionopentomorethanonebasestationresultsinfewerlostcalls,whichisveryimportanttotheoperator.

Toachievegoodsystemperformancewithafrequencyre-useof1andpowercontrol,softandsofterhandoverisrequired.Softandsofterhandoverenablesthehandsettomaintainthecontinuityandqualityoftheconnectionwhilemovingfrom.onecelltoanother.

Duringsofthandover,thehandsetwillmomentarilyadjustitspowertothebasestationthatrequiresthesmallestamountoftransmitpowerandthepreferredcellmaychangeveryrapidly.Thedifferencebetweensoftandsofterhandoveristhatduringsofthandover,thehandsetisconnectedtomultiplecellsatdifferentbasestations,whileduringsofterhandover,thehandsetisconnectedtomultiplecellsatthesamebasestation.Adrawbackwithsofthandoveristhatitrequiresadditionalhardwareresourcesonthenetworkside,asthehandsethasmultipleconnections.Inawell-designedradionetwork,30%-40%oftheuserswillbeinsoftorsofterhandover.Asanexampleofsoftorsofterhandover.

3.3HandovertoGSM（inter-systemhandover）

WhenWCDMAwasstandardizedakeyaspectwastoensurethatexistinginvestmentscouldbere-usedasmuchaspossible.Oneexampleishandoverbetweenthenew（WCDMA）networkandtheexisting（GSM）network,whichcanbetriggeredbycoverage,capacityorservicerequirements.HandoverfromWCDMAtoGSM,forcoveragereasons,isinitiallyexpectedtobeveryimportantsinceoperatorsareexpectedtodeployWCDMAgraduallywithintheirexistingGSMnetwork.WhenasubscribermovesoutoftheWCDMAcoveragearea,ahandovertoGSMhastobeconductedinordertokeeptheconnection.

HandoverbetweenGSMandWCDMAcanalsohaveapositiveeffectoncapacitythroughthepossibilityofloadsharing.IfforexamplethenumbersofsubscribersintheGSMnetworkisclosetothecapacitylimitinonearea,handoverofsomesubscriberstotheWCDMAnetworkcanbeperformed.

Anotherfunctionthatisrelatedtointer-systemhandoveristhecompressedmode.WhenperforminghandovertoGSM,measurementshavetobemadeinordertoidentifytheGSMcelltowhichthehandoverwillbemade.Thecompressedmodeisusedtocreatethemeasurementperiodsforthehandsettomaketherequiredmeasurements.

3.4Inter-frequencyhandover（intra-systemhandover）

Theneedforinter-frequencyhandoveroccursinhighcapacityareaswheremultiple5MHzWCDMAcarriersaredeployed.Inter-frequencyhandover,whichishandoverbetweenWCDMAcarriersondifferentfrequencies,hasmanysimilaritieswithGSMhandover,forexamplethecompressedmodefunctionality.

3.5Channeltypeswitching

InWCDMAtherearedifferenttypesofchannelsthatcanbeusedtocarrydatainordertomaximizethetotaltrafficthroughput.Thetwomostbasiconesarecommonchannelsanddedicatedchannels.Channeltypeswitchingfunctionalityisusedtomovesubscribersbetweenthecommonandthededicatedchannel,dependingonhowmuchinformationthesubscriberneedstotransmit.Thededicatedchannelisusedwhenthereismuchinformationtotransmit,suchasavoiceconversationordownloadingawebpage.Itutilizestheradioresourcesefficientlyasitsupportsbothpowercontrolandsofthandover.

3.6Admissioncontrol

Asthereisaverycleartrade-offbetweencoverageandcapacityinWCDMAsystems,theadmissioncontrolfunctionalityisusedtoavoidsystemoverloadandtoprovidetheplannedcoverage.Whenanewsubscriberseeksaccesstothenetwork,admissioncontrolestimatesthenetw