通信工程外文文献Word格式文档下载.docx

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ReviewofUMTS

1.1UMTSNetworkArchitecture

TheEuropean/Japanese3GstandardisreferredtoasUMTS.UMTSisoneofanumberofstandardsratifiedbytheITU-TundertheumbrellaofIMT-2000.Itiscurrentlythedominantstandard,withtheUSCDMA2000standardgainingground,particularlywithoperatorsthathavedeployedcdmaOneastheir2Gtechnology.Attimeofwriting,Japanisthemostadvancedintermsof3Gnetworkdeployment.Thethreeincumbentoperatorstherehaveimplementedthreedifferenttechnologies:

J-PhoneisusingUMTS,KDDIhasaCDMA2000network,andthelargestoperatorNTTDoCoMoisusingasystembrandedasFOMA（FreedomofMultimediaAccess）.FOMAisbasedontheoriginalUMTSproposal,priortoitsharmonizationandstandardization.

TheUMTSstandardisspecifiedasamigrationfromthesecondgenerationGSMstandardtoUMTSviatheGeneralPacketRadioSystem（GPRS）andEnhancedDataforGlobalEvolution（EDGE）,asshowninFigure.ThisisasoundrationalesinceasofApril2003,therewereover847MillionGSMsubscribersworldwide1,accountingfor

68%oftheglobalcellularsubscriberfigures.Theemphasisisonkeepingasmuchof

theGSMnetworkaspossibletooperatewiththenewsystem.

WearenowwellontheroadtowardsThirdGeneration（3G）,wherethenetworkwillsupportalltraffictypes:

voice,videoanddata,andweshouldseeaneventualexplosionintheservicesavailableonthemobiledevice.ThedrivingtechnologyforthisistheIPprotocol.Manycellularoperatorsarenowatapositionreferredtoas2.5G,withthedeploymentofGPRS,whichintroducesanIPbackboneintothemobilecorenetwork.Thediagrambelow,Figure2,showsanoverviewofthekeycomponentsinaGPRSnetwork,andhowitfitsintotheexistingGSMinfrastructure.

TheinterfacebetweentheSGSNandGGSNisknownastheGninterfaceandusestheGPRStunnelingprotocol（GTP,discussedlater）.Theprimaryreasonfortheintroductionofthisinfrastructureistoofferconnectionstoexternalpacketnetworks,suchastheInternetoracorporateIntranet.

ThisbringstheIPprotocolintothenetworkasatransportbetweentheSGSNandGGSN.Thisallowsdataservicessuchasemailorwebbrowsingonthemobiledevice,withusersbeingchargedbasedonvolumeofdataratherthantimeconnected.

Thedominantstandardfordeliveryof3GnetworksandservicesistheUniversalMobileTelecommunicationsSystem,orUMTS.ThefirstdeploymentofUMTSistheRelease’99architecture,shownbelowinFigure3.

Inthisnetwork,themajorchangeisintheradioaccessnetwork（RAN）withtheintroductionofCDMAtechnologyfortheairinterface,andATMasatransportinthetransmissionpart.Thesechangeshavebeenintroducedprincipallytosupportthetransportofvoice,videoanddataservicesonthesamenetwork.Thecorenetworkremainsrelativelyunchanged,withprimarilysoftwareupgrades.However,theIPprotocolpushesfurtherintothenetworkwiththeRNCnowcommunicatingwiththe3GSGSNusingIP.

ThenextevolutionstepistheRelease4architecture,Figure4.Here,theGSMcoreisreplacedwithanIPnetworkinfrastructurebasedaroundVoiceoverIPtechnology.

TheMSCevolvesintotwoseparatecomponents:

aMediaGateway（MGW）andanMSCServer（MSS）.Thisessentiallybreaksaparttherolesofconnectionandconnectioncontrol.AnMSScanhandlemultipleMGWs,makingthenetworkmorescaleable.

SincetherearenowanumberofIPcloudsinthe3Gnetwork,itmakessensetomergethesetogetherintooneIPorIP/ATMbackbone（itislikelybothoptionswillbeavailabletooperators.）ThisextendsIPrightacrossthewholenetwork,allthewaytotheBTS.ThisisreferredtoastheAll-IPnetwork,ortheRelease5architecture,asshowninFigure5.TheHLR/VLR/EIRaregeneralisedandreferredtoastheHLRSubsystem（HSS）.

Nowthelastremnantsoftraditionaltelecommunicationsswitchingareremoved,leavinganetworkoperatingcompletelyontheIPprotocol,andgeneralisedforthetransportofmanyservicetypes.Real-timeservicesaresupportedthroughtheintroductionofanewnetworkdomain,theIPMultimediaSubsystem（IMS）.

Currentlythe3GPPareworkingonRelease6,whichpurportstocoverallaspectsnotaddressedinfrozenreleases.SomecallUMTSRelease64GanditincludessuchissuesasinterworkingofhotspotradioaccesstechnologiessuchaswirelessLAN.

1.2UMTSFDDandTDD

LikeanyCDMAsystem,UMTSneedsawidefrequencybandinwhichtooperatetoeffectivelyspreadsignals.Thedefiningcharacteristicofthesystemisthechiprate,whereachipisthewidthofonesymboloftheCDMAcode.UMTSusesachiprateof3.84Mchips/sandthisconvertstoarequiredspectrumcarrierof5MHzwide.Sincethisiswiderthanthe1.25MHzneededfortheexistingcdmaOnesystem,theUMTSairinterfaceistermed‘wideband’CDMA.

ThereareactuallytworadiotechnologiesundertheUMTSumbrella:

UMTSFDDandTDD.FDDstandsforFrequencyDivisionDuplex,andlikeGSM,separatestrafficintheuplinkanddownlinkbyplacingthematdifferentfrequencychannels.Thereforeanoperatormusthaveapairoffrequenciesallocatedtoallowthemtorunanetwork,hencetheterm‘pairedspectrum’.TDDorTimeDivisionDuplexrequiresonlyonefrequencychannel,anduplinkanddownlinktrafficareseparatedbysendingthematdifferenttimes.TheITU-Tspectrumusage,asshowninFigure6,forFDDis1920-980MHzforuplinktraffic,and2110-2170MHzfordownlink.Theminimumallocationanoperatorneedsistwopaired5MHzchannels,oneforuplinkandonefordownlink,ataseparationof190MHz.However,toprovidecomprehensivecoverageandservices,itisrecommendedthatanoperatorbegiventhreechannels.Consideringthespectrumallocation,thereare12pairedchannelsavailable,andmanycountrieshavenowcompletedthelicencingprocessforthisspectrum,allocatingbetweentwoandfourchannelsperlicence.Thishastendedtoworkoutacostlyprocessforoperators,sincetheregulatoryauthoritiesinsomecountries,notablyinEurope,haveauctionedtheselicencestothehighestbidder.Thishasresultedinspectrumfeesashighastensofbillionsofdollarsinsomecountries.

TheTimeDivisionDuplex（TDD）system,whichneedsonlyone5MHzbandinwhichtooperate,oftenreferredtoasunpairedspectrum.ThedifferencesbetweenUMTSFDDandTDDareonlyevidentatthelowerlayers,particularlyontheradiointerface.Athigherlayers,thebulkoftheoperationofthetwosystemsisthesame.Asthenamesuggests,theTDDsystemseparatesuplinkanddownlinktrafficbyplacingthemindifferenttimeslots.Aswillbeseenlater,UMTSusesa10msframestructurewhichisdividedinto15equaltimeslots.TDDcanallocatethesetobeeitheruplinkordownlink,withoneormorebreakpointsbetweenthetwoinaframedefined.Inthisway,itiswellsuitedtopackettraffic,sincethisallowsgreatflexibilityindynamicallydimensioningforasymmetryintrafficflow.

TheTDDsystemshouldnotreallybeconsideredasanindependentnetwork,butratherasasupplementforanFDDsystemtoprovidehotspotcoverageathigherdatarates.Itisratherunsuitableforlargescaledeploymentduetointerferencebetweensites,sinceaBTSmaybetryingtodetectaweaksignalfromaUE,whichisblockedoutbyarelativelystrongsignalatthesamefrequencyfromanearbyBTS.TDDisidealforindoorcoverageoversmallareas.

SinceFDDisthemainaccesstechnologybeingdevelopedcurrently,theexplanationspresentedherewillfocuspurelyonthissystem.

1.3UMTSBearerModel

TheproceduresofamobiledeviceconnectingtoaUMTSnetworkcanbesplitintotwoareas:

theaccessstratum（AS）andthenon-accessstratum（NAS）.Theaccessstratuminvolvesallthelayersandsubsystemsthatoffergeneralservicestothenon-accessstratum.InUMTS,theaccessstratumconsistsofalloftheelementsintheradioaccessnetwork,includingtheunderlyingATMtransportnetwork,andthevariousmechanismssuchasthosetoprovidereliableinformationexchange.Allofthenon-accessstratumfunctionsarethosebetweenthemobiledeviceandthecorenetwork,forexample,mobilitymanagement.Figure7showsthearchitecturemodel.TheASinteractswiththeNASthroughtheuseofserviceaccesspoints（SAPs）.

UMTSradioaccessnetwork（UTRAN）providesthisseparationofNASandASfunctions,andallowsforASfunctionstobefullycontrolledandimplementedwithintheUTRAN.ThetwomajorUTRANinterfacesaretheUu,whichistheinterfacebetweenthemobiledevice,orUserEquipment（UE）andtheUTRAN,andtheIu,whichistheinterfacebetweentheUTRANandthecorenetwork.Bothoftheseinterfacescanbedividedintocontrolanduserplaneseachwithappropriateprotocolfunctions.

ABearerServiceisalinkbetweentwopoints,whichisdefinedbyacertainsetofcharacteristics.InthecaseofUMTS,thebearerserviceisdeliveredusingradioaccessbearers.

ARadioaccessbearer（RAB）isdefinedastheservicethattheaccessstratum（i.e.UTRAN）providestothenon-accessstratumfortransferofuserdatabetweentheUserEquipmentandCoreNetwork.ARABcanconsistofanumberofsubflows,whicharedatastreamstothecorenetworkwithintheRABthathavedifferentQoScharacteristics,suchasdifferentreliabilities.AcommonexampleofthisisdifferentclassesofbitswithdifferentbiterrorratescanberealisedasdifferentRABsubflows.RABsubflowsareestablishedandreleasedatthetimetheRABisestablishedandreleased,andaredeliveredtogetheroverthesametransportbearer.

ARadioLinkisdefinedasalogicalassociationbetweenasingleUserEquipment（UE）andasingleUTRANaccesspoint,suchasanRNC.Itisphysicallycomprisedofoneorm