葛彦麟 基于GPRS的无线显示系统的设计与实现 外文翻译 第二稿.docx

葛彦麟 基于GPRS的无线显示系统的设计与实现 外文翻译 第二稿.docx

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葛彦麟基于GPRS的无线显示系统的设计与实现外文翻译第二稿

东南大学成贤学院

外文翻译材料

电子工程系自动化专业

学生姓名：

葛彦麟

学号：

01207111　　

设计地点：

东南大学成贤学院

指导教师：

汪力纯

TheoreticalStudyoftheGeneralPacketRadioService

Abstract:

Ascommunicationstechnologycontinuestomaturea-ndusers’qualityhavebecomeincreasinglydemanding,newda-taapplicationsareemergingandarereachingthegeneralpublic.ThroughtheuseofGPRS（GeneralPacketRadioService）techn-ology,theexistingGSMnetworkscanbeeasilyachievedwithsimplehigh-speeddatapacketaccess,enablingmobileoperatorstorespondrapidlytomarketdemandsandgaincompetitiveadva-ntage.ThispaperaimstoprovideacomprehensiveyetsimpleoverviewoftheGPRSsystemfromtheuser’sandfromthearch-itecturalperspectives.

Keywords:

GPRSnetworks;Voice/dataintegration;Multiple-channels;Packetradio;Telecommunications

1Introduction

Inthelate1800s,Marconididthepioneerworkestablishingthefirstsuccessfulradiolinkbetweenaland-basedstationandatugboat.Sincethen,wirelesscommunicationsystemshavebeendevelopingandevolvingwithafuriouspace.Thenumberofmobilesubscribershasbeengrowingtremendouslyinthepastdecades.Intheearlystages,wirelesscommunicationsystemsweredominatedbymilitaryusageandsupportedaccordingtomilitaryneedsandrequirements.Duringthelasthalfacentury,withincreasingcivilapplicationsofmobileservices,commercialwirelesscommunicationsystemshavebeentakingthelead.Theearlywirelesssystemsconsistedofabasestationwithahigh-powertransmitterandservedalargegeographicarea.Eachbasestationcouldserveonlyasmallnumberofusersandwascostlyaswell.

ThesystemswereisolatedfromeachotherandonlyafewofthecommunicatedwiththePSTNs（PublicSwitchedTelephoneNetworks）.Today,thecellularsystemsconsistofaclusterofba-sestationwithlowpowerradiotransmitters.Eachbasestations-ervesasmallcellwithinalargegeographicarea.Thetotalnum-berofusersservedisincreasedbecauseofchannelreuseandal-solargerfrequencybandwidth.

GlobalsystemformobilecommunicationsistheEuropeanstandardforcellularcommunicationsdevelopedbytheETSI（EuropeanTelecommunicationsStandardsInstitute）.ThroughoutEuropeandtherestoftheworld,GSMhasbeenwidelyadopted.Ithasalreadybeenimplementedinover100countries.ThemostimportantserviceinGSMisvoicetelephony.VoiceisdigitallyencodedandcarriedbytheGSMnetworkasadigitalstreaminacircuit-switchedmode.

Dataserviceshavebeengainingpopularityincellularnetworkssincetheywerefirstintroducedthroughthecircuit-switchedconnection.Ascellularcarriers,however,startdeployingthepacketdataservicesuchasGPRS（GeneralPacketRadioService）,carrierscanprovidedataservicesinamorebandwidth-efficientwayoverthecellularnetwork,therebyincreasingtheradioresourceutilization.

2GPRS-relatedtechnologies

GSMoffersdataservicesalreadybuttheyhavebeenconstrainedbytheuseofcircuit-switcheddatachannelsovertheairinterfaceallowingamaximumbitrateof14.4kbit/s.Forthisreason,theGSMstandardhascontinueditsnaturalevolutionto

accommodatetherequirementforhigherbitrates.TheHSCSD（High-speedCircuit-switchedData）areonesolutionthataddressthisrequirementbyallocatingmoretimeslotspersubscriberandthusbetterrates.ItremainshoweverinsufficientforburstydataapplicationssuchasWebbrowsing.Moreover,HSCSDrelyoncircuit-switchingtechniquesmakingitunattractiveforsubscriberswhowanttobechargedbasedonthevolumeofthedatatraffictheyactuallyuseratherthanonthedurationoftheconnection.Inturn,serviceprovidersneedeffectivemeanstosharethescarceradioresourcesbetweenmoresubscribers.Inacircuit-switchedmode,achannelisallocatedtoasingleuserforthedurationoftheconnection.Thisexclusiveaccesstoradioresourcesisnotnecessaryfordataapplicationswiththeuseofpacketswitchedtechniques.

GPRSstandsoutasonemajordevelopmentintheGSMstandardthatbenefitsfrompacketswitchedtechniquestoprovidemobilesubscriberswiththemuchneededhighbitratesforburstydatatransmissions.ItispossibletheoreticallyforGPRSsubscriberstouseseveraltimeslots（packetdatachannels）simultaneouslyreachingabitrateofabout170kbit/s.Volume-basedchargingispossiblebecausechannelsareallocatedtousersonlywhenpacketsaretobesentorreceived.Burstydataapplicationsmakeitpossibletobalancemoreefficientlythenetworkresourcesbetweenusersbecausetheprovidercanusetransmissiongapsforothersubscriberactivities.

2.1BasicPrinciple

Inacellularnetwork,anentiregeographicareaisdividedintocells,witheachcellbeingservedbyabasestation.Becauseofthelowtransmissionpoweratthebasestation,thesamechannelscanbereusedagaininanothercellwithoutcausingtoomuchinterference.Theconfigurationandplanningofthecellischosentominimizetheinterferencefromanothercellandthusmaximumcapacitycanbeachieved.Thecellisusuallydepictedasahexagon,butinrealitytheactualshapevariesaccordingtothegeographicenvironmentandradiopropagation.Channelallocationischosenbasedonthedensityoftheusers.Ifacellhasmanyuserstoserve,usuallymorechannelsareallocated.Thechannelsarethenreusedinadjacentcellsorclusterofcells.Thespatialseparationofthecellswiththesameradiochannels,inconjunctionwiththelowtransmissionpowerandantennaorientation,keepstheco-channelinterferenceatanacceptablelevel.

Mobilityisoneofthekeyfeaturesinwirelesscommunicationsystems.Thereisaneedtotracktheusersmovingintodifferentcellsandchangingradiochannels.Amobileswitchedtoanotherchannelinadifferentcelliscalledhandoff.Asignalingandcallprocessingprocedureisneededtosupportusermobilityandhandoffsuchthatamobilephonecanbecompletedsuccessfully.Pagingisanotherkeyfeatureincellularsystems.Itusesacommonsharedchanneltolocatetheuserswithintheserviceareaandtobroadcastsomesignalingmessages.

2.2MultipleAccessTechnique

Multipleaccessisatechniquetoallowuserstoshareacommunicationmediumsothattheoverallcapacitycanbeincreased.Therearethreecommonlyusedmultipleaccessschemes:

FDMA（FrequencyDivisionMultipleAccess）,TDMA（TimeDivisionMultipleAccess）,CDMA（CodeDivisionMultipleAccess）.InFDMA,eachcallisassigneditsownbandoffrequencyforthedurationofthecall.Theentirefrequencybandisdividedintomanysmallindividualchannelsforuserstoaccess.InTDMA,userssharethesamebandoffrequencies.

Eachcallisassignedadifferenttimeslotforitstransmission.InCDMA,userssharethesamebandoffrequenciesandtimeslots.Eachcallisassignedauniquecode,whichcanspreadthespectrumtotheentirefrequencyband.Thespectrumspreadcallsaresentontopofeachothersimultaneously,andareseparatedatthereceiverbyaninverseoperationoftheuniquecodes.Acombinationofthethreemultipleaccessschemescanalsobeapplied.

2.3PowerControl

PowercontrolisoneofthemostimportantdesignfeaturesinwirelesscommunicationincludingFDMA,TDMA,andCDMAsystems.Itensureseachusertransmitsandreceivesataproperenergyleveltoconveyinformationsuccessfullywhilereducingtheinterferencetootherusers.PowercontrolisneededinFDMAandTDMAsystemsbecauseoftheco-channelinterferencemanagement.Thistypeofinterferenceiscausedbythefrequencyreuseinthelimitedavailablespectrum.Viaaproperpowerleveladjustment,theco-channelinterferencecanbereduced.Thisallowsahigherfrequencyreusefactorandthusincreasesthesystemcapacity.owercontrolisthemostessentialrequirementinCDMAsystems.Withoutpowercontrol,allthemobilestransmittothebasestationwiththesamepowernottakingintoaccountpathlossandfadingeffect.Mobilesclosetothebasestationwillcausesignificantinterferencetomobilesthatarefartherawayfromthebasestation.Thiseffectistheso-callednear/fareffect.Therefore,awell-designedpowercontrolalgorithmiscrucialforproperoperationofaCDMAsystem.Intheabsenceofpowercontrol,thesystemcapacityisverylowcomparedtoothersystems.Anotheradvantageofpowercontrolisthatitcanprolongbatterylifebyusingaminimumrequiredtransmissionpower.Powercontrolonareverselinkismorestringentthanonaforwardlinkbecauseofthenear/fareffect.Onaforwardlink,powercontrolisstillnecessarytoreducetheinter-cellinterference.

Powercontrolcanbeoperatedinacentralizedformoradistributedform.Acentralizedcontrollerobtainstheinformationofalltheestablishedconnectionsandchannelgains,andcontrolsthetransmissionpowerlevel.Thecentralizedapproachcanoptimizethepowerusageoftheentireorpartofthenetworkandthusisveryefficient.Itrequiresextensivecontrolsignalinginthenetwork,however,andisdifficulttoapplyinpractice.

3GPRSarchitecture

GPRSisconsideredasaserviceorfeatureofGSM.ItwasdesignedbyETSItobeimplementedovertheexistinginfrastructureofGSMwithoutinterferingwiththealreadyexistingservices.TheaimisquickGPRSdeploymentwithminorimpactonexistingGSMPLMNcomponents.Fig.1illustratesthelogicalarchitectureofaGSMnetworksupportingGPRS.

Figure1.ArchitectureofGPRSnetwork

3.1MobileStation

GPRSandGSMsystemsprovideinter-workingandsharingofresourcesdynamicallybetweenusers.Forthisreason,threetypesofterminalshavebeendefined:

aclass-AMScancarryacircuit-switchedandapacketswitchedconnect