GPRS外文翻译理论研究通用分组无线业务.docx
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GPRS外文翻译理论研究通用分组无线业务.docx
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GPRS外文翻译理论研究通用分组无线业务
山东科技大学
外文翻译
学院名称电气信息系
专业班级电气工程及其自动化10-4班
学生姓名梅玉金
学号201003201414
指导教师姚福强翟绪梅
TheoreticalStudyoftheGeneralPacketRadioService
Abstract:
Ascommunicationstechnologycontinuestomatureandusers’qualityhavebecomeincreasinglydemanding,newdataapplicationsareemergingandarereachingthegeneralpublic.ThroughtheuseofGPRS(GeneralPacketRadioService)technology,theexistingGSMnetworkscanbeeasilyachievedwithsimplehigh-speeddatapacketaccess,enablingmobileoperatorstorespondrapidlytomarketdemandsandgaincompetitiveadvantage.ThispaperaimstoprovideacomprehensiveyetsimpleoverviewoftheGPRSsystemfromtheuser’sandfromthearchitecturalperspectives.
Keywords:
GPRSnetworks;Voice/dataintegration;Multiple-channels;Packetradio;Telecommunications
1Introduction
Inthelate1800s,Marconididthepioneerworkestablishingthefirstsuccessfulradiolinkbetweenaland-basedstationandatugboat.Sincethen,wirelesscommunicationsystemshavebeendevelopingandevolvingwithafuriouspace.Thenumberofmobilesubscribershasbeengrowingtremendouslyinthepastdecades.Intheearlystages,wirelesscommunicationsystemsweredominatedbymilitaryusageandsupportedaccordingtomilitaryneedsandrequirements.Duringthelasthalfacentury,withincreasingcivilapplicationsofmobileservices,commercialwirelesscommunicationsystemshavebeentakingthelead.Theearlywirelesssystemsconsistedofabasestationwithahigh-powertransmitterandservedalargegeographicarea.Eachbasestationcouldserveonlyasmallnumberofusersandwascostlyaswell.
ThesystemswereisolatedfromeachotherandonlyafewofthecommunicatedwiththePSTN(PublicSwitchedTelephoneNetworks).Today,thecellularsystemsconsistofaclusterofba-sestationwithlowpowerradiotransmitters.Eachbasestationservesasmallcellwithinalargegeographicarea.Thetotalnumberofusersservedisincreasedbecauseofchannelreuseandal-solargerfrequencybandwidth.
GlobalsystemformobilecommunicationsistheEuropeanstandardforcellularcommunicationsdevelopedbytheETSI(EuropeanTelecommunicationsStandardsInstitute).ThroughoutEuropeandtherestoftheworld,GSMhasbeenwidelyadopted.Ithasalreadybeenimplementedinover100countries.ThemostimportantserviceinGSMis
voicetelephony.VoiceisdigitallyencodedandcarriedbytheGSMnetworkasadigitalstreaminacircuit-switchedmode.
Dataserviceshavebeengainingpopularityincellularnetworkssincetheywerefirstintroducedthroughthecircuit-switchedconnection.Ascellularcarriers,however,startdeployingthepacketdataservicesuchasGPRS(GeneralPacketRadioService),carrierscanprovidedataservicesinamorebandwidth-efficientwayoverthecellularnetwork,therebyincreasingtheradioresourceutilization.
2GPRS-relatedtechnologies
GSMoffersdataservicesalreadybuttheyhavebeenconstrainedbytheuseofcircuit-switcheddatachannelsovertheairinterfaceallowingamaximumbitrateof14.4kb/s.Forthisreason,theGSMstandardhascontinueditsnaturalevolutiontoaccommodatetherequirementforhigherbitrates.TheHSCSD(High-speedCircuit-switchedData)areonesolutionthataddressthisrequirementbyallocatingmoretimeslotspersubscriberandthusbetterrates.ItremainshoweverinsufficientforburstydataapplicationssuchasWebbrowsing.Moreover,HSCSDrelyoncircuit-switchingtechniquesmakingitunattractiveforsubscriberswhowanttobechargedbasedonthevolumeofthedatatraffictheyactuallyuseratherthanonthedurationoftheconnection.Inturn,serviceprovidersneedeffectivemeanstosharethescarceradioresourcesbetweenmoresubscribers.Inacircuit-switchedmode,achannelisallocatedtoasingleuserforthedurationoftheconnection.Thisexclusiveaccesstoradioresourcesisnotnecessaryfordataapplicationswiththeuseofpacketswitchedtechniques.
GPRSstandsoutasonemajordevelopmentintheGSMstandardthatbenefitsfrompacketswitchedtechniquestoprovidemobilesubscriberswiththemuchneededhighbitratesforburstydatatransmissions.ItispossibletheoreticallyforGPRSsubscriberstouseseveraltimeslots(packetdatachannels)simultaneouslyreachingabitrateofabout170kb/s.Volume-basedchargingispossiblebecausechannelsareallocatedtousersonlywhenpacketsaretobesentorreceived.Burstydataapplicationsmakeitpossibletobalancemoreefficientlythenetworkresourcesbetweenusersbecausetheprovidercanusetransmissiongapsforothersubscriberactivities.
2.1BasicPrinciple
Inacellularnetwork,anentiregeographicareaisdividedintocells,witheachcellbeingservedbyabasestation.Becauseofthelowtransmissionpoweratthebasestation,thesamechannelscanbereusedagaininanothercellwithoutcausingtoomuch
interference.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.PowercontrolisneededinFDMAandTDMA
systemsbecauseoftheco-channelinterferencemanagement.Thistypeofinterferenceiscausedbythefrequencyreuseinthelimitedavailablespectrum.Viaaproperpowerleveladjustment,theco-channelinterferencecanbereduced.Thisallowsahigherfrequencyreusefactorandthusincreasesthesystemcapacity.OwnercontrolisthemostessentialrequirementinCDMAsystems.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-switchedandapacketsw
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