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Word下载.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Word下载.docx
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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
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