外文翻译--全球移动通信系统概要.doc

OverviewoftheGlobalSystemforMobileCommunications1HistoryofGSMDuringtheearly1980s,analogcellulartelephonesystemswereexperiencingrapidgrowthinEurope,particularlyinScandinaviaandtheUnitedKingdom,butalsoinFranceandGermany.Eachcountrydevelopeditsownsystem,whichwasincompatiblewitheveryoneelsesinequipmentandoperation.Thiswasanundesirablesituation,becausenotonlywasthemobileequipmentlimitedtooperationwithinnationalboundaries,whichinaunifiedEuropewereincreasinglyunimportant,buttherewasalsoaverylimitedmarketforeachtypeofequipment,soeconomiesofscaleandthesubsequentsavingscouldnotberealized.TheEuropeansrealizedthisearlyon,andin1982theConferenceofEuropeansPostsandTelegraphs(CEPT)formedastudygroupcalledtheGroupSpecialMobile(GSM)tostudyanddevelopapan-Europeanpubliclandmobilesystem.Theproposedsystemhastomeetcertaincriteria:GoodsubjectivespeechqualityLowterminalandservicecostSupportforinternationalroamingAbilitytosupporthandheldterminalsSupportforrangeofnewservicesandfacilitiesSpectralefficiencyISDNcompatibilityIn1989,GSMresponsibilitywastransferredtotheEuropeanTelecommunicationStandardsInstitute(ETSI),andphaseIoftheGSMspecificationswerepublishedin1990.Commercialservicewasstartedinmid-1991,andby1993therewere36GSMnetworksin22countries.AlthoughstandardizedinEurope,GSMisnotonlyaEuropeanstandard.Over200GSMnetworksareoperationalin110countriesaroundtheworld.Inthebeginningof1994,therewere1.3millionsubscribersworldwide,whichhadgrowntomorethan55millionbyOctober1997.WithNorthAmericamakingadelayedentryintotheGSMfieldwithaderivativeofGSMcalledPCS1900,GSMsystemsexistoneverycontinent,andtheacronymGSMnowaptlystandsforGlobalSystemforMobilecommunications.ThedevelopersofGSMchoseanunproven(atthetime)digitalsystem,asopposedtothethen-standardanalogcellularsystemslikeAMPSintheUnitedStatesandTACSintheUnitedKingdom.Theyhadfaiththatadvancementsincompressionalgorithmsanddigitalsignalprocessorswouldallowthefulfillmentoftheoriginalcriteriaandthecontinualimprovementofthesystemintermsofqualityandcost.Theover8,000pagesofGSMrecommendationstrytoallowflexibilityandcompetitiveinnovationamongsuppliers,butprovideenoughstandardizationtoguaranteeproperinterworkingbetweenthecomponentsofthesystem.Thisdonebyprovidingfunctionalandinterfacedescriptionsforeachofthefunctionalentitiesdefinedinthesystem.2GeneralConcepts2.1AnalogVersusDigitalTelephonySystemsFirst-generationsystemswereanalog.Duringtheearly1980stheseunderwentrapiddevelopmentinEurope.AlthoughtheNMTsystemwasusedbyalltheNordiccountries,andtheTACSsystemintheUnitedKingdomandItaly,therewasavarietyofsystemsandnocompatibilityamongthem.Comparedwiththesesystems,themainadvantagesofferedbyGSM,whichisthemostimportantofthesecond-generationdigitalsystems,are:Standardization；Capacity；Quality；Security.Standardizationguaranteescompatibilityamongsystemsofdifferentcountries,allowingsubscriberstousetheirownterminalsinthosecountriesthathaveadoptedthedigitalstandard.Thelackofstandardizationinthefirst-generationsystemlimitedservicetowithinthebordersofacountry.Mobilityisimproved,sinceroamingisnolongerlimitedtoareascoveredbyacertainsystem.Callscanbechargedandhandledusingthesamepersonalnumberevenwhenthesubscribermovesfromonecountrytoanother.Standardizationalsoallowstheoperatortobuyentitiesofthenetworkfromdifferentvendors,sincethefunctionalelementsofthenetworkandtheinterfacesbetweentheseelementsarestandardized.Thismeansthatamobilephonefromanymanufacturerisabletocommunicatewithanynetwork,evenifthisnetworkisbuiltwithentitiesfromdifferentvendors.Thisleadstoalargeeconomyofscaleandresultsincostreductionforboththeoperatorandthesubscriber.Furthermore,thephonecostisalsoreduced,becauseGSMisaninternationalstandard,producedquantitiesaregreaterandthelevelofcompetitionishigh.Withrespecttocapacity,theuseoftheradioresourceismuchmoreefficientinadigitalsystemsuchasGSMthaninananalogsystem.Thismeansthatmoreuserscanbeallocatedinthesamefrequencybandwidth.Thisispossiblewiththeuseofadvanceddigitaltechniques,suchasvoicecompressionalgorithms,channelcoding,andmultipleaccesstechniques.Notethatcapacitygainsarealsoachievedwithradiofrequencyreuse,whichhadalsousedinanalogsystems.Frequencyreusemeansthatagivencarriercanbeemployedindifferentareas.Thequalityindigitaltransmissionsystemsisbetter,thankstothechannelcodingschemesthatincreasetherobustnessinthefaceofnoiseanddisturbancessuchasinterferencecausedbyotherusersorothersystems.Thequalityimprovementisalsoduetotheimprovedcontroloftheradiolink,andadaptationstopropagationconditions,withadvancedtechniquessuchaspowercontrolorfrequencyhopping.Thiswillbeexplainedingreaterdetailinthenextsection.Intermsofsecurity,powerfulauthenticationandencryptiontechniquesforvoiceanddatacommunicationsareenabledwithGSM,whichguaranteesprotectedaccesstothenetwork,andconfidentiality.2.2CellularTelephonyInmobileradiosystems,oneofthemostimportantfactorsisthefrequencyspectrum.Inordertomakethebestuseofthebandwidth,thesystemisdesignedbymeansofthedivisionoftheserviceareaintoneighboringzones,orcells,whichintheoryhaveahexagonalshape.EachcellhasaBaseTransceiverStation(BTS),whichtoavoidinterferenceoperatesonasetofradiochannelsdifferentfromthoseoftheadjacentcells.Thisdivisionallowsfortheuseofthesamefrequenciesinnonadjacentcells.Agroupofcellsthatasawholeusetheentireradiospectrumavailabletotheoperatorisreferredtoasacluster.Theshapeofacellisirregular,dependingontheavailabilityofaspotfortheBTS,thegeographyoftheterrain,thepropagationoftheradiosignalinthepresenceofobstacles,andsoon.Indenseurbanareas,forinstance,wherethemobiletelephonytrafficisimportant,thediameterofthecellsisoftenreducedinordertoincreasecapacity.Thisisallowedsincethesamefrequencychannelsareusedinasmallerarea.Ontheotherhand,reducingthecelldiameterleadstoadecreaseinthedistancenecessarytoreusethefrequencies(thatis,thedistancebetweentwoco-channelcells),increasingco-channelinterference.Inordertominimizethelevelofinterference,severaltechniquesareusedontheradiointerface.AbasicexampleofclusterorganizationisshowninFigure1.Inthisexample,weseeareuseatternforsevendifferentfrequencies,f1tof7.Thesefrequenciescorrespondtothebeaconcarrierofeachcell,onwhichsignalinginformationaboutthecellisbroadcast(seeSection2.7).Itcanbeseenfromthisfigurethatagivencarriercanbereusedintwoseparategeographicalareas,aslongastheseareasarefarenoughfromeachothertoreducetheeffectofinterference.Withthistechniqueofdividingtheareaincellsandclusters,theoperatorcanincreasetheareaitisabletocoverwithalimitedfrequencybandwidth.Figure1ExampleofaCellPlanning2.3PublicLandMobileNetworkApublicLandMobileNetwork(PLMN)isanetworkestablishedforthepurposeofprovidinglandmobiletelecommunicationsservicestothepublic.Itmaybeconsideredasanextensionofafixednetwork,suchasthePublicSwitchedTelephoneNetwork(PSTN),orasanintegralpartofthePSTN.2.4MultibandMobilePhonesBecauseoftheincreasingdemandonthemobilenetworks,todaythemobilestations(MSs)tendtobemultiband.Indeed,toavoidnetworksaturationindenselypopulatedregions,mo