码分多址通信系统扩频通信外文翻译Word文件下载.docx

码分多址通信系统扩频通信外文翻译Word文件下载.docx

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班级2010级

学生姓名

学号

课题码分多址通信系统的建模、仿真和设计

——初始化模块、基站接收模块

指导教师

2014年06月10日

译文原文

1.1Thebasicconceptofspread-spectrumcommunication

Spreadspectrumcommunication’sbasiccharacteristics,isusedtotransmitinformationtothesignalbandwidth（W）isfargreaterthanpracticalrequiredminimum（effective）bandwidth（

）,astheradioofprocessinggain

.

Aswewellknow,theordinaryAM,FM,orpulsecodemodulation,GPvalueintheareamorethan10times,collectively,the“narrow-bandcommunication”,andspread-spectrumcommunicationGPvaluesashundredoreventhousandsoftimes,canbecalled“broadbandcommunication”.

Duetothespread-spectrumsignal,itisverylowpowertransmitters,transmissionspacemostlydrownedinthenoise,itisdifficulttointerceptedbytheotherreceiver,onlyspreadingcodeswiththesame（orrandomPNcode）receiver,Gaincanbedealtwith,anddespreadingresumetheoriginalsignal.

1.2Thetechnologysuperiorityofspread-spectrumcommunication.

Stronganti-interference,biterrorrateislow.Asnotedabove,thespreadspectrumcommunicationsystemduetotheexpansionofthetransmittersignalspectrum,thereceiverdespreadingreductionsignalproducedspreadinggain,therebygreatlyenhancingitsinterferencetolerance.Underthespreadinggain,orevennegativeinthesignal-to-noiseratioconditions,canalsosignalfromthenoisedrownedoutExtraction,inthecurrentbusinesscommunicationssystems,spreadspectrumcommunicationssystems,spreadspectrumcommunicationisonlyabletoworkinanegativesignal-to-noiseratioundertheconditionsofcommunication.

Anti-multi-pathinterferencecapability,increasethereliabilityofsystem.Spread-spectrumsystemsasusedinthePNhasagoodcorrelation,correlationisveryweak.Differentpathstothetransmissionsignalcaneasilybeseparatedandmayintimeandre-alignmentphase,formationofseveralsuperimposedsignalpower,therebyimprovingthesystem’sperformancetoreceiveincreasedreliabilityofthesystem.

Easytousethesamefrequency,improvingthewirelessspectrumutilization.Wirelessspectrumisveryvaluable,althoughlong-wavemicrowavehavetobeexploited,andstillcannotmeettheneedsofcommunity.Tothisend,countriesaroundtheworldaredesignedspectrummanagement,userscanonlyusethefrequencyapplications,relyonthechanneltopreventthedivisionbetweenthechannelinterference.

Duetotheuseofspread-spectrumcommunicationrelatedreceivethishigh-tech,lowsignaloutputpower（“aW,asageneral-100mW）,andwillworkinthechannelnoiseandthermalnoiseinthebackground,easytoduplicateinthesameareausingthesamefrequency,cannowallsharethesamenarrow-bandfrequencycommunicationresources.

Spread-spectrumcommunicationisdigitalcommunication,particularlyfordigitalvoiceanddatatransmissionwiththeirownencryption,onlyinthesamePNcodecommunicationbetweenusers,isgoodforhidingandconfidentialinnature,facilitatingcommunicationbusiness.Easytousespread-spectrumCDMAcommunications,voicecompressionandmanyothernewtechnologies,moreapplicabletocomputernetworksanddigitizationofvoice,imageinformationtransmission.

Communicationinthemostdigitalcircuits,equipment,highlyintegrated,easyinstallation,easymaintenance,butalsoverycompactandreliable.Theaveragefailureratenotimewasverylong.

1.3Spreadspectrumcommunicationsystem

Spreadspectrumcommunication,namely,spreadspectrumcommunications（Spreadspectrumcommunication）,withfiber-opticcommunications,satellitecommunications,withaccesstotheinformationageasthethreemajorhigh-techcommunicationstransmission.Spreadspectrumcommunicationistosendtheinformationtobepseudo-randomdataiscoded（Spreadspectrumsequence:

spreadsequence）modulation,spreadspectrumandthentherealizationoftransmission;

thereceivingendisusingthesamemodemcodeandrelatedprocessing,therestorationoftheoriginaldata.Spreadspectrumcommunicationsystemhasthreemaincharacteristics.

（1）Carrierisanunpredictable,orso-calledpseudo-randombroadbandsignal.

（2）Carrierdatabandwidththanthemodulationbandwidthismuchwilder.

（3）Receivingprocessisgeneratedbylocalbroadbandcarriersignalandreceivingacopyofthesignaltothebroadbandsignaltoachieve.

Themainwayofspreadspectrumareasfollows:

DirectSequenceSpreadSpectrum（DSSS）usinghigh-speedpseudo-randomcodeontothelow-speeddatatransmissionspreadspectrummodulation;

Frequency-hoppingsystemusingpseudo-randomcodetocontrolthecarrierfrequencyinawiderbandofthechange;

THisthedatatransmissiontimeslotisapseudo-random;

chirpfrequencysystemisalinearextensionoftheprocessofchange.Combinationofanumberofwaysofhybridsystemsareoftenapplied.

Themostimportantmeasurepfspread-spectrumsystemisanindicatorofspreadinggain,alsoknownasprocessinggain.Itispreciselybecauseofthespreadspectrumsystemitselfwithitsperformancecharacteristicswithaseriesofadvantages.

1.4Codedivisionmultipleaccess

Codedivisionmultipleaccess（CDMA）isachannelaccessmethodusedbyvariousradiocommunicationtechnologies.ItshouldnotbeconfusedwiththemobilephonestandardscalledcdmaOne,CDMA2000（the3GevolutionofcdmaOne）andWCDMA（the3GstandardusedbyGSMcarrier）,whichareoftenreferredtoassimplyCDMA,anduseCDMAasanunderlyingchannelaccessmethod.

Oneoftheconceptsindatacommunicationistheideaofallowingseveraltransmitterstosendinformationsimultaneouslyoverasignalcommunicationchannel.Thisallowsseveraluserstoshareabandoffrequencies（seebandwidth）.Thisconceptiscalledmultipleaccess.CDMAemploysspread-spectrumtechnologyandaspecialcodingscheme（whereeachtransmitterisassignedacode）toallowmultipleusertobemultiplexedoverthesamephysicalchannel.Bycontrast,timedivisionmultipleaccess（FDMA）dividesitbyfrequency.CDMAisaformofspread-spectrumsignaling,sincethemodulatedcodedsignalhasamuchhigherdatabandwidththanthedatabeingcommunicated.

1.5Spread-spectrumcharacteristicofCDMA

Mostmodulationschemestrytominimizethebandwidthofthissignalsincebandwidthisalimitedresource.However,spreadspectrumuseatransmissionbandwidththatisseveralordersofmagnitudegreaterthantheminimumrequiredsignalbandwidth.Oneoftheinitialreasonsfordoingthiswasmilitaryapplicationsincludingguidanceandcommunicationsystems.Thesesystemweredesignedusingspreadspectrumbecauseifitssecurityandresistancetojamming.AsynchronousCDMAhassomelevelofprivacybuiltinbecausethesignalisspreadusingapseudo-randomcode;

thiscodemakesthespreadspectrumsignalsappearrandomorhavenoise-likeproperties.Areceivercannotdemodulatethistransmissionwithoutknowledgeofthepseudo-randomsequenceusedtoencodethedata.CDMAalsoresistanttojamming.Ajammingsignalonlyhasafiniteamountofpoweravailabletojamthesignal.Thejammercaneitherspreaditsenergyovertheentirebandwidthofthesignalorjamonlypartoftheentiresignal.

CDMAcanalsoeffectivelyrejectnarrowbandinterference.Sincenarrowbandinterferenceaffectsonlyasmallportionofthespreadspectrumsignal,itcaneasilyberemovedthroughnotchfilteringwithoutmuchlossofinformation.Convolutionencodingandinterleavingcanbeusedtoassistinrecoveringthislostdata.CDMAsignalarealsoresistanttomultipathfading.Sincethespreadspectrumsignaloccupiesalargebandwidthonlyasmallportionofthiswillundergofadingduetomultipathatanygivetime.Likethenarrowbandinterferencethiswillresultinonlyasmalllossofdataandcanbeovercome.

AnotherreasonCDMAisresistanttomultipathinterferenceisbecausethedelayedversionsofthetransmittedpseudo-randomcode,andwillthusappearasanotheruser,whichisignoredatthereceiver.Inotherwords,aslongasthemultipathchannelinducesatleastonechipofdelay,天themultipathchannelinducesatleastonechipofdelay,themultipathsignalswillarriveatthereceiver.inotherwords,aslongasthemultipathchannelinducesatleastonechipofdelay,themultipathsignalswillarriveatthereceiversuchthattheyareshiftedintimebyatleastonechipfromtheintendedsignal.Thecorrelationpropertiesofthepseudo-randomcodesaresuchthatthisslightdelaycausesthemultipathtoappearuncorrelatedwiththeintendedsignal,anditisthusignored.

SomeCDMAdevicesusearakereceiver,whichexploitsmultipathdelaycomponentstoimprovetheperformanceofthesystem.Arakereceivercombinestheinformationfromseveralcorrelators,eachonetunedtoadifferentpathdelay,producingastrongerversionofthesignalthanasimplereceiverwithasignalcorrelationtunedtothepathdelayofthestrongestsignal.

Frequencyreuseistheabilitytoreusethesameradiochannelfrequencyatothercellsiteswithinacellularsystem.IntheFDMAandTDMAsystemsfrequencyplanningisandimportantconsideration.Thefrequenciesusedindifferentcellsmustbeplannedcarefullytoensuresignalsfromdifferentcellsdonotinterferewitheachother.InaCDMAsystem,thesamefrequencycanbeusedineverycell,becausechannelizationisdoneusingthepseudo-randomcodes.ReusingthesamefrequencyineverycelleliminatestheneedforfrequencyplanninginaCDMAsystem;

however,planningofthedifferentpseudo-randomsequencesmustbedonetoensurethatthereceivedsignalfromonecelldoesnotcorrelatewiththesignalfromanearbycell.

Sinceadjacentcellusethesamefrequencies,CDMAsy