译文 李哲.docx
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译文 李哲.docx
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译文李哲
毕业设计(论文)译文
题目名称:
扩频技术
院系名称:
电子信息学院
班级:
信息091
学号:
200900484131
学生姓名:
李哲
指导教师:
王新强
2013年2月
SpreadSpectrumTechniques
ByWikipedia
Abstract:
Spread-spectrumtechniquesaremethodsbywhichasignal(e.g.anelectrical,electromagnetic,oracousticsignal)generatedinaparticularbandwidthisdeliberatelyspreadinthefrequencydomain,resultinginasignalwithawiderbandwidth.Thesetechniquesareusedforavarietyofreasons,includingtheestablishmentofsecurecommunications,increasingresistancetonaturalinterferenceandjamming,topreventdetection,andtolimitpowerfluxdensity(e.g.insatellitedownlinks).
HistoryFrequencyhopping:
Theconceptoffrequencyhoppingwasfirstalludedtointhe1903U.S.Patent723,188andU.S.Patent725,605filedbyNikolaTeslainJuly1900.Teslacameupwiththeideaafterdemonstratingtheworld'sfirstradio-controlledsubmersibleboatin1898,whenitbecameapparentthewirelesssignalscontrollingtheboatneededtobesecurefrom"beingdisturbed,intercepted,orinterferedwithinanyway."Hispatentscoveredtwofundamentallydifferenttechniquesforachievingimmunitytointerference,bothofwhichfunctionedbyalteringthecarrierfrequencyorotherexclusivecharacteristic.Thefirsthadatransmitterthatworkedsimultaneouslyattwoormoreseparatefrequenciesandareceiverinwhicheachoftheindividualtransmittedfrequencieshadtobetunedin,inorderforthecontrolcircuitrytorespond.Thesecondtechniqueusedavariable-frequencytransmittercontrolledbyanencodingwheelthatalteredthetransmittedfrequencyinapredeterminedmanner.Thesepatentsdescribethebasicprinciplesoffrequencyhoppingandfrequency-divisionmultiplexing,andalsotheelectronicAND-gatelogiccircuit.
FrequencyhoppingisalsomentionedinradiopioneerJohannesZenneck'sbookWirelessTelegraphy(German,1908,EnglishtranslationMcGrawHill,1915),althoughZenneckhimselfstatesthatTelefunkenhadalreadytrieditseveralyearsearlier.Zenneck'sbookwasaleadingtextofthetime,anditislikelythatmanylaterengineerswereawareofit.APolishengineer,LeonardDanilewicz,cameupwiththeideain1929.Severalotherpatentsweretakenoutinthe1930s,includingonebyWillemBroertjes(Germany1929,U.S.Patent1,869,695,1932).DuringWorldWarII,theUSArmySignalCorpswasinventingacommunicationsystemcalledSIGSALYforcommunicationbetweenRooseveltandChurchill,whichincorporatedspreadspectrum,butduetoitstopsecretnature,SIGSALY'sexistencedidnotbecomeknownuntilthe1980s.
ThemostcelebratedinventionoffrequencyhoppingwasthatofactressHedyLamarrandcomposerGeorgeAntheil,whoin1942receivedU.S.Patent2,292,387fortheir"SecretCommunicationsSystem".LamarrhadlearnedabouttheproblematdefensemeetingsshehadattendedwithherformerhusbandFriedrichMandl,whowasanAustrianarmsmanufacturer.TheAntheil-Lamarrversionoffrequencyhoppingusedapiano-rolltochangeamong88frequencies,andwasintendedtomakeradio-guidedtorpedoesharderforenemiestodetectortojam.Thepatentcametolightduringpatentsearchesinthe1950swhenITTCorporationandotherprivatefirmsbegantodevelopCodeDivisionMultipleAccess(CDMA),acivilianformofspreadspectrum,thoughtheLamarrpatenthadnodirectimpactonsubsequenttechnology.ItwasinfactongoingmilitaryresearchatMITLincolnLaboratory,MagnavoxGovernment&IndustrialElectronicsCorporation,ITTandSylvaniaElectronicSystemsthatledtoearlyspread-spectrumtechnologyinthe1950s.Parallelresearchonradarsystemsandatechnologicallysimilarconceptcalled"phasecoding"alsohadanimpactonspread-spectrumdevelopment.
Commercialuse
The1976publicationofSpreadSpectrumSystemsbyRobertDixon,ISBN0-471-21629-1,wasasignificantmilestoneinthecommercializationofthistechnology.Previouspublicationswereeitherclassifiedmilitaryreportsoracademicpapersonnarrowsubtopics.Dixon'sbookwasthefirstcomprehensiveunclassifiedreviewofthetechnologyandsetthestageforincreasingresearchintocommercialapplications.
Initialcommercialuseofspreadspectrumbeganinthe1980sintheUSwiththreesystems:
EquatorialCommunicationsSystem'sverysmallaperture(VSAT)satelliteterminalsystemfornewspapernewswireservices,DelNorteTechnology'sradionavigationsystemfornavigationofaircraftforcropdustingandsimilarapplications,andQualcomm'sOmniTRACSsystemforcommunicationstotrucks.IntheQualcommandEquatorialsystems,spreadspectrumenabledsmallantennasthatviewedmorethanonesatellitetobeusedsincetheprocessinggainofspreadspectrumeliminatedinterference.TheDelNortesystemusedthehighbandwidthofspreadspectrumtoimprovelocationaccuracy.
In1981,theFederalCommunicationsCommissionstartedexploringwaystopermitmoregeneralcivilusesofspreadspectruminaNoticeofInquirydocket.ThisdocketwasproposedtoFCCandthendirectedbyMichaelMarcusoftheFCCstaff.Theproposalsinthedocketweregenerallyopposedbyspectrumusersandradioequipmentmanufacturers,althoughtheyweresupportedbythethenHewlett-PackardCorp.ThelaboratorygroupsupportingtheproposalwouldlaterbecomepartofAgilent.
TheMay1985decisioninthisdocketpermittedunlicenseduseofspreadspectrumin3bandsatpowersupto1Watt.FCCsaidatthetimethatitwouldwelcomeadditionalrequestsforspreadspectruminotherbands.Theresultingrules,nowcodifiedas47CFR15.247permittedWi-Fi,Bluetooth,andmanyotherproductsincludingcordlesstelephones.Theseruleswerethencopiedinmanyothercountries.Qualcommwasincorporatedwithin2monthsafterthedecisiontocommercializeCDMA.
Spread-spectrumtelecommunications
Thisisatechniqueinwhicha(telecommunication)signalistransmittedonabandwidthconsiderablylargerthanthefrequencycontentoftheoriginalinformation.
Spread-spectrumtelecommunicationsisasignalstructuringtechniquethatemploysdirectsequence,frequencyhopping,orahybridofthese,whichcanbeusedformultipleaccessand/ormultiplefunctions.Thistechniquedecreasesthepotentialinterferencetootherreceiverswhileachievingprivacy.Spreadspectrumgenerallymakesuseofasequentialnoise-likesignalstructuretospreadthenormallynarrowbandinformationsignaloverarelativelywideband(radio)bandoffrequencies.Thereceivercorrelatesthereceivedsignalstoretrievetheoriginalinformationsignal.Originallythereweretwomotivations:
eithertoresistenemyeffortstojamthecommunications(anti-jam,orAJ),ortohidethefactthatcommunicationwaseventakingplace,sometimescalledlowprobabilityofintercept(LPI).
Frequency-hoppingspreadspectrum(FHSS),direct-sequencespreadspectrum(DSSS),time-hoppingspreadspectrum(THSS),chirpspreadspectrum(CSS),andcombinationsofthesetechniquesareformsofspreadspectrum.Eachofthesetechniquesemployspseudorandomnumbersequences—createdusingpseudorandomnumbergenerators—todetermineandcontrolthespreadingpatternofthesignalacrosstheallotedbandwidth.Ultra-wideband(UWB)isanothermodulationtechniquethataccomplishesthesamepurpose,basedontransmittingshortdurationpulses.WirelessEthernetstandardIEEE802.11useseitherFHSSorDSSSinitsradiointerface.
Notes
∙Techniquesknownsince1940sandusedinmilitarycommunicationsystemsince1950s
∙"Spread"radiosignaloverawidefrequencyrangeseveralmagnitudeshigherthanminimumrequirement.Thecoreprincipleofspreadspectrumistheuseofnoise-likecarrierwaves,and,asthenameimplies,bandwidthsmuchwiderthanthatrequiredforsimplepoint-to-pointcommunicationatthesamedatarate.
∙Twomaintechniques:
1.Directsequence(DS)
2.Frequencyhopping(FH)
∙Resistancetojamming(interference).DSisbetteratresistingcontinuous-timenarrowbandjamming,whileFHisbetteratresistingpulsejamming.InDSsystems,narrowbandjammingaffectsdetectionperformanceaboutasmuchasiftheamountofjammingpowerisspreadoverthewholesignalbandwidth,whenitwilloftennotbemuchstrongerthanbackgroundnoise.Bycontrast,innarrowbandsystemswherethesignalbandwidthislow,thereceivedsignalqualitywillbeseverelyloweredifthejammingpowerhappenstobeconcentratedonthesignalbandwidth.
∙Resistancetoeavesdropping.Thespreadingcode(inDSsystems)orthefrequency-hoppingpattern(inFHsystems)isoftenunknownbyanyoneforwhomthesignalisunintended,inwhichcaseit"encrypts"thesignalandreducesthechanceofanadversary'smakingsenseofit.What'smore,foragivennoisepowerspectraldensity(PSD),spread-spectrumsystemsrequirethesameamountofenergyperbitbeforespreadingasnarrowbandsystemsandthereforethesameamountofpowerifthebitratebeforespreadingisthesame,butsincethesignalpowerisspreadoveralargebandwidth,thesignalPSDismuchlower,oftensignificantlylowerthanthenoisePSD,thereforetheadversarymaybeunabletodetermineifthesignalexistsatall.However,formission-criticalapplications,particularlythoseemployingcommerciallyavailableradios,spread-spectrumradiosdonotintrinsicallyprovideadequatesecurity;"...justusingspread-spectrumradioitselfisnotsufficientforcommunicationssecurity"
∙Resistancetofading.Thehighbandwidthoccupiedbyspread-spectrumsignalsoffersomefrequencydiversity,i.e.itisunlikelythatthesignalwouldencounterseveremultipathfadingoveritswholebandwidth,andinothercasesthesignalcanbedetectedusinge.g.aRakereceiver.
∙Multipleaccesscapability.Multipleuserscantransmitsimultane
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