无线传感器网络测距技术外文翻译文献.docx

无线传感器网络测距技术外文翻译文献.docx

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无线传感器网络测距技术外文翻译文献

无线传感器网络测距技术外文翻译文献

（文档含中英文对照即英文原文和中文翻译）

原文：

RANGINGTECHNIQUESFORWIRELESSSENSORNETWORKS

TheRFlocationsensorsoperatingindifferentenvironmentscanmeasuretheRSS,AOA,phaseofarrival（POA）,TOA,andsignatureofthedelay-powerprofileaslocationmetricstoestimatetherangingdistance[4,7].Thedeploymentenvironment（i.e.,wirelessRFchannel）willconstraintheaccuracyandtheperformanceofeachtechnique.Inoutdooropenareas,theserangingtechniquesperformverywell.However,asthewirelessmediumbecomesmorecomplex,forexample,denseurbanorindoorenvironments,thechannelsuffersfromseveremultipathpropagationandheavyshadowfadingconditions.Thisfindinginturnimpactstheaccuracyandperformanceinestimatingtherangebetweenapairofnodes.Forthisreason,thischapterwillfocusitsrangingandlocalizationdiscussiononindoorenvironments.ThisisimportantbecausemanyoftheWSNapplicationsareenvisionedfordeploymentinroughterrainandclutteredenvironmentsandunderstandingoftheimpactofthechannelontheperformanceofrangingandlocalizationisimportant.Inaddition,rangemeasurementsusingPOAandAOAinindoorandurbanareasareunreliable.Therefore,wewillfocusourdiscussionontwopracticaltechniques,TOAandRSS.Thesetworangingtechniques,whichhavebeenusedtraditionallyinwirelessnetworks,haveagreatpotentialforuseinWSNlocalization.

TheTOAbasedrangingissuitableforaccurateindoorlocalizationbecauseitonlyneedsafewreferencesandnopriortraining.Byusingthistechnique,however,thehardwareiscomplexandtheaccuracyissensitivetothemultipathconditionandthesystembandwidth.ThistechniquehasbeenimplementedinGPS,PinPoint,WearNet,IEEE802.15.3,andIEEE802.15.4systems.TheRSSbasedranging,ontheotherhand,issimpletoimplementandisinsensitivetothemultipathconditionandthebandwidthofthesystem.Inaddition,itdoesnotneedanysynchronizationandcanworkwithanyexistingwirelesssystemthatcanmeasuretheRSS.Foraccurateranging,however,ahighdensityofanchorsorreferencepointsisneededandextensivetrainingandcomputationallyexpensivealgorithmsarerequired.TheRSSranginghasbeenusedforWiFipositioninginsystems,forexample,Ekahau,NewburyNetworks,PanGo,andSkyhook.

ThissectionfirstintroducesTOAbasedrangingandthelimitationsimposedbythewirelesschannel.ThenitwillbecomparedwiththeRSScounterpartfocusingontheperformanceasafunctionofthechannelbehavior.Whatisintroducedhereisimportanttotheunderstandingoftheunderlyingissuesindistanceestimation,whichisanimportantfundamentalbuildingblockinWSNlocalization.

TOABasedRanging

InTOAbasedranging,asensornodemeasuresthedistancetoanothernodebyestimatingthesignalpropagationdelayinfreespace,whereradiosignalstravelattheconstantspeedoflight.Figure8.3showsanexampleofTOAbasedrangingbetweentwosensors.TheperformanceofTOAbasedrangingdependsontheavailabilityofthedirectpath（DP）signal[4,14].Initspresence,forexample,shortdistanceline-of-sight（LOS）conditions,accurateestimatesarefeasible[14].Thechallenge,however,isranginginnon-LOS（NLOS）conditions,whichcanbecharacterizedassite-specificanddensemultipathenvironments[14,22].Theseenvironmentsintroduceseveralchallenges.ThefirstcorruptstheTOAestimates

duetothemultipathcomponents（MPCs）,whicharedelayedandattenuatedreplicasoftheoriginalsignal,arrivingandcombiningatthereceivershiftingtheestimate.Thesecondisthepropagationdelaycausedbythesignaltravelingthroughobstacles,whichaddsapositivebiastotheTOAestimates.ThethirdistheabsenceoftheDPduetoblockage,alsoknownasundetecteddirectpath（UDP）[14].Thebiasimposedbythistypeoferrorisusuallymuchlargerthanthefirsttwoandhasasignificantprobabilityofoccurrenceduetocabinets,elevatorshafts,ordoorsthatareusuallyclutteringtheindoorenvironment.

InordertoanalyzethebehavioroftheTOAbasedranging,itisbesttoresorttoapopularmodelusedtodescribethewirelesschannel.Inatypicalindoorenvironment,thetransmittedsignalwillbescatteredandthereceivernodewillreceivereplicasoftheoriginalsignalwithdifferentamplitudes,phases,anddelays.Atthereceiver,thesignalsfromallthesepathscombineandthisphenomenonisknownasmultipath.InordertounderstandtheimpactofthechannelontheTOAaccuracy,weresorttoamodeltypicallyusedtocharacterizemultipatharrivals.Formultipathchannels,theimpulserespons

characterizesthearrivalpaths,theirrespectiveamplitudes,anddelays.Mathematically,itcanberepresentedasasummationofallthearrivingmultipathcomponentsor

，（8.1）

whereLpisthenumberofMPCs,and

and

areamplitude,phase,andpropagationdelayofthekthpath,respectively[7,23].Let

and

denotetheDPamplitudeandpropagationdelay,respectively.ThedistancebetweenthesensornodeandtheRPoranchoris

wherevisthespeedofsignalpropagation.IntheabsenceoftheDP,rangingcanbeachievedusingtheamplitudeandpropagationdelayofthenon-directpath（NDP）componentgivenby

and

respectively;resultinginalongerdistance

where

.ForthereceivertoidentifytheDP,theratioofthestrongestMPCtothatoftheDPgivenby

，（8.2）

mustbelessthanthereceiverdynamicrangekandthepoweroftheDPmustbegreaterthanthereceiversensitivity

.Theseconstraintsaregivenby

，（8.3a）

（8.3b）

where

.

Ingeneral,rangingandlocalizationaccuracyisconstrainedbytherangingerror,whichisdefinedasthedifferencebetweentheestimatedandtheactualdistance;thatis,.

.（8.4）

Inanindoorenvironment,thenode/MTwillexperienceavaryingerrorbehaviordependingontheavailabilityoftheDPandinthecaseofitsabsenceonthecharacteristicsoftheDPblockage.Itispossibletocategorizetheerrorbasedonthefollowingrangingstates[24].InthepresenceoftheDP,both（8.3a）and（8.3b）aremetandthedistanceestimateisveryaccurate,yielding

（8.5a）

wheretherandombiasinducedbythemultipath,

isthebiascorrespondingtothepropagationdelaycausedbyNLOSconditions,andzisazero-meanadditivemeasurementnoise.Ithasbeenshownthat

isindeedafunctionofthebandwidthandthesignaltonoiseratio（SNR）[14],whilebpdisdependantonthemediumoftheobstacles.WhenthenodeexperiencessuddenblockageoftheDP,Eq.（8.3a）isnotmetandtheDPisshadowedbysomeobstacle,buryingitspowerunderthedynamicrangeofthereceiver.Inthissituation,therangingestimateexperiencesalargererrorcomparedtoEq.（8.5a）.EmphasizingthatrangingisachievedthroughtheNDPcomponent,theestimateisthengivenby

（8.6a）

（8.6b）

where

isadeterministicadditivebiasrepresentingthenatureoftheblockage.Unlikethemultipathbiases,butsimilartothebiasesinducedbythepropagationdelay,thedependenceof

onthesystembandwidthandSNRhasitsownlimitationsasreportedinRef.[14].Formally,theserangingstatescanbedefinedas

（8.7a）

（8.7b）

Figures8.4and8.5providesamplechannelprofilesofthesetworangingsituations[24].

TheperformanceofTOAbasedrangingcanbedeterminedbytheCramer-Raolowerbound（CRLB）,whichhasbeenstudiedextensivelyforexistingsystems.ThevarianceofTOAestimation

isboundedbytheCRLB[25]

（8.8）

whereTisthesignalobservationtime,

istheSNR,

isthefrequencyofoperation,andwisthesystembandwidth.

Inpractice,TOAcanbeobtainedbymeasuringthearrivaltimeofawide-bandnarrowpulse,whichcanbeobtainedeitherbyusingspreadspectrumtechnologyordirectly.

DirectSpreadSpectrum.

OneTOAestimationtechniquebasedonthedirectspreadspectrum（DSS）widebandsignalhasbeenusedinGPSandotherrangingsystemsformanyyears.Insuchasystem,asignalcodedbyaknownpseudorandom（PN）sequenceistransmittedandareceivercross-correlatesthereceivedsignalwithalocallygeneratedPNsequenceusingaslidingcorrelatororamatchedfilter.Thedistancebetweenthetransmitterandthereceiverisdeterminedfromthearrivaltimeofthefirstcorrelationpeak.Becauseoftheprocessinggainofthecorrelationatthereceiver,DSSrangingsystemsperformmuchbetterthancompetingsystemsinsuppressinginterferencefromotherradiosystemsoperatinginthesamefrequencyband.Intheseband-limitedsystems,super-resolutiontechniquesforTOAestimationhavebeenappliedsuccessfully.Resultshaveshownthatthesehigh-resolutionalgorithmscanprovideimprovedaccuracy[25].

Ultra-WidebandRanging.

ApromisingalternativetoDSSsystemsisultra-wideband（UWB）ranging[26].AccordingtoEq.（8.8）,itisclearthatinmultipathpropagationenvironments,theperformanceofTOAestimationisinverselyrelatedtothesystembandwidth.Increasingthesystembandwidth（i.e.,narrowertime-domainpulse）resultsinhighertimeresolutionandthusbetterrangingaccuracy.Asaresult,thesesystemshaveattractedconsiderableattentioninrecentyears[16,22,26].ForUWBapplications,theFCCregulationallocatedanunlicensedflatfrequencyband3.1–10.6GHzforwhichtherearetwoproposals:

directsequence（DS）–UWBandmultibandorthogonalfrequencydivisionmultiplexing（MB–OFDM）.Theformerispulsebased,whichutilizeslargebandwidths,forexample,3GHz,whilethelatteroccupiesabandwidthof528MHz.Theacc