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》由会员分享,可在线阅读,更多相关《无线传感器网络测距技术外文翻译文献.docx(16页珍藏版)》请在冰豆网上搜索。
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