外文翻译基于无线传感器网络的智能家居系统设计.docx
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外文翻译基于无线传感器网络的智能家居系统设计.docx
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外文翻译基于无线传感器网络的智能家居系统设计
ASimpleEnergyModelforWirelessMicrosensorTransceivers
Abstract—Thispaperdescribesthemodellingofshortrangetransceiversformicrosensorapplications.Asimpleenergymodelisderivedandusedtoanalyzethetransceiverbatterylife.Thismodeltakesintoaccountenergydissipationduringthestart-up,receive,andtransmitmodes.Itshowsthatthereisasignificantfixedcostinthetransceiverenergyconsumptionandthisfixedcostcanbedrivendownbyincreasingthedatarateofthetransceiver.
I.Introduction
Wirelessmicrosensornetworkscanprovideshort-rangeconnectivitywithsignificantfaulttolerances.Thesesystemsfindusageindiverseareassuchasenvironmentalmonitoring,industrialprocessautomation,andfieldsurveillance.Asanexample,TableIshowsadetailedspecificationforasensorsystemusedinafactorymachinemonitoringenvironment.
Themajorcharacteristicsofamicrosensorsystemarehighsensordensity,shortrangetransmissions,andlowdatarate.Dependingontheapplication,therecanalsobestringentBERandlatencyrequirements.Duetothelargedensityandtherandomdistributednatureofthesenetworks,batteryreplacementisadifficulttask.Infact,aprimaryissuethatpreventsthesenetworkstobeusedinmanyapplicationareasistheshortbatterylife.Therefore,maximizingthebatterylifetimeofthesensornodesisimportant.Figure1showsthepeakcurrentconsumptionlimitwhena950mAhbatteryisusedastheenergysource.Asseeninthefigure,batterylifecanvarybyordersofmagnitudedependingonthedutycycleofeachoperation.Toallowforhighermaximumpeakcurrent,itisdesirabletohavethesensorremainintheoff-stateforaslongaspossible.However,thelatencyrequirementofthesystemdictateshowoftenthesensorneedstobeactive.Fortheindustrialsensorapplicationdescribedabove,thesensorneedstooperateevery5mstosatisfythelatencyrequirement.Assumingthatthesensoroperatesfor100µsevery5ms,
thedutycycleis2%.Toachieveaone-yearbatterylife,thepeakcurrentconsumptionmustbekeptunder5.4mA,whichtranslatestoapproximately10mWat2Vsupply.Thisisadifficulttargettoachieveforsensorsthatcommunicateatgiga-Hertzcarrierfrequencies.
Therehasbeenactiveresearchinmicrosensornetworksoverthepastyears.Gupta[1]andGrossglauser[2]establishedinformationtheoreticboundsonthecapacityofad-hocnetworks.Chang[3]andHeinzelman[4]suggestedalgorithmstoincreaseoverallnetworklife-timebyspreadingworkloadsevenlyamongallsensors.Muchoftheworkinthisarea,especiallythosethatdealwithenergyconsumptionofsensornetworks,requireanenergymodel[5].ThispaperdevelopsarealisticenergymodelbasedonthepowerconsumptionofastateoftheartBluetooth
transceiver[6].Thismodelprovidesinsightsintohowtominimizethepowerconsumptionofsensornetworksandcanbeeasilyincorporatedintoworkthatstudiesenergylimitedwirelesssensornetworks.Theoutlineofthispaperisasfollows.SectionIIderivesthetransceivermodel.SectionIIIappliesthismodeltoanalyzingthebatterylifetimeoftheBluetoothtransceiver.SectionIVinvestigatesthedependenciesinthemodelandshowshowtomodifythedesignoftheBluetoothtransceivertoimprovethebatterylife.SectionVshowsthebatterylifeimprovementrealizedbyapplyingtheresultsinSectionIV.SectionVIsummarizesthepaper.
II.MicrosensorTransceiverModelling
Thissectionderivesasimpleenergymodelforlowpowermicrosensors.Figure2showsthemodelofthesensornode.Itincludesasensor/DSPunitfordataprocessing,D/AandA/Dfordigital-to-analogandanalog-to-digitalconversion,andawirelesstransceiverfordatacommunication.Thesensor/DSP,D/A,andA/Doperateatlowfrequencyandconsumelessthan1mW.Thisisoveranorderofmagnitudelessthanthepowerconsumptionofthetransceiver.Therefore,theenergymodelignoresthecontributionsfromthesecomponents.Thetransceiverhasthreemodesofoperation:
start-up,receive,andtransmit.Eachmodewillbedescribedandmodelled.
A.Start-upMode
Whenthetransceiverisfirstturnedon,ittakessometimeforthefrequencysynthesizerandtheVCOtolocktothecarrierfrequency.Thestart-upenergycanbemodelledasfollows:
wherePLOisthepowerconsumptionofthesynthesizerandtheVCO.Thetermtstartistherequiredsettlingtime.RFbuildingblocksincludingPA,LNA,andmixerhavenegligiblestart-uptimeandthereforecanremainintheoff-stateduringthestart-upmode.
B.ReceiveMode
Theactivecomponentsofthereceiverincludesthelownoiseamplifier(LNA),mixer,frequencysynthesizer,VCO,intermediate-frequency(IF)amplifier(amp),anddemodulator(Demod).Thereceiverenergyconsumptioncanbemodelledasfollows:
wherePRXincludesthepowerconsumptionoftheLNA,mixer,IFamplifier,anddemodulator.Thereceiverpowerconsumptionisdictatedbythecarrierfrequencyandthenoiseandlinearityrequirements.Oncetheseparametersaredetermined,tothefirstorderthepowerconsumptioncanbeapproximatedasaconstant,fordataratesupto10’sofMb/s.Inotherwords,thepowerconsumptionisdominatedbytheRFbuildingblocksthatoperateatthecarrierfrequency.TheIFdemodulatorpowervarieswithdatarate,butitcanbemadesmallbychoosingalowIF.
C.TransmitMode
Thetransmitterincludesthemodulator(Mod),frequencysynthesizerandVCO(sharedwiththereceiver),andpoweramplifier(PA).ThedatamodulatestheVCOandproducesaFSKsignalatthedesireddatarateandcarrierfrequency.AsimpletransmitterenergymodelisshowninEquation(3).Themodulatorconsumesverylittleenergyandthereforecanbeneglected.
PLOcanbeapproximatedasaconstant.PPAdependsonadditionalfactorsandneedstobemodelledmorecarefullyasfollows:
whereηisthePAefficiency,risthedatarate,disthetransmissiondistance,andnisthepathlossexponent.γPAisafactorthatdependsonEb/NO,noisefactorFofthereceiver,linkmarginLmar,wavelengthofthecarrierfrequencyλ,andthetransmit/receiveantennagainsGT,GR:
FromEquations(3)and(4),thetransmitterpowerconsumptioncanbewrittenasaconstanttermplusavariableterm.Theenergymodelthusbecomes
III.BluetoothTransceiver
HerewedemonstratehowtheabovemodelcanbeusedtocalculatethebatterylifetimeofaBluetoothtransceiver[6].ThisisoneofthelowestpowerBluetoothtransceiversreportedinliterature.Theenergyconsumptionofthetransceiverdependsonhowitoperates.Assuminga100-bitpacketisreceivedanda100-bitpacketistransmittedevery5ms,Figure3shows
thetransceiveractivitywithinonecycleofoperation.Thetransceivertakes120µstostartup.Operatingat1Mb/s,thereceivertakes100µstoreceivethepacket.Thetransceiverthenswitchestothetransmitmodeandtransmitsasame-lengthpacketatthesamerate.A10µsinterval,tswitch,betweenthereceiveandthetransmitmodeisallowedtoswitchchannelortoabsorbanytransientbehavior.Therefore,theenergydissipatedinonecycleofoperationissimply
BoththeaveragepowerconsumptionandthedutycyclecanbefoundFromFigure3.Knowingthatthetransceiveroperatesat2V,thelifetimefora950mAhbatteryiscalculatedtobeapproximately2-months.
IV.EnergyOptimization
ThemicrosensorsystemdescribedinSectionIrequiresabatterylifeofoneyearorbetter.AlthoughtheBluetoothtransceiverdescribedinthelastsectionfallsshortofthisrequirement,itservesasastartingpointformakingimprovements.ThissectionexaminesEopindetailandsuggestswaystoincreasethebatterylifebyconsideringbothcircuitandsystemimprovements.
A.Start-upEnergy
Thestart-upenergycanbeasignificantpartofthetotalenergyconsumption,especiallywhenthetransceiverisusedtosendshortpacketsinburstmode.FortheBluetoothtransceiver,Estartaccountsfor20%ofEop.Thestart-upenergybecomesnegligibleifthefollowingconditionisheldtrue:
Forthereceive/transmitschemeshowninFigure3,therighthand-sideofEquation(8)isevaluatedtobeapproximately450µs.TokeepEstartanorderofmagnitudebelowEop,itisdesirabletohaveastart-uptimeoflessthan45µs.Chohasdemonstrateda5.8GHzfrequencysynthesizerim-
plementationwithastart-uptimeunder20µs[7].
B.PowerAmplifier
ThePApowerconsumptionisgivenby
whereηisthepowerefficiencyandPoutistheRFoutputpower.Poutcanbedeterminedbylink-budgetanalysis.ForaBluetoothtransceiver,therequiredPoutis1mW[8].
Thisenablesamaximumtransmissiondistanceof10meters,whichisadequateformicrosensorapplications.NotethatPoutissmallascomparedtoPLO.TheBluetoothtransceiverdiscussedinSectionIIhasamaximumRFoutputpowerof1.6mWandaPApowerconsumptionof10mW,sotheefficiencyisat16%.Atfrequenciesaround2GHz,thePAefficiencycanvaryfrom10%[9]to70%[10]dependingonlinearity,circuittopology,andtechnology.SinceFSKsignalhasaconstantenvelope,nonlinearPA’scanbeusedsothatbetterefficiencycanbeachieved.Aswillbeshowninthenextsection,PAefficiencyhasasignificantimpactonthebatterylife.
C.DataRate
AssumingapacketoflengthLpktistransmittedatdatrater,thenthetransmittimeis
Thetransmitterenergyconsumptioncanbere-writtenas
Equation(12)showsthatthecontributionofthefixedcostPLOcanbereducedbyincreasingthedatarate.Theenergyperbit,Ebit,isdefinedasEopdividedbythetotalnumberofbitsreceivedandsentduringonecycleofoperation.Assumingapacketoflength
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