无线传感器网络节点的物理层设计_英文_钟子果

JournalofSoutheastUniversity(EnglishEdition)󰀁Vo.l22,No.1,pp.21-25Mar.2006󰀁ISSN1003󰀁7985

Physicallayerdesignofwirelesssensornetworknodes

ZhongZiguo󰀁HuAiqun󰀁WangDan

(DepartmentofRadioEngineering,SoutheastUniversity,Nanjing210096,China)

Abstract:Majorconsiderationdimensionsforthephysicallayerdesignofwirelesssensornetwork(WSN)nodesisanalyzedbycomparingdifferentwirelesscommunicationapproaches,diversematurestandards,

importantradiofrequency(RF)parametersandvariousmicrocontrollerunit(MCU)solutions.AnimplementationoftheWSNnodeispresentedwithexperimentalresultsandanovel󰀁oneprocessorworkingat

twofrequencies󰀁energysavingstrategy.ThelifetimeestimationissueisanalyzedwithconsiderationtotheperiodicallistenrequiredbycommonWSNmediaaccesscontrol(MAC)algorithms.Itcanbeconcludedthat

thestartuptimeoftheRFwhichdeterminesthebestsleeptimeratioandtheshortestbackoffslottimeofMAC,theRFfrequencyandmodulationmethodswhichdeterminatetheRXandTXcurren,tandtheoverallenergy

consumptionofthedualfrequencyMCUSOC(systemonchip)arethemostessentialfactorsfortheWSNnodephysicallayerdesign.

Keywords:wirelesssensornetworknode;physicallayer;radiofrequency;energyconsumption;nodelifetime

Received2005-06-30.Foundationitem:TheNationalHighTechnologyResearchandDeve-lopmentProgramofChina(863Program)(No.2003AA143040).Biographies:ZhongZiguo(1981󰀁),male,graduate;HuAiqun(corres-pondingauthor),male,doctor,professor,aqhu@seu.edu.cn.󰀁󰀁Thewirelesssensornetwork(WSN)hasbeen

showntobeapromisingprospectinvariousapplica-

tions.InitialresearchonWSNwasmainlymotivated

bymilitaryapplications:e.g.smartdust[1]fundedby

theDefenseAdvancedResearchProjectsAgency

(DARPA).Withtheexplosivedevelopmentofmicro

electro-mechanicalsystems(MEMS),integratedcir-

cuitsandwirelesscommunications,lowcos,tlowpower

consumptionandtinysizecomponentsareofftheshelf

now.WSNhasbecomemoreavailableincivilapplica-

tions:e.g.animalobservation[2],glaciermonitoring[2],

nuclearpowerplantmonitoring[3],etc.

ThedesignofWSNnodespresentsabigcha-l

lengebecauseofthelimitedresourcesofeachnode.A

lotofworkhasbeendoneinthisarea,suchasthe󰀁Mote󰀁sfamily󰀁inthesmartdustprojectinBerkeley,

thePicoRadioprojec,talsoinBerkeley,theiBadgein

UCLA,MIT󰀁sPushPin,andWiseNetinSwitzerland.

Massivedeploymentofthenodesrequiresthecostof

eachnodetobeaslowaspossible;theinconvenience

ofreplacingthebatteriesandtherelativelyslowlyim-

provedbatterytechnologyadvocateminimalpower

consumptionorenergyextractingtechniques;smallsize

isalwaysagoodadvantageduringdeploymen.tInthe

WSN,thecommunicationdatarateisusuallynotanes-sentialfactorsinceWSNisdeployedusuallynotfor

datatransmissionbutforeventdetecting.

Inthispaper,WSNnodephysicallayerdesign

considerationsarediscussedindetailandanimplemen-

tationispresentedfollowedbytestresultsandanalysis

ofthenode󰀁slifetimeestimation.

1󰀁DesignConsiderationDimensions

Withoutdoub,tagoodWSNnodedesignneeds

notonlyconsiderthephysicallayer,butalsootherop-

timizeddesignspaces[2]suchastheoptimizedMAC

layeralgorithm,thesmartroutestrategy,andtheeff-i

cientsoftwaresystem,etc.Inthissection,wewillcon-

centrateonsomebasicconsiderationdimensionsforthe

designofthephysicallayer.Massivemanufacturecan

settlethecostproblem;hereenergyisthemajorfactor.

1󰀁1󰀁Architectureofthenode

Fig.1showsthesystemarchitectureofageneric

WSNnode.

Fig.1󰀁WSNnodearchitectureThebasicpartsarecomposedoffourentities.The

powersupplyandmanagementmode,lusuallysome

kindofbattery,providesthenodeselectricalenergy.Thecommunicationblockcomprisesawirelesscom-

municationchannelintheformsofradio,laser,infra-

red,etc.Theprocessingandcontrolunitshouldatleast

includeamicrocontroller,astoragememoryandthein-

terfacetothesensorblock.Thelastblockisagroupof

sensorsorotherdeviceswhichcollectthedesireddata.

Sincedifferentapplicationsemploydiversesensors,this

paperwillnotgofurtherthanthesensorpar.t

1󰀁2󰀁Communicationapproaches

FortheWSN,accordingtospecificapplication

scenes,largequantitiesofnodesmightbedeployede-i

therindoorsoroutdoors,eitherdeliberatelyorrandom-

ly,eitherfixedorwithmobility.Therefore,thewire-

lesscommunicationapproachesusedintheWSN

shouldmeetvariousandevenroughenvironments.

Here,fivecommonwirelessapproachesarelistedand

analyzed.

1)Infrared

Infrarediswidelyusedinremotecontrolapplica-

tions.Theadvantagesofinfraredareasfollows:the

costoftheinfrareddevicesisextremelylow;noanten-

naisneeded,soatinysizeofthenodecanbe

achieved.Thedefectsareasfollows:notenergyeff-i

cient(a10m20󰀁directionaltransmitterconsumes

about60mW);shortcommunicationrange(usually

lessthan1mofthediffusedtypeandseveralmetersof

thedirectionaltype);poorabilityoftravelingroundor

penetratingtheobstacles.Insum,infraredisnotagood

solutionfortheWSN.

2)Laser(opticalmethods)

Themeritsoflaserareasfollows:energyefficient

becauseofitsextremelyfocusedenergy;securitydueto

nobroadcastingtransmission;noantennaisneeded;a

smalllaseremitterisalsocheap.Theshortcomingsare

asfollows:lineofsight(LOS)isneeded;thenodes

mustbecarefullyplacedsothatthelaserbeamcanbe

received;itissensibletoatmosphericconditions.Due

tothoseshortcomings,laserisnotattractiveinthe

WSN.

3)Inductiveandcapacitivecoupling

MostpassiveRFIDsystemsusethisapproach.Its

disadvantageissmallworkingdistances,usuallyonly

abouttensofcentmieters.TheWSNrequiresacommun-i

cationrangefromseveralmeterstohundredsofmeters.

4)Sound

Soundorultrasoundistypicallyusedforcommu-

nicationunderwater,butnotformostWSNs.

5)Radiofrequency(RF)

ThebenefitsofRFareasfollows:omn-idirectiontransmissionwithabilityofdiffractingorpenetrating

theobstacles(NLOS);lowerpowerconsumptionthan

thatofinfrared;relativelylongfunctionrange(1~

100m@0dBm).ThebiggestchallengeforRFisthe

antennadesign.Tooptimizetheantennagain,thePCB

antennaneeds󰀁/4inlengthforwhiporstubshape(󰀁

isthewavelengthofthecarrier),whichaddstothe

sizeofthenode.TheadvantagesofRFhavemadeit

anidealtestingplatformfortheWSN.Actually,most

oftheWSNprojectsemployedtheRFsolution.

1.3󰀁Radiocommunicationsolutions

1)Matureshortdistanceradiostandards

WPAN(wirelesspersonalareanetworks,IEEE

802󰀁15Group)andWLAN(wirelesslocalareane-t

works,IEEE802󰀁11Group)arenowadaysmajor

shortrangewirelessnetworks.The802󰀁11series

(802󰀁11b/a/g,etc.)isdevelopedfora󰀁wireless

Ethernet󰀁,whichdoesnottakeintoconsiderationthe

energybuthasahighdatarate(11to54Mbit/s,con-

sumingmorethan1Winactivemodeandmorethan

50mW[4]inpowersavingmode).Therefore,the

WLANdevicesarenotsuitablefortheWSNnodes.

802󰀁15seriesincludes802󰀁15󰀁1/2(Bluetooth),

802󰀁15󰀁3(UWB),and802󰀁15󰀁4(ZigBee).The

UWBaimsatmultimediaapplicationsandisnotener-

gysaving.SomeprojectsdiduseBluetoothtoimple-

mentWSN[5],butbecauseofitsshortrange(about10

m)andlackofextremepowerefficiency(refertothe

BluetoothchipBRF6150inTab.1),theBluetoothis

alsonotaremarkablesolution.ZigBeeisregardedas

themostsuitablestandardfortheWSN.ThePHY

layerofZigBeeisalmostsuitableforWSN(notthe

best,seetheZigBeechipsinTab.1).TheMAClayer

isnotenergyefficientenoughforitstraditional

CSMA-CAalgorithm[6].

2)Offtheshelfcomponents

LotsofSDR(shortdistanceradio)chipsare

availabletoday.Tab.1listsseveralmarketleading

chipswiththeirfeatures.

3)RFdeviceconsiderations

Therearemainlyfivedimensionsneededtobe

consideredtomakeagoodchoicefortheRFpar.t

①Operationfrequency󰀁TheWSNisexpected

tooperateinthe433MHz/868MHz/2󰀁4GHz/5GHz

ISMbands.Ingenera,lhigherbandscanprovidehigh-

erdataratesandallowforsmallerantennasize,buta

higherfrequencyneedsahigherclockrate[7],which

meanshigherpowerconsumption,andbringsgreater

oscillatorphasenoise[7],whichreducesthereception22ZhongZiguo,HuAiqun,andWangDan