无线传感器网络节点的物理层设计_英文_钟子果

Jour na l of Southeast U niversity (Eng lis h Edition) V o.l 22,N o .1,pp .21-25M ar .2006 I SSN 1003 7985

Physical layer design of wireless sensor net work nodes

Z hong Z iguo

Hu A iqun W ang D an

(D epart m ent o f R adio Eng ineer i ng ,So ut heast U n i ve rsit y,N an jing 210096,Ch i na )

Abstract :M ajor consi der ation di m ensi ons fo r the phy sical l ayer design of w irel ess sensor net w ork (W SN )

nodes is analyzed by co m paring differe nt w ireless co mm unication approaches ,di verse m ature sta ndar ds ,i m po rtant radi o frequency (RF )para m eters and various m icr ocontr o ller unit (M CU )so lutions .An i m p l e m entation of theW SN node is presente d w ith experi m ental r esults and a novel one processo rw o r ki ng at t w o frequenc i es energy sav i ng strategy .The lifeti m e esti m ation issue is anal yzed w ith consi derati on to t he peri odical listen required by co mm onW SN m edi a access contro l (M AC )a l gorith m s .It can be concluded that the startup ti m e of t he RF which deter m ines the best sleep ti m e rati o and t he shortest backoff sl o t ti m e ofM AC,the RF frequency and m odulati on m ethods w hic h deter m i nate the RX and TX curren,t and the overall energy consu m pti on of t he dual fr equency M CU SOC (syste m on chi p)are them o st essential factors for theW SN node phy si ca l layer design .K ey words :w ir e l ess senso r net w ork node ;phy si cal layer ;radi o f r equency ;e nergy consu m pti on ;node lifeti m e

Recei ved 2005-06-30.

F oundati on ite m:Th e National H i gh T echnology Res earc h and D eve -l opm en t Progra m of Ch i na (863Program )(No.2003AA 143040).B i ographi es :Z hong Z i guo (1981 ),m ale ,g raduat e ;H u A iqun (corres -pond i ng aut hor),m ale ,doct or ,profess or ,aqhu @s eu .edu .cn .

The w ireless senso r net w o r k (W SN )has been show n to be a prom isi n g prospect in vari o us applica -ti o ns .I n itia l researc h on W SN w as m a i n l y m o ti v ated

by m ilitary app lications :e .g .s m art dust [1]

funded by t h e D efense Advanced Research Pr o j e cts A gency (DARPA ).W ith the exp l o sive developm ent o f m i c ro electro -m echanica l syste m s (M E M S ),integ rated cir -cuits and w ire l e ss co mm un ications ,low co s,t l o w pow er consu m pti o n and ti n y size com ponen ts are off t h e shelf now.W SN has becom e m ore available in civ il applica -ti o ns :e .g .an i m a l obser va ti o n [2],g lacier m on ito ri n g [2],

nuclear pow er plantm on ito ri n g [3]

,etc .

The design of W SN nodes presents a big cha-l

lenge because o f the li m ited resources o f each node .A l o t of w ork has been done i n this area ,such as the M o te s fa m ily i n the s m art dust pro ject i n B er keley ,t h e P i c o Radio pro jec,t also i n B erke ley ,t h e i B adge in UCLA,M I T s PushPi n ,and W ise Ne t i n Sw itzerland .M assi v e deploy m ent o f the nodes requires the co st of each node to be as l o w as po ssible ;the i n conven ience of rep lac i n g the ba tteries and the re l a ti v e ly sl o w ly i m -proved battery techno logy advoca te m ini m al pow er consu m pti o n o r energy ex tracti n g techniques ;s m a ll size is a l w ay s a good advantage during deploy m en.t In the W SN,the co mm unication data ra te is usua lly no t an es -sentia l factor since W SN is deployed usua ll y no t fo r

da ta trans m issi o n but fo r event detecti n g .

I n this paper ,W SN node physica l layer desi g n consi d erati o ns are d iscussed in de tail and an i m ple m en -tation is presented fo ll o w ed by test results and analy sis o f the node s lifeti m e esti m ation.

1 D esign Consideration D i m ensions

W ithout doub,t a good W SN node desi g n needs no t only consi d er t h e physical l a yer ,but a lso other op -ti m ized design spaces [2]such as the opti m ized MAC layer a l g o rithm,the s m art r o ute strategy ,and t h e eff-i

c ient so ft w are sy ste m,etc .In this section ,w e w ill con -centrate on som e basic consideration

d i m ensions fo r th

e desi g n o

f the phy sica l layer .M assive m anufacture can settle the co st pr ob l e m;here energy is t h e m a j o r facto r .1 1 A rchitecture o f the node

Fig .1show s the syste m architecture o f a generic W SN node .

Fig .1 W S N node a rchitecture

The basic parts are com po sed o f fo ur entiti e s .The pow er supp l y and m anage m ent m ode,l usua ll y som e kind o f battery ,prov ides the nodes e lectrical ener g y .

The comm unicati o n block co m prises a w ir e less com-m un ication channe l i n the for m s o f radio,l a ser,i n fra-red,etc.The pr o cessi n g and contro l unit should at least i n clude a m i c rocontro ll e r,a storag e m e m o ry and t h e i n-terface to t h e sensor b l o ck.The l a st bl o ck is a group of sensors or o ther dev ice sw hich co llect the desir ed da ta. Since d ifferent applicati o ns e m p l o y diverse senso rs,th is paper w ill no t go further t h an the senso r par.t

1 2 Co mm un ication approaches

Fo r the W SN,acco rdi n g to s pec ific app lication scenes,lar g e quantities of nodesm igh t be dep l o y ed e-i ther i n doors or outdoors,e ither de liberate l y o r random-l y,either fixed or w ith m ob ility.Therefore,t h e w ire-less co mm un ication approaches used i n the W SN sho ul d m eet vari o us and even rough env ironm ents.

H ere,five comm on w ire l e ss approaches are listed and ana lyzed.

1)I nfrared

Infrar ed is w ide l y used i n re m o te contro l applica-ti o ns.The advan tages o f infrar ed ar e as fo ll o w s:the co st of the i n frared dev ices is ex tr e m ely l o w;no anten-na is needed,so a ti n y si z e o f the node can be ach ieved.The defects are as fo ll o w s:no t energy eff-i cient(a10m20 directiona l trans m itter consu m es about60mW);s hort co mm un ication range(usually less than1m o f t h e d iffused type and sev era lm eters of t h e directi o nal type);poo r ability o f trave ling r o und or pene trating t h e obstacles.I n su m,infrar ed is no t a good so l u tion fo r the W SN.

2)Laser(optica lm ethods)

The m erits of laser are as fo ll o w s:energy efficient because o f its extre m e l y focused energy;security due to no br o adcasti n g trans m ission;no antenna is needed;a s m a ll laser e m itter is also cheap.The sho rtco m i n gs are as fo ll o w s:li n e o f sight(LO S)is needed;the nodes m ust be carefully placed so tha t the laser bea m can be rece i v ed;it is sensi b le to at m o spheric cond itions.Due t o tho se sho rtco m i n gs,laser is no t attracti v e i n the W SN.

3)I nductive and capaciti v e coup ling

M o st passi v e RFI D sy ste m s use this approach.Its disadvantage is s m a ll w o r king distances,usuall y on l y about tens of centm i eters.TheW SN requires a co mm un-i cation range fro m severalm eters to hundreds of m eters.

4)Sound

Sound o r u ltra sound is typica lly used for comm u-n icati o n under w ater,but no t fo r m o stW SN s.

5)Radio frequency(RF)

The benefits o f RF are as fo llow s:o m n-i d irection

trans m issi o n w ith ab ility of diffracting o r pene trati n g the obstac les(NLO S);low er pow er consum ption t h an tha t o f infrared;re l a ti v ely long functi o n range(1~ 100m@0dBm).The b iggest challenge fo r RF is the antenna design.To op ti m ize the antenna g ain,the PCB antenna needs /4i n length fo r w h i p or stub shape( is the w ave leng th o f the carrier),w h ich adds to the size o f the node.The advan tage s o f RF have m ade it an ideal testing p l a tfo r m fo r the W SN.A ctually,m ost o f theW SN pro jects e m ployed the RF so lution.

1.3 Rad i o comm un i c ation so lution s

1)M ature sho rt d istance radio standards

W PAN(w ireless persona l area net w o rks,I EEE 802 15G roup)and W LAN(w ireless l o ca l area ne-t w o rks,I EEE802 11G ro up)are now aday s m ajo r sho rt range w ireless net w orks.The802 11series (802 11b/a/g,etc.)is developed for a w ire l e ss E ther net ,w hich do es no t take i n to consideration the energy but has a high data ra te(11to54M b it/s,con-sum ing m o re t h an1W i n acti v e m ode and m ore t h an 50mW[4]in pow er sav i n g m o de).Therefo re,the W LAN dev ices are no t su itab le fo r the W SN nodes. 802 15series inc l u des802 15 1/2(B l u e too th), 802 15 3(UW B),and802 15 4(Z igB ee).The UW B a i m s atm ulti m ed ia app lications and is no t ener-gy sav i n g.Som e pro jects did use B lue too t h to i m p l e-m en tW SN[5],but because of its sho rt range(about10 m)and lac k o f ex tre m e pow er efficiency(refer to the B luetoo th chip BRF6150in Tab.1),t h e B l u etoo th is a lso no t a re m ar kab l e so lution.Z i g B ee is regarded as the m o st suitable standard fo r the W SN.The P HY layer of Z igB ee is a l m o st suitab l e for W SN(no t the be st,see the Z igB ee ch i p s i n Tab.1).TheM AC lay er is no t energy effic ient enough fo r its trad itiona l

C S M A-CA a l g o rithm[6].

2)O ff the she lf com ponents

Lo ts o f SDR(sho rt d istance radio)chips are ava ilable today.T ab.1lists severa l m ar ket lead i n g chipsw it h their features.

3)RF dev ice consi d erations

There are m a i n l y five di m ensions needed to be considered to m ake a g ood cho ice for the RF par.t

①O peration frequency The W SN is expected to operate i n the433M H z/868MH z/2 4GH z/5GH z IS M bands.I n genera,l higher bands can prov i d e h igh-er data rates and a ll o w fo r s m a ller an tenna size,but a h i g her frequency needs a higher clock rate[7],w hich m eans h igher pow er consu m ption,and bring s g rea ter o scill a tor phase no ise[7],w hich reduces t h e recepti o n

22Zhong Z i g uo,H u A i q un,andW ang Dan

sensitiv ity.In T ab.1,the2 4GH z dev ice s generally consum e m ore pow er espec i a lly i n RX m ode,and w ith l o w er RX sensitiv ity.

②M odulation The m ore co m plex t h e m odula-ti o n is,the m o re pow er is needed.C onstant envelope m odulations t h at a ll o w high-efficient non li n ear P A s are favo red[7].There fo re,FSK is a good so l u tion fo r W SN s stri n gent low pow er require m en.t

③Startup ti m e W ith regar d to the M AC l a yer strategy,the node s inW SN w ill be i n a periodic listen and sleep cy cle[8]i n o r der to save pow er.A cco r d i n g to t h e research on ourM AC l a yer pr o to co,l w e conclude t h at t h e startup ti m e o f the com ponent play s an e ssen-tial ro l e in theW SN.Since t h e startup ti m e is the ti m e per i o d that t h e node needs to s w itch bet w een sl e ep state and active state,it deter m ines the best ti m e ra ti o o f sl e ep m ode i n the cyc le,the evok i n g ti m e needed for w aking up the neighbo ri n g nodes and t h e m i n i m a l ti m e slo t of t h e M AC backoff strateg y.

④S leep current The node in W SN w ill be i n sleep state m o st o f ti m e;therefo re,the sleep current is an i m po rtant para m e ter i n the design.

⑤RSS I(receive signa l streng t h indicato r) The RSSI si g nal is useful fo r fast carrier sense,w h ich is an i n d ispensable part i n the M AC backo ff a l g o rit h m.

Tab.1 R ad i o com ponen ts com parison

C o m ponen ts C C1010(8051+RF)CC2420(Z i gB ee)nRF24E1(8051+RF)BRF6150(B luet oo t h)M C13192(Z igB ee)TDA5250

Frequ ency/GH z0 3to12 42 42 42 40 868 M odulati on FSK O-Q PSK(D SSS)GFSK G FSK(F H SS)O-QPSK(D SSS)A SK/FSK Vo ltage/V2 7to3 62 1t o3 61 9t o3 61 7to3 62 0t o3 42 1to5 5 TX curren t/mA10 4(0dBm/433M H z)17 4(0dBm)13(0dBm)2530(0dBm)9 4(6dBm) R X cu rrent/mA9 1(433M H z)19 718(250K b it/s)37379(3V FSK) RX sensiti v it y/dBm-107@1 2K bit/s-94-90-85-92-109 Data rate/(K b it s-1)76 8(m ax)2501000(m ax)723 225064(m ax) Start up ti m e/ s250(RX)/270(TX)<192<200NA2 5 1041 2 104 S l eep curren t/ A0 2(RF on l y)12(8051+RF)252 59 RSS I Yes Y es N o NA N o Y es

1 4 Processing un its and sensor i n terfaces

The MC U i n aW SN node serves as bo t h the node contro ller and t h e da ta processor.H ere,the processi n g ab ility of the M C U is not an acute issue,but since the M C U should never be turned off(m ost o f the ti m e in-i dle m ode),it occupies a b i g part i n po w er consum p-ti o n.Thus,the pow er consum ption i n idle sta te is an i m portant para m eter for the cho ice o fMCU.On t h e oth-er hand,the cos,t the pow er and the size li m itations of t h e node argue that theMCU can on ly be an8b it or16 bit lo w-end m icroprocessor SOC so l u tion:the MCU chip i n tegrates the code and data m e m ories,the ADC con-verters and UART/SPI/GPI O i n terfaces.

There is ano t h er recently appeari n g so l u ti o n:the MCU e mbedded in the RF c h ips such as CC1010, nRF9E5,nRF24E1,etc.Co m pared w ith the traditi o na l so l u ti o n of MC U connecting w ith an RF ch i p,t h e ad-vantages of th is integrati o n are as fo ll o w s:better syste m reliab ility due to fe w er chips and fe w er i n ter-ch i p con-nections;l o w er po w er consum ption for i n ner ch i p data transfer and shared clock oscillati o n parts;s m aller PCB size.T ab.2lists the co m parisons of so m e practica l MCU so l u ti o ns.

Tab.2 M CU com paris on s

M CU A T m ega16L(8bit)P89LPC912(8b i t)M SP430F147(16b it)CC1010(8051core)nRF24E1(8051core) M axi m u m perfor m ance8M IPS@8M H z9M I PS@18M Hz8M I PS@8M Hz5M IPS@24MH z5M IPS@20M Hz Fl as h/EEP ROM/RA M16K B/512B/1K B1KB/0/128B32K B+256B/0/1KB32KB/0/2K B+128B0/0/4KB+256B AD C8channe,l10bit C o m parator12bit AD C,comparator3channe,l10bit9channe,l10b it

I O P WM,UART,SPI,IIC,G PI O P WM,UART,SPI,G PI O U SAR T,SPI,G PI O P W M,U ART,SPI,G PI O P WM,UA RT,GPI O

V oltage/V2 7t o5 52 4t o3 31 8t o3 62 7t o3 61 9to3 6

Curre nt(active/i d l e)3 8mA/1 2mA@4MHz,3V

1 1mA/0 35mA@1MH z,3V

7mA/1 5mA@12MHz,3 6V

4mA/1mA@7 373MHz,3 6V

280 A/1 6 A

@1MHz,2 6V

14 8mA/12 8mA@14 7456MHz

1 3mA/29 4 A@32K Hz

3mA/2 A

@16MHz,3V

1 5 Po w er supply and m anage m en t issues

The po w er supply block consists o f a battery and re lative circuits.B atteries can be d i v i d ed into t w o clas-ses:non-rechargeable and rechargeable.I n genera,l the non-rechargeab le has higher energy density and l o w er cos.t Itm ay be possi b le to ex tend the lifeti m e of a sen-sor node by extracti n g energy fro m the env ironm en,t for i n stance,so lar li g h,t v i b rations,RF,etc.But the high cost of these tec hniques h i n ders the i m ple m entati o n.

Po w er m anage m ent strateg ies are consi d ered i n

23

Physica l layer desi g n ofw ireless sensor net w or k nodes

pr i o r research[7,9,10].A lthough it is usually acco m-plished by the app licati o n layer,there are also som e m ethods i n the physical layer to reduce po w er con-sum ption.I n Re fs.[9,10],dual processor solutions are suggested for po w er sav i n g because the pow er consum p-ti o n of the m icr opr ocessor is in d irect proporti o n to the frequency at w hich it w orks.One high frequency pro-cessor,wh ich serves for the RF m ode,l can be in sleep m ode w hen t h ere is no data to transfer;one lo w fre-quency processor contro ls the node and does so m e data processi n g.H o w ever,this design results in several proble m s:the cost o f the node ascends;the reliab ility of t h e node descends;the size o f the node PCB i n crea-ses.There are so m e dev ices such as CC1010t h at can use t w o crystal clock inputs(one is32kH z,and the other m ay be3MH z),and w e can m ake the node w ith on l y one M C U but w or k at t w o different c l o ck frequen-cies so as to ach i e ve the sa m e results as usi n g dual pro-cessors.One processor work i n g at t w o frequenc ies i s a m uch better solution than dua l processors.

2 A n I m p le m entation of the W S N N odes

2 1 So l u tion and desi g n

Accordi n g to the consi d erati o ns m entioned above, to m ini m ize po w er consu m ption,node size and node cos,t and to guarantee the re li a b ility o f t h e device,w e prefer an SOC so l u tion.For the frequency band,433/868 MH z i s chosen.It is best that there is no hardware or fir m w are MAC layer in the chip so that our o w n MAC and route pr otoco ls later can be dep l o yed on t h isW SN node p latf o r m.So,the Z i g Bee dev ices are deserted and t h e ex istence of RSSI signa l and short startup ti m e are requ ired.F i n ally w e acco mp lished our prototype W SN node(see Fig.2)usi n g CC1010.The node i n tegrated a co m pact PCB antenna,a te mperature sensor,and a DC-DC converter.The RF part o f CC1010was configured to w or k at868.277MH z

.

Fig.2 W S N node

2 2 Experi m enta l results

W e conducted a node to node open air LOS data transfer test on Nove mber20th,2005.The resu ltsw ere the re li a ble co mmun icati o n d istances bet w een t w o nodes,wh ich are sho w n in Tab.3.

Tab.3 Po i nt t o po i nt comm un ica ti on rang e(LO S) m Data rate/(b it s-1)

RF pow er/dBm

-20-100

19200253646

9600333749

2400474858

600484981

3 L ifeti m e Esti m ation

There are four w or k i n g states o f each node and the pow er consum pti o ns of each state are listed in Tab.4. The ti m e rati o in Tab.4m eans the assum ed ti m e per-cent of each state.W e took into consi d eration the per-i odic listen and sleep cycles[8]requ ired by the MAC layer a l g orithm,so state3o w ns a ti m e ratio as h i g h as 5%and1%.The lifeti m e T lifeti m e can be esti m ated as

T lifeti m e=

C

battery

I average

(1)

I aver age= 4i=1P i I i(2) where C battery is the capac ity of the battery(mA h), I average is the average current consumpti o n(mA),P i is the ti m e rati o i n t h e state i,and I i is the current con-sumpti o n i n the state i.

Tab.4 F our states o f t he node w ith the pow er co nsu m pti on o f each state(868 277M H z)

M CU R F S t at e C urrent/

mA

Ti m e rat i o

A/%

T i m e rat i o

B/%

I avar age/mA Lifeti m e/h

Id l e(29 4 A@32k H z)Pow er dow n(0 2 A)1S l eep0 029693 598 80.906286(CR2330) A cti ve(3 7mA@3 686M H z)Pow er dow n(0 2 A)2D ata process3 710 1(rati o A)1434(CR123A) A cti ve(3 7mA@3 686M H z)RX(11 9mA)3RX15 6510.2011427(CR2330) A cti ve(3 7mA@3 686M H z)TX(8 6mA@-20dBm)4TX12 30 50 1(rati o B)7138(CR123A)

As sho wn in Tab.4,t h e lifetm i e was calculated based on t w o types of batteries(C R2330L-i Mn co i n ba-t tery:260mA h,m ax3mA conti n uous curre n;t CR123A L-i M n co l u mn battery:1300mA h,m ax1A conti n uous cur-rent)a nd under d iffer ent state tm i e rati o s.Fr o m the life-tm i e estm i ation,w e can confir m that the m ost essential design factors for po wer sav i n g are as follo ws:the RF star-tup tm i e,which decides wh i c h tm i e ratio can be achieved by the MAC algorith m;the RX current(i n c l u di n g MC U acti v e current),si n ce t h e node has to li s ten to the med i a

24Zhong Z i g uo,H u A i q un,andW ang Dan

periodica lly;and the sleep curren,t which occupies the bi g gest tm i e par.t

The lifeti m e eval u ation i n Re.f[9]is prob le m atic for its lack o f consi d eri n g the node s period ica l RX state.The m u lt-i hop feature of the W SN requ ires that t h e node shou l d act as both a data source and a ne-t w ork router,so each node shou l d period ica lly listen to t h e m edia prepar i n g for data re lay.Th is err or in R e.f [9]leads to t h e m uch longer butm istaken esti m ated l-i feti m e(13 1years).

4 Conclusion

In th i s paper,the basi c d i m ensi o ns for the W SN node physica l layer desi g n are discussed,and an i m ple-m entation is presented together w ith t h e test results and life ti m e eva l u ation ana l y sis.

To m eet the ener gy,size and cost design challen-ges,every aspect o f the node syste m shou l d be caref u ll y considered.For t h e physical layer,the startup ti m e of t h e RF wh ich dec i d es the best sleep ti m e ratio and the shortest backoff slot ti m e o fMAC can be ach ieved.The RF frequency and m odulation m ethods wh ich deter m-i nate the RX and TX c urren,t the dua lwork i n g frequen-cy M C U SOC w hich contri b utes to the reliab ility and o-vera ll energy consum ptions,are t h e t h ree m ost essential facto rs for th is cha llenge.

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无线传感器网络节点的物理层设计

钟子果 胡爱群 王 丹

(东南大学无线电工程系,南京210096)

摘要:比较和分析了无线传感器网络节点物理层设计的各种无线通信技术、现有的成熟标准、重要的射频参数以及不同的处理器方案,对节点物理层设计的各个主要方面进行了较为全面的研究.介绍了无线传感器网络节点的一种实现,给出了相关的测试数据以及一种 单处理器双工作频率 的新节能策略.分析了符合传感器网络MAC算法中节点周期性侦听要求的节点寿命估算问题.指出无线传感器网络节点物理层设计的关键考虑因素为:RF启动时间(决定M AC协议可以实现的最佳节点睡眠时间比例和最小的退避时间颗粒)、RF载波频率和调制方式(决定射频收发功耗)以及处理器工作在2个频率的片上系统的总体功耗.

关键词:无线传感器网络节点;物理层;无线电射频;能耗;节点寿命

中图分类号:TN9225

Physica l layer desi g n ofw ireless sensor net w or k nodes