End-to-end Power Optimization in Non-Homogenous Relay Environment for Wireless Body Area Networks (WBANs)

Transcription

1 End-to-end Power Optmzaton n Non-Homogenous Relay Envronment for Wreless Body Area Networks (WBANs) Dan Lu 12, Yshuang Geng 2 and and Kaveh Pahlavan 2 1 College of Informaton Engneerng, Dalan Ocean Unversty (DLOU), Dalan, Chna, Center of Wreless Informaton Network Studes (CWINS), Worcester Polytechnc Insttute (WPI), Worcester, MA, Emal: {dlu2, ygeng and kaveh}@wp.edu Abstract Wreless body area networks (WBANs) s a specal desgned sensor network to connect varous medcal sensors for health whch emerges as the natural byproduct of exstng sensor network technology and bomedcal engneerng. In ths paper, the tradeoff between the total power consumpton and the endto-end rate n non-homogenous relay envronment for WBANs has been nvestgated. We consder such scenaro nvolved two communcaton lnks, mplant to body surface and body surface to off body. WCE emts RF sgnal from nsde of human GI tract, the external AP that s desgned destnaton to collect the transmtted bts by an on-body relay node whch s placed on the belt of the human body. The energy consumpton to delver one bt and the end-to-end rate for the whole relay network s formulated. The total energy and end-to-end rate tradeoff has been studed wth bandwdth, propagaton dstance and data rate for each lnk. The mnmum energes over all possble data rate s obtaned for dfferent condtons. I. INTRODUCTION Wreless body area networks (WBANs) s a specfcally desgned sensor network to connect varous medcal sensors for health whch emerges as the natural byproduct of exstng sensor network technology and bomedcal engneerng. WBANs comprses a seres of mnature sensor each of whch has ts own energy supply, consstng of storage and energy scavengng devces. Each node has enough ntellgence to carry out ts task. These nodes can be placed nsde, on the surface of, or around the body, whch can provde long term health montorng of patents wthout constranng ther normal actvtes as an alternatve to tradtonal practces of montorng a patents physologcal condton [1]. The wreless capsule endoscopy (WCE) s an mportant part of WBANs whch provdes a nonnvasve way to nspect the entre Gastrontestnal (GI) tract. It s capable to provde the nteror vew of ntestnal system and at the same tme assocate the vew wth locaton and moton nformaton of the capsule [2][3]. As the capsule moves through the ntestnes, t takes pctures, whch are transmtted to a small ntermedate data recorder that the patent wears on hs/her belt. The capsule s dsposable and does not need to be recovered by the patent or medcal personnel. So power consumpton of the WCE s another core research topc [4]. Besdes WCE, other nodes n WBANs are also requred to operate under strct resource constrants. The most lmted factor s power source, whch determnes maxmum power consumpton of the system. Specfcally, power for transmttng data from WBANs node to the remote Access Pont (AP) must be preserved so that battery lfe s extended. In addton, the total power consumpton to transmt nformaton bts at the end-to-end rate s also an mportant consderaton. The relay nodes play an mportant role n reducng the transmsson power of bosensors and energy consumpton of networks [5]. The advantages of resortng to cooperatve communcatons for WBANs n terms of mnmzed energy consumpton are nvestgated n [6]. The dynamc resource management n hgher-level protocols by nvestgatng the trade-off between connecton latency and energy consumpton s addressed [7]. A protocol named Dstance Aware Relayng Energy effcent (DARE) to montor patents n mult-hop Body Area Sensor Networks (BASNs) s proposed to ncrease network lfetme and effcently reduce the energy consumpton [8]. Prevous research has manly focused on desgnng algorthm and protocol to mnmze the energy for WBANs nodes and leverage the end-to-end energy-bandwdth trade-off n mult-hop wreless networks n homogenous envronment [9]. In ths paper, we consder the trade-off between the total power consumpton and the end-to-end rate n nonhomogenous relay envronment for WBANs and select WCE applcaton as a case study, for whch the total power consumpton for sngle bt transmsson only ncludes the data forwardng cost n each ndvdual lnk. In the WCE case, the relay node s placed on the belt of the human body, over whch the WCE can communcate wth external AP. We frst derve the expresson for the relatonshp between the overall energy per bt to nose power spectral densty rato and end-to-end rate. Then we use the WCE system to demonstrate the total energy and end-to-end rate trade-off wth varous bandwdth, deployment and data rate for ndvdual lnk. As far as we can see, no such research on the overall energy per bt to nose power spectral densty rato and the end-to-end rate has been conducted n the perspectve of non-homogenous envronment for WBANs. The rest of ths paper s organzed as follows. We descrbe the system model n Secton II and the problem formulaton n wreless body area networks on the total power and the endto-end rate n secton III. In secton IV, we provde numercal results and conclude the paper n secton V.

2 TABLE I: IEEE rado propagaton channel models. Scenaro Descrpton Frequency Band Channel Model S1 Implant to mplant MHz CM1 S2 Implant to body surface MHz CM2 S3 Implant to external MHz CM2 S4 Body surface to body surface (LOS) 13.5,50,400,600,900MHz, 2.4, GHz CM3 S5 Body surface to body surface (NLOS) 13.5,50,400,600,900MHz, 2.4, GHz CM3 S6 Body surface to external (LOS) 900MHz, 2.4, GHz CM4 S7 Body surface to external (NLOS) 900MHz, 2.4, GHz CM4 II. SYSTEM MODEL Consder a wreless relay network where a source node communcates wth a destnaton node over a dstance d, through a multhop route of N relay nodes. For the th hop ( [1, N + 1]), only drect connectvty between the ( 1)th and th nodes s allowed and any cooperatve scheme explotng dversty s prohbted. Assume the wreless relay network works n non-homogeneous envronment, where the propagaton loss of each hop can be ndependently modeled and calculated but at least two out of then hops have dfferent physcal channel characterstcs. Wth such communcaton system and non-homogeneous medum, the ultmate goal of ths work s to nvestgate the end-to-end energy consumpton varaton n varous condtons, whch supports the relay network power optmzaton. A. WBANs Communcaton System Model The WBANs communcaton system employed n ths study s the typcal clncal wreless capsule endoscopy system, whch can be abstracted nto a 2-hop relay network wth three crtcal components, namely, source node, the capsule pll that emts RF sgnal from nsde of human GI tract; Relay node, the nformaton forwardng devce weared on the specfc belt of patents; Destnaton node, the external AP that s desgned to collect the transmtted nformaton of nteror ntestnal envronment. The clncal system has been plotted n Fg. 1, n whch the specfc belt s assumed to be at the average heght of small and large ntestne. WCE AP Relay node Fg. 1: A typcal relay network confguraton for WBANs. The unqueness of such wreless relay network s the nonhomogenety, that s, the 2 hops operate n dfferent propagaton medums. MICS band technologes have been used for the frst hop nsde the lossy human body, whle the second hop works on regular envronment wth modern ndoor wreless access technologes. Note that even though the 2-hop specal case has been employed for numercal analyss, the dervaton ncluded n followng sectons works n general multhop cases. Also, relay node s located on the border lne of two dfferent propagaton medums for the WBANs applcaton n ths work but such stuaton s not requred as long as the propagaton loss of each hop can be properly expressed n the form of eq. (1). The abstracted wreless relay network has been plotted n Fg. 2, where the dstance of hop 1 and hop 2 s gven as d 1 and d 2 respectvely. The n-body regon has been marked wth lght blue and the dstance-power gradent α 1 and α 2 are also dfferent. B. Rado Propagaton Channel Models Consderng the WBANs communcaton system descrbed n the prevous subsecton, t s necessary to understand the physcal rado propagaton medum. We brng about ths subsecton to provde lteratures of WBANs channel modelng. The statstcal RF channel model has been frst proposed by Natonal Insttute of Standards Technology (NIST) n 2007 and then fnalzed as the IEEE standard n 2012 after multple revsons [10]. The BAN channel model apples to multple frequency bands varyng throughout MICS, ISM and even UWB. It has been also parttoned nto seven scenaros from S1 to S7, and four categores from CM1 to CM4. Detaled channel models has been shown n Table I [10]. The n-body communcaton lnk for WCE s a typcal mplementaton of the mplant to body surface scenaro (S2), for whch the standard focuses on the narrow band CM2 channel from 402 to 405MHz. Apart from that, for the offbody communcaton from relay node to external AP, the CM4 wdeband channel for (S6) scenaro can be employed. The propagaton loss for both channels can be expressed as L p (d) = P t P r (d) = L p (d 0 )+10αlog 10 d d 0 +ζ (1) where L p (d) denotes to the pathloss between two neghborng nodes separated at dstance d, P t and P r (d) are the transmt power and receve power respectvely, L p (d 0 ) s the pathloss at reference dstance d 0, α s the dstance-power gradent, and ζ denotes to the shadow fadng effect. Necessary parameters for CM2 and CM4 channel can be found from [10] where we select deep tssue case for the

3 d 1 α 1 α 2 d2 Fg. 2: Equvalent Illustraton of 2-D network model for relay WBANs n non-homogenous envronment. n-body propagaton. Note that for the off-body wdeband channel, only the parameters for LOS scenaro (S6) have been consdered n ths work for the purpose of clarty and smplcty. In the human body caused NLOS scenaro, creepng wave has been observed and the parameters need proper modfcaton [11]. III. FORMULATION OF TOTAL ENERGY CONSUMPTION In ths secton, we provde the defnton and formulaton of the total energy consumpton for our non-homogeneous relay network. Theoretcally, the total energy consumpton ncludes the transmsson energy aganst the lossy propagaton medum and the processng energy aganst the crcutry cost. In ths work, we only take the transmsson energy nto consderaton due to the fact that processng energy s a hardware related extra lnear term n the formulaton and s trval n terms of magntude. A. End-to-End Rate The end-to-end rate s defned as the effectve datarate to transfer certan amount of nformaton bts across the entre network wthn a constant tme T. It can be regarded as a general performance metrc of the relay network. The defnton of end-to-end rate can be derved from the followng equatons { TRe = T1R1 = T2R2 =... = TNRN T = T 1 +T T N (2) By nducton, R e can be proved to be R e = 1 N =1 R 1 Wth such defnton of effectve end-to-end rate, t s reasonable to evaluate the energy-end-to-end rate trade-off of the relay network and therefore propose power optmzaton mechansms. B. End-to-End Energy Consumpton Consder the th lnk whch s communcatng at bt rate R b wth a per bt transmsson energy consumpton of E b tx,, the total energy consumpton per bt can be modeled as E b tot = d 1 α 1 d 2 α 2 (3) N Etx, b (4) =1 where N s the total number of lnks. Obvously, t s dffcult to drectly compute the transmsson energy consumpton but we know that the transmsson power consumpton s postvely related to the rado propagaton channel. From the pathloss model n eq. (1), the determnstc porton of transmsson power for the th lnk, P tx,, can be expressed as P tx, = P rx, +L 0, +10α log 10 (d ) (5) where P rx, s the sgnal strength at the recever sde; L 0, s the pathloss at reference dstance for the th lnk; α and d are dstance-power gradent and TX-RX dstance, respectvely. Performng a lnearzaton on the decbal power readngs n eq. (5), we obtan the lnear transmsson power of th lnk as 10 P tx, 10 = 10 P rx, L 0, α log 10 (d) (6) Usng a superscrpt to label the lnear transmsson power, eq. (6) can be re-wrtten as P l tx, = P l rx,l l 0,10 α log 10 (d) (7) From the energy power relatonshp, we express the energy consumpton per symbol transmsson as E s tx, = P l tx, T s = Pl tx, R s Erx, s = Prx, l T s = Pl rx, R s (9) where Etx, s s the energy consumpton to transmt a symbol on the th lnk, Erx, s s the energy consumpton to receve a symbol, T s the symbol duraton and R s s the symbol rate of th lnk. To solve the energy consumpton per symbol transmsson, t s ntutve to go wth Shannon s theorem as C = Rb = log W W 2 (1+ Es rx, ) (10) N 0 where C s the capacty of the physcal channel of th lnk, W s the bandwdth of that channel, N 0 s the background nose level. Note that we assume that maxmum nformaton transfer rate s achevable so that C = R b. The dervaton does not lose generalty wth that assumpton due to the fact a scale factor on R b can be easly added as a compensaton. Solvng Erx, s, we have (8) Erx, s = N 0 (2 R b W 1) (11) Substtutng eq. (11) nto (9) we know that Prx, l = W N 0 (2 R b W 1) (12) Wth the Ptx, l, Pl rx, relatonshp n eq. (7), we have Ptx, l = W N 0 L l 0,10 α log 10 (d) (2 R b W 1) (13) Fnally, substtute eq. (13) nto (8), the energy consumpton to transmt a symbol on the th lnk, Etx, s, can be gven as E s tx, = N 0 L l 0,10 α log 10 (d) (2 R b W 1) (14)

4 Wth our prevous assumpton of maxmum bandwdth effcency C = R b, we have Es tx, = Rb Eb tx, /W and the energy consumpton per bt transmsson can be gven as Etx, b = N 0 L l 0,10 α log 10 (d) (2 R b W 1) W R b (15) and the overall energy per bt to nose power spectral densty rato Etot/N b 0 can be gven as E b tx, = L l N 0,10 α log 10 (d) (2 R b W 1) W 0 R b (16) Consderng eq. (15) and (16), the overall energy per bt to nose power spectral densty rato E b tot n a nonhomogeneous relay network wll be affected by four dfferent aspects. (1) The bandwdth allocaton of each lnk, W = {W 1,W 2,...,W N }; (2) The channel characterstcs of each lnk, ᾱ = {α 1,α 2,...,α N }; (3) The actual propagaton dstance of each lnk, d = {d1,d 2,...,d N }; (4) The actual bt rate of each lnk, R = {R1,R 2,...,R N }. Aspects (1), (2) and (4) depends on the communcaton technologes selected for each lnk, take the WCE case as an example, the n-body propagaton lnk works on MHz MICS channel whle the off-body lnk may works n 2.4GHz ISM band or even UWB band. Aspect (3) depends on the deployment of relay networks and the dstance between source and destnaton. IV. NUMERICAL RESULTS FOR POWER OPTIMIZATION It s explaned n the prevous secton that the overall energy per bt to nose power spectral densty rato Etot/N b 0 depends on for dfferent aspects, namely, bandwdth W, channel characterstcs ᾱ, actual propagaton dstance d and bt rate of each lnk R. In ths secton, we provde numercal smulaton results to nvestgate the nfluence of these aspects. Snce the bandwdth W and the channel characterstcs ᾱ are hghly correlated wth each other, we do not solate them from each other. A. Energy-Bandwdth Tradeoff The numercal result presented n ths subsecton shows the energy-bandwdth trade-off for the 2-hop relay network n nonhomogeneous envronment. Accordng to the practcal WCE applcatons, we fx the n-body propagaton to MICS band wth bandwdth W 1 = 3MHz and employ α 1 = 4.26 for deep tssue propagaton [10]. After that we vary the bandwdth of off-body propagaton W 2 from 1MHz to 1GHz to nvestgate the change of E b tot. The value of power gradent α 2 s selected accordng to the frequency dependent channel model n [12]. In-body propagaton dstance has been fxed as 0.25m and off-body propagaton dstance d 2 s 1m (we denote 2- hop dstance as d 1 =0.25:1.0 n the followng secton for smplcty). Fnally the data rates for these 2 lnks satsfy eq. (3) wth R 1 :R 2 =1:1. The overall energy per bt to nose power spectral densty rato E b tot has been plotted n Fg. (3) aganst effectve end-to-end rate R e. Three regons can be observed n Fg. (3). E tot W 1 =3MHz W 2 =1 to 1000MHz W 2 =200 to 1000MHz W 2 =20 to 100MHz W 2 =1 to 10MHz Effectve Datarate R [bps] e Fg. 3: Energy-bandwdth trade-off for the relay network. E tot =0.05:1.2) =0.15:1.1) =0.25:1.0) =0.35:0.9) =0.45:0.8) Effectve Datarate, R x 10 7 e [bps] Fg. 4: Energy-deployment trade-off for the relay network. The frst regon s for narrow band off-body propagaton where W 2 ranges from 1 to 10MHz. For each curve wth specfc W 2, a mnmum E b tot can be observed at certan effectve data rate R e. Wth the ncrement of W 2, R e wth mnmum E b tot ncreases, ndcatng that for greater W 1 : W 2 rato, hgher optmal end-to-end data rate can be acheved wth mnmzed energy per bt to nose power spectral densty rato. In addton, t s worth mentonng, for narrow band off-body propagaton regon, the value of mnmum E b tot has very lttle varaton. The second regon s for wde band off-body propagaton where W 2 ranges from 20 to 100MHz. In that regon, smlar trend as the frst regon can be stll observed but the value of mnmum E b tot slghtly ncreases wth the ncrement of W 2. Also, the R e wth mnmum E b tot ncreases slower wth the ncrement of W 2. Fnally, UWB off-body propagaton wth W 2 from 200 to 1000MHz has been plotted n the thrd regon. Stll the same trend can be observed as both E b tot and R e for mnmum E b tot ncreases wth the ncrement of W 2.

5 E tot R 1 :R 2 =1:1 R 1 :R 2 =10:1 to 1: Effectve Datarate R [bps] e Fg. 5: Energy-datarate trade-off for the relay network. B. Energy-Deployment Tradeoff Ths subsecton reports the energy-deployment trade-off for the 2-hop relay network n non-homogeneous envronment. To analyze the effect of deployment, we frst fx W 1 :W 2 =3:7 wth properly selected ᾱ values; R 1 :R 2 =1:1 and R 1, R 1 satsfy eq. (3). Then we setup d 1 rato to represent varous system deployment. For the abdomen of a adult male ndvdual, the n-body propagaton dstance usually ranges n [0.05m, 0.45m]. Also, we set the dstance between on-body relay node and external AP to be wthn [0.8m, 1.2m]. Gven the propagaton dstances, the d 1 rato goes from 0.05:1.2 to 0.45:0.8. Wth these setup, the overall energy per bt to nose power spectral densty rato E b tot has been plotted n Fg. (4) aganst effectve end-to-end rate R e. As can be seen from Fg. (4), wth a greater d 1 rato, the R e for mnmal E b tot decreases and the mnmal E b tot also decreases. Such trend ndcate that when d 1 and d 2 get mbalanced, the mnmal energy consumpton can be hgher. It s worth mentonng that ths observaton can be only appled to the WCE case. In the general sense, t s possble to have reversed trends for d 1 smaller than specfc constant c or d 1 larger than the constant c (c=1:1 for homogeneous case). We leave ths problem open for future works. C. Energy-Datarate Tradeoff Ths subsecton reports the energy-datarate trade-off for the 2-hop relay network n non-homogeneous envronment. To nvestgate the effect of data rate, we frst fx W 1 :W 2 =3:3 wth properly selected ᾱ values; d 1 =0.25:1.0. The R 1 :R 2 rato vares from R 1 :R 2 =1:10 to R 1 :R 2 =10:1. Note that R 1 and R 2 always satsfy eq. (3). The overall energy per bt to nose power spectral densty rato E b tot has been plotted n Fg. (5) aganst effectve end-to-end rate R e. Observaton of energy-datarate trade-off s that the balance between R 1 and R 2 s mportant. When R 1 :R 2 =1:1, maxmzed effcency can be acheved as the mnmal E b tot of 11.2dB provdes the maxmum R e of over 6MHz. If R 1 :R 2 <1:1, as the blue curves n Fg. (5), lower energy consumpton can be acheved wth the sacrfce of end-to-end data rate R e. However, when R 1 :R 2 >1:1, more energy s requred and R e stll goes lower than R 1 :R 2 =1:1, whch s not recommended n the mplementaton of relay networks. V. CONCLUSION In ths paper, we consdered the trade-off between the total power consumpton and the end-to-end rate n nonhomogenous relay envronment for WBANs where the relay node s placed on the belt of the human body by whch the WCE can communcate wth external AP. We formulated the energy consumpton to delver one bt and the end-to-end rate for the whole relay network. 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