On the Use of Adaptive OFDM to Preserve Energy in Ad Hoc Wireless Networks

Transcription

1 O the Use of Adaptive OFDM to Preserve Eergy i Ad Hoc Wireless etworks Kamol Kaemarugsi ad Prashat Krishamurthy Telecommuicatios Program, School of Iformatio Sciece, Uiversity of Pittsburgh 135 orth Bellefield Aveue, Pittsburgh, Pesylvaia Tel , Fax {kakst11, prashk}@pitt.edu Abstract - Orthogoal frequecy divisio multiplexig (OFDM) is the physical layer i emergig wireless local area etworks that are also beig targeted for ad hoc etworkig. Mobile devices i ad hoc etworks eed to coserve their eergy because of the limited battery power. Adaptive OFDM is a techique that ca improve the performace of OFDM i terms of icreasig the capacity for a give trasmit power by exploitig the chael coditio over a lik. We believe that adaptive OFDM ca be also exploited i ad hoc etworks to improve the eergy performace of mobile devices. I this paper, we evaluate the improvemet i performace of adaptive OFDM over o-adaptive OFDM i ad hoc etworks usig simulatios. 1. ITRODUCTIO Orthogoal frequecy divisio multiplexig (OFDM) is importat i wireless etworks because it ca be used adaptively i a dyamically chagig chael. OFDM has also bee selected as the stadard physical layer for IEEE 80.11a ad IEEE 80.11g wireless local area etworks (WLAs). At the same time, ad hoc etworkig usig WLAs has received attetio i recet years [1]. I ad hoc etworks, eergy efficiecy, throughput, ad routig efficiecy are importat performace metrics. Eergy efficiecy is especially importat for mobile devices with limited battery power. Cross-layer protocol desig ad optimizatio ca prolog the battery life of mobile devices [1]. Researchers have ivestigated makig OFDM or discrete multitoe (DMT) adaptive to maximize chael capacity by usig adaptive modulatio ad loadig algorithms. Bits ad power loadig algorithms cotrol how the umber of bits ad power are allocated across all subcarriers i OFDM such that the capacity over the lik is maximized or the trasmit power is miimized. Several loadig algorithms have bee proposed i the literature. For istace, the algorithm of Fischer ad Huber [] is well kow for its simplicity ad efficiecy for discrete multitoe loadig. Campello i [3] also proved the ear optimality of the discrete greedy loadig algorithm ad suggested algorithms for efficiet loadig with low complexity. However, it is ot clear how the ad hoc etwork performace costraits ca impact makig OFDM adaptive. I ad hoc etworkig these measures may ot be sufficiet to exted the battery life. This paper is a prelimiary effort to uderstad whether adaptive 1

2 OFDM ca impact the eergy cosumptio i ad hoc etworks ad how OFDM ca be made adaptive. As a first step to uderstadig this problem, we compare the use of adaptive ad oadaptive OFDM i ad hoc etworks i terms of eergy cosumptio ad bit error rates usig simulatios with Qualet. We oly employ primitive cross-layerig i that we preempt trasmissio if there are isufficiet power resources at the trasmitter based o chael kowledge. I sectio of this paper, we discuss the otio of adaptive OFDM ad bit loadig algorithms for adaptive OFDM. I Sectio 3, we provide a brief review of eergy coservatio techiques i ad hoc etworks. Sectio 4 discusses the details of the simulatio, the set up, ad the results. Sectio 5 cosiders approaches for future work.. ADAPTIVE OFDM OFDM is a sub-class of multicarrier modulatio (MCM) that combies parallel data trasmissio with frequecy divisio multiplexig (FDM) techique ad allows spectral overlap of subchaels. The idea is to trasmit sigle highrate data stream over multiple parallel low-rate data streams [4]. The low-rate data streams are modulated oto orthogoal subcarriers i order to avoid adjacet carrier iterferece ad improve spectrum efficiecy. Due to the loger symbol period o each subcarrier, the OFDM sigal is more robust agaist large multipath delay spreads that are ormally ecoutered i wireless eviromets. With a cyclic prefix or a repetitio of part of OFDM symbol added at the begiig of the symbol, the iter-symbol iterferece (ISI) caused by multipath fadig ca be further reduced. Digital modulatio schemes such as phase shift keyig (PSK) or quadrature amplitude modulatio (QAM) are usually used o each subcarrier; however, the modulatio techique does ot have to be the same for all subcarriers..1 The Chael State Iformatio Adaptive OFDM takes advatage of the idepedece of subcarriers by assigig higher eergy ad larger umber of bits to subchaels that have better quality or higher SR ad assigig less eergy ad bits or oe at all to the poor quality subchaels. This techique is a wellkow result from iformatio theory that states that the chael capacity ca be maximized by a water-fillig or water-pourig techique [3]. A importat assumptio is that the trasmitter has the chael state iformatio (CSI) i order to perform power or bit allocatio to achieve the maximum chael capacity. This may be difficult to achieve i practice, but it is possible that the estimated chael iformatio ca be obtaied at the trasmitter usig feedback from the receiver or i the case of reciprocal chaels. ote that the CSI is oly applicable betwee a give commuicatio pair ad differet CSI are required for differet pairs of commuicatig odes. The chael state should also chage slowly compared to the frame duratio [5]. Otherwise, adaptive OFDM may perform worse tha o-adaptive OFDM. I idoor wireless local area etworks, it is likely that the chael chages very slowly because the mobility of the ode is limited. Both

3 adaptive ad o-adaptive OFDM require chael kowledge at the receiver i order to detect the correct trasmit symbol o each subchael. This ca be achieved by usig kow traiig symbols or blid detectio [6], [7]. All OFDM systems require chael codig (such as covolutioal codes) to maitai low bit error rates [8].. Loadig Algorithms The water-fillig power distributio is kow to be the optimal solutio for ay spectrally shaped chael [3]. The resultig bits or power allocatio maximizes the iformatio capacity ad it is called the capacity-achievig distributio. A greedy algorithm ca be used to fid the optimal solutio for this problem. For a large umber of bits ad subchaels o the order of 1000s, the greedy algorithm is iefficiet due to operatios ivolvig chael gai sortig ad the umber of iteratios [3]. However, i the case of small umber of bits ad chaels such as those i IEEE 80.11a, the umber of bits is betwee 48 ad 88 (to be loaded o 48 subchaels for data rates betwee 6 Mbps ad 54 Mbps) [9]. For istace, 48 bits per oe OFDM symbol with 4 µsec symbol duratio is eeded to achieve a 6 Mbps data rate that icludes a covolutioal rate ½ code. A umber of loadig algorithms have bee proposed i the literature such as the algorithms by Fischer ad Huber [], ad Campello [3]. They propose differet approaches to solve the loadig problem such as miimizig the bit-error-rate (BER) (rather tha maximizig the SR or the capacity of the chael). Most algorithms try to avoid itese sortig ad searchig that causes iefficiecy i greedy-like algorithms. The result is a slightly suboptimal allocatio, but with a dramatically reduced computatioal complexity [3]. The authors predict that with advaces i digital sigal processig, the loadig algorithms will be able to perform i real time ad be suitable for operatios such as those evisaged i this paper. A OFDM lik ca be modeled as a group of parallel AWG chaels. The widebad radio chael is partitioed ito discrete arrowbad subchaels with chael badwidth of f Hz. Each chael is free of iter-symbol iterferece whe f is small ad the chael respose appears to be flat for each chael. I the case of sigle carrier commuicatios, the iformatio capacity for a ideal chael with AWG follows Shao's iformatio capacity theorem. I practical systems, a quatity called SR gap is itroduced ad used to determie the efficiecy of a modulatio or ecodig scheme compared to the ideal scheme [3]. For a practical modulatio or ecodig scheme, the system ca trasmit at most R bits/trasmissio with the lowest acceptable error rate. The SR gap is defied as a ratio of ideal SR at which the system ca trasmit at C bits/trasmissio over a practical SR at which the system ca trasmit R bits/trasmissio. It is a measure of how well the practical system compares to a ideal modulatio system. The chael capacity i bits per trasmissio ca be calculated by [10]. α C = log ( 1 + SR ) (1) 3

4 ote that α is the dimesio of modulatio scheme, i.e. α = for M-QAM modulatio scheme. Rearragig Equatio 1 eables us to express the SR as SR = C α 1. Usig a similar expressio for the SR of practical systems, the SR gap, deoted by Γ, ca be calculated as Γ = C α R α 1 SR = R α. () 1 1 The SR for additive white Gaussia oise (AWG) with oise variace of σ per dimesio = H ca be defied as SR, where H is chael gai ad ε is the trasmit power per symbol. Therefore, for a particular combiatio of ecodig scheme ad modulatio with - dimesioal symbol costellatio, the SR gap ca be used to determie the data rate for subchael i multicarrier commuicatios [3] as R α σ H ε G ε = log 1 + = log 1 +, (3) Γ Γσ H σ where =. If T sym is the OFDM symbol G duratio, the data rate of OFDM over all subchaels is R = 1 = R T 1. By rearragig Equatio 3 the eergy fuctio ca be writte as a fuctio of bits per subcarrier as ε sym R Γ R ( 1) = ( 1 Γσ ε = ) (4) H G.3 Campello s Algorithm Campello suggests that the water-fillig problem ca be formulated i two ways from the optimizatio perspective [3]. First, a bit rate maximizatio problem ca be formulated by maximizig the total umber bits across all OFDM subcarriers i Equatio 3 subject to the costrait that a fixed amout of power is available to the trasmitter. This is similar to the classical waterfillig formulatio i [10]. Secod, a eergy miimizatio problem ca be formulated by miimizig the total amout of power o all OFDM subcarriers i Equatio 4 subject to the costrait of a fixed amout of bits trasmitted per OFDM symbol. Give a eergy fuctio ε(r ) for a particular modulatio ad codig techique where R is the umber of bits o subcarrier, R Total is the total umber of bits per OFDM symbol, E Total is the fixed amout of power available, ad B is the fixed umber of bits per symbol per secod, the formulatio of these optimizatio problems are described below. ote that the resultig bit allocatio should be a positive iteger. Bit Rate Maximizatio or Water-fillig Problem ( ) Total Maximize R ε = R (5) = 1 + Subject to ε E ad Z (6) =1 Total Eergy Miimizatio Problem R Miimize ε ( R ) = E (7) = 1 Total + Subject to R B ad Z (8) =1 = The solutio to either oe of the above formulatios ca be foud by formig a R 4

5 Lagragia equatio ad takig the partial derivative with respect to the multiple costrait variables, i.e. ε for bit rate maximizatio ad R for eergy miimizatio [3]. For istace, the Lagragia equatio for bit-rate maximizatio is J = G ε E log Total = Γ λ 1 = 1 ε. (9) Assumig that the SR gap is equal for every subchael, the solutio to the problem cosists of a water-fillig costat K. The subchael eergy allocatio ca be calculated usig this costat ad the SR of the subchael. The solutio to the optimizatio i Equatio 5 is (a) K 1 1 = ETotal + Γ = 1 G (10) (b) + Γ ε = K, = 1, G,..., (11) where (x) + = x if x > 0, otherwise (x) + = 0. Ay subchael that has egative eergy allocatio will be tured off by the trasmitter. ote that the amout of eergy used is measured i joules = watts secods. A example of the solutio for eergy miimizatio is show i Figure 1 for OFDM with 64 subcarriers. Figure 1a represets the chael frequecy respose for a three equal-tap-gai chael model. Figure 1b represets the cotiuous bit loadig result from the eergy miimizatio algorithm i [3]. Figure 1c represets the discretized bit loadig result ad Figure 1d represets the correspodig power allocatio. I this example, the oise variace is assumed to be 1 for all subchaels. (c) (d) Figure 1. Example of Eergy Miimizatio Loadig, (a) Chael Respose, (b) Cotiuous Bit Allocatio, (c) Discrete Bit Allocatio, (d) Power Allocatio of Discrete Bit Allocatio. 5

6 The dual formulatios of water-fillig solutio ca be applied at the physical layer to either maximize the data rate or miimize the eergy o each frame trasmissio. These two alteratives are ivestigated i the ext sectio for their impact o the eergy cosumptio of a ad hoc wireless ode. The questio that this paper would like to aswer is how much of eergy ca be preserved by employig adaptive OFDM o the physical layer. Aother questio regardig the cross-layer protocol desig is how the chael iformatio gai from the loadig algorithm will help improve the eergy preservatio of ad hoc wireless etwork. 3. TECHIQUES FOR REDUCIG EERGY I AD HOC ETWORKS Due to the emergece of small mobile devices with limited battery capacity, eergy-aware protocols are key to the success of this techology. It is suggested that the eergy optimizatio should be doe across all protocol layers i a cross-layer approach [1]. Each cross-layer protocol stack should adapt its operatios to the etwork load, the eergy budget, ad lik characteristics. The cooperatio ad exchage of ecessary iformatio betwee layers must be allowed for ay cross-layer protocol to adapt to global system costraits ad characteristics. Eergy o ad hoc wireless devices is cosumed differetly durig the trasmittig state, the receivig state, ad the idle state. It has bee show from early measuremet results that the power used durig the idle state of a mobile ode domiates the overall (total) eergy cosumptio, while the output power durig the trasmittig ad receivig state add additioal eergy cosumptio to that of the idle state [11]. I the literature three separately eergy preservatio approaches are suggested at differet layers for ad hoc wireless etworks [1]. For istace, power savig protocols ad power cotrol protocols are suggested at the MAC layer ad a maximum lifetime routig protocol is suggested at the etwork layer. These approaches oly focus o the protocols at the medium access cotrol layer ad above. They try to miimize the eergy cosumptio i differet parts of ad hoc systems by maximizig the idle state, miimizig the trasmit power, ad usig routig kowledge to exted the etwork lifetime. 3.1 Eergy preservatio at the MAC layer Power savig protocols ad power cotrol protocols are categorized as eergy efficiet techiques at the MAC layer. The power savig protocol puts most of the ad hoc odes ito sleep mode as ofte as possible. It is more suitable for etworks with a cetralized cotrol that is eeded to maitai the coectivity of all adjacet odes that go ito sleep mode. To implemet this approach i a peer-to-peer ad hoc etwork will be quite complex due to the schedulig of the sleep time. It also limits the capacity of ad hoc etworks because these odes caot forward frames durig their sleepig period. It is a tradeoff betwee etwork capacity ad eergy preservatio [13]. There is also sigificat cost of chagig the ode state from sleep to idle ad vice versa that may outweigh the power savig techique [14]. O the other had, power cotrol protocols reduce the 6

7 trasmit power to levels that ca just maitai the coectivity betwee adjacet ad hoc odes. This approach ca miimize the eergy cosumptio due to trasmissio ad additioally improve the etwork capacity by miimizig the iterferece betwee trasmissios [13]. 3. Eergy preservatio at the routig layer Istead of focusig o power cosumptio at each mobile ode, a eergy coservig routig approach tries to create eergy aware routig mechaisms for ad hoc wireless etworks. For example, i the maximum lifetime routig protocol, a selectio is made from differet routig metrics such as miimum eergy routig, maxmi routig, ad miimum cost routig to preserve the eergy i forwardig packets [1]. The ad hoc etwork routig protocol should cosider both the cost of trasmittig each packet ad the residual eergy of odes that will be used to further forward packets. All three approaches discussed so far focus maily o specific layer of the protocol stack ad do ot cosider ay cooperatio betwee the techiques [1]. 3.3 Adaptive vs. o-adaptive OFDM This paper suggests a adaptive protocol layer that fits ito cross-layer desig criteria at the physical layer with a primitive cooperatio betwee the physical layer ad MAC layer. A potetial stroger cooperatio is possible with the bit rate ad power budget parameters as the iformatio exchaged betwee the physical ad MAC layer. Give that the radio chael characteristics ca be estimated at the receiver, a adaptive OFDM physical layer ca exploit the temporal fluctuatios of the chael i frequecy selective fadig media. The service requested by the MAC layer ca be supported by this smarter physical layer equipped with a bit loadig algorithm. I our work, usig adaptive OFDM, a extra chael capacity is gaied durig a short period whe the chael is cosidered good for trasmissio. Durig this time, the physical layer ca trasmit a MAC frame faster usig the same trasmit power level as it could without adaptive OFDM. This beefit ca be coverted ito the savig of eergy cosumed for trasmissio. The loadig algorithm based o bit rate maximizatio [3] is selected as our choice of study. The idea here is to push as may bits across the chael as possible while the trasmitter has the opportuity to do so thereby reducig the chael holdig time o average. We assume that advaces i digital sigal processig techiques allow the radio chael to be estimated fast eough i a slowly chagig wireless eviromet. To solve the problem of chael state iformatio at the trasmitter, we add extra iformatio i the MAC header of the IEEE protocol withi the request-to-sed (RTS) ad clear-to-sed (CTS) packets ad the details are discussed i the ext sectio. Sice the maximum allowable data rate i fadig chaels ca be higher or lower tha the MAC layer s miimum request rate, the MAC protocol i this paper decides o allowig the commuicatio over the lik by choosig to reply or ot reply with a CTS frame. By this, the MAC layer avoids a loger 7

8 trasmit time with smaller data rate that could cosume more eergy. 4. SIMULATIO AD RESULTS The performace of a ad hoc wireless etwork usig adaptive OFDM is evaluated with the Qualet packet level simulator. Below we describe the parameters ad sceario used i our simulatios at various layers of the protocol stack. 4.1 Radio chael model A two-ray path loss model is assumed with a shadow fadig sigma of 1 db which is suitable for idoor eviromets [16]. The thermal oise floor is calculated from the Boltzma costat k = W/(Hz K ) at T = 90 K, oise factor F = 10, ad a effective oise badwidth i BW Hz usig the followig equatio W = F k T BW = BW Watt. The simulator has a radio model with capture capability that ca receive the strog radio sigal amog iterfererig sigals [17]. Packet error is based o the SR threshold i.e., a packet is assumed to be i error if the SR is below a threshold of 10 db above the oise level. The physical ad MAC parameters follow the IEEE 80.11a specificatios [9] ad are summarized i Table 1. However, the rate fall back feature is ot used. Table 1. IEEE 80.11a Specificatios Physical Layer Ceter Frequecy 5. GHz Chael Badwidth 0 MHz Miimum Data Rate 6 Mbps Receiver Threshold -8 dbm Atea height 1.5 m The Rayleigh multipath fadig is modeled with three tap gais accordig to the JTC idoor office areas Chael A, although this ot strictly for a 5 GHz frequecy bad [18]. The tap parameters are show i Table 1. Each tap is a radom process geerated before the actual simulatio usig Jakes method [19]. Durig the simulatio, a set of tap gais is radomly selected from a pool to simulate the Rayleigh fadig betwee each pair of odes. Table. JTC Idoor Office Areas Chael A Tap o. Relative Delay (sec) Average Power (db) The maximum Doppler shift frequecy of the model is set to f d = 30 Hz for slow time-varyig chaels which correspods to the maximum mobile speed of v = 1.73 m/s at f c = 5. GHz. Each OFDM symbol has symbol duratio T sym = 4 µsec. Assumig a maximum data frame legth of 4096 bytes ad each OFDM symbol ca support 4 ucoded bits, the frame duratio is approximately T frame = ( µsec)/(4) msec. The ormalized maximum Doppler rate is f d T frame which is close to the reasoable values for the rate adaptive physical layer system i [0]. 4. Physical layer model A cotiuous bit loadig algorithm based o Campello s bit rate maximizatio algorithm [3] is implemeted i the simulatio for both trasmitter ad receiver. It is possible to fid the maximum umber of bits per OFDM symbol. The digital 8

9 modulatio scheme o each subchael is assumed to scale the costellatio from 1 bit to higher bits per symbol usig BPSK ad M-QAM modulatios. The SR gap is assumed to be 8.8 db for u-coded QAM bits with error rate P e of 10-6 as give i [7]. The resultig bit rate is based o a cotiuous bit distributio ad eeds to be discretized by roudig dow to the earest iteger. Campello [3] suggests a Eergy Tighte Algorithm to reallocate the left-over eergy from the roudig bit to guaratee a optimal solutio. Assumig the loadig calculatio ca be doe i real time due to the small umber of bits ad subcarriers (as discussed previously) we igore the eergy spet o this calculatio as ot sigificat. The bit loadig ca oly be doe o a data frame sice the trasmitter ca gai the chael iformatio oly after the CTS frame has arrived. Therefore, all sigalig (cotrol) frames the RTS ad CTS are ot adaptive. Due to the multiple receiver possibility, the MAC broadcast frames are also o-adaptive. Both cotrol ad broadcast frames are set at a rate of 6 Mbps. The power cosumptio is based o the estimated values give i Agere s product specificatio 003 [1]. The estimated active receive ad trasmit power cosumptio of a 80.11a stadard device is give as 951 mw ad 141 mw respectively. The idle state power cosumptio is assumed to be equal to power cosumptio i receive mode although this is a over-estimate. We use per packet eergy cosumptio i this paper. The eergy cosumed by each frame is liearly depedet o its trasmissio duratio, which depeds o the istataeous data rate ad frame size for a give trasmit power. The trasmit eergy is calculated by multiplyig the trasmit power cosumptio of 141 mw by the duratio of the frame. The eergy cosumptio rates for both trasmit, receive, ad idle states are assumed to be costat over time. 4.3 MAC layer model The IEEE MAC layer is based o Carrier Sese Multiple Access with Collisio Avoidace (CSMA/CA). The simulatio operates oly i the distributed coordiatio fuctio (DCF) mode. It also has the request-to-sed (RTS) ad clear-to-sed (CTS) cotrol sigalig to avoid the hidde termial problem ad to carry extra iformatio for chael estimatio procedures as i [0]. The overhead iformatio is the SR level ad chael impulse respose estimated at the receiver. Figure illustrates a adaptive OFDM trasmissio procedure. time Perform Bit Loadig T T T T RTS CTS w/ CSI Adaptive DATA ACK Figure : Procedure of DATA frame trasmissio The etwork allocatio vector (AV) duratio for each data frame is calculated from the smallest data rate of 6 Mbps. We do ot vary it accordig R R R R Chael Estimatio ad Bit Loadig for MAC s decisio Chael is good. 9

10 to the variable duratio of the adaptive OFDM frame. This causes a loger waitig time due to the AV i eighborig odes, but it does ot cause ay problems to the trasmissio. The data frame duratio will be guarateed to have at least 6 Mbps of badwidth i our modificatio to the MAC protocol i the case of adaptive OFDM. We ote that this study does ot attempt to maximize capacity, but oly evaluate the eergy savigs from adaptive OFDM. 4.4 etwork layer model The etwork layer is the iteret protocol (IP) ad the trasport layer is UDP. The routig protocol is ad hoc o demad distace vector (AODV) i uicast mode []. This protocol discovers a route wheever there is a request by issuig a Route Request (RREQ) message. The routig table o each ode is filled by both RREQ message ad the reply iformatio o the uicast Route Reply (RREP) message from the eighborig odes. The old route i the table is elimiated based o the sequece umber ad its activity. I this study, we do ot modify this protocol to lear of the chage from the physical layer. The routig protocol parameters are set accordig to the values i []. 4.5 etwork topology Four ad hoc odes are placed i a simple rectagular topology. The odes are assumed to be statioary which is the case for most idoor operatios of today. The distace betwee two closest odes is varied from 5 meters to 00 meters. The simulatio study compares the eergy reductio achieved by adaptive OFDM over oadaptive OFDM whe the received SR is chaged due to the distace. The simulatio duratio is 10 sec. Each experimet has 10 repetitios ad we calculate the 95% cofidece iterval of the mea value of the eergy cosumed. We assume that odes i the etwork always have packets for trasmissio. Each ode has a costat bit rate (CBR) packet geerator which geerates a 00-byte packet every 90 msec or a data rate of kbps. The traffic is oly oe hop from the origi ode ad oly 4 CBR streams from ode 1 to ode, ode to ode 3, ode 3 to ode 4, ad ode 4 to ode 1 are preset. Each ode caot trasmit ad receive at the same time. 4.6 Results The simulatio results of the average trasmit eergy cosumed per ode is show i Figure 3. Average trasmit eergy cosumptio i mwhr per ode a-5m -5m 95% Upper C. I. 95% Lower C. I. Mea a-50m -50m a-100m -100m a-150m -150m a-00m -00m Figure 3. Compariso of trasmit eergy cosumptio Here adaptive ad o-adaptive odes i each experimet are deoted with letter a ad followig by the distace, respectively. At each distace poit, the adaptive OFDM physical layer cosumes less eergy. This is because o average 10

11 a shorter DATA frame is trasmitted by adaptive OFDM. It ca exploit the temporal fadig diversity i radio chael. Sigificat reductios i eergy ragig from 76.94% dow to 63.67% are possible depedig o the distace betwee odes ad the received SR. Figure 4 shows the average umber of RTS frames that are retrasmitted due to timeout. This is compared because the MAC layer usig adaptive OFDM has a optio ot to reply with a CTS message whe the chael caot support the miimum data rate. The result reports that o average the umber of RTS timeouts is higher oly i the mea for adaptive OFDM, but there is o statistical differece. That is, the eergy cosumptio for RTS frame trasmissio is higher, but ot sigificat ad does ot domiate the overall trasmit power cosumptio. Average umber of RTS retrasmissio due to timeout a-5m -5m 95% Upper C. I. 95% Lower C. I. Mea a-50m -50m a-100m -100m a-150m -150m a-00m -00m Figure 5. Compariso of RTS retrasmissios 5. DISCUSSIO AD FUTURE WORK Takig the advatage of radio chael i additio to mitigatig its harshess ca provide beefits for ad hoc wireless etworks. However, adaptive OFDM mechaism is ot sufficiet to miimize the eergy cosumptio i a ad hoc wireless etwork. The first step toward iformatio exchage ad Physical ad MAC layer cooperatio is suggested i this work by cotrollig the RTS/CTS sigalig based o the kowledge of the physical layer. Based o our simulatio with IEEE MAC, the etwork throughput is expected to icrease if we reduce the AV time accordig to the shorter DATA frame duratio. The eighborig odes o loger have to wait for a loger AV as i o-adaptive OFDM. Power cotrol mechaisms as i [13] could be used with adaptive OFDM to decide a suitable trasmissio power depedig o the rage of commuicatio betwee mobile odes. etwork layer cooperatio ca improve the eergy coservatio further. The impact of adaptive OFDM with such cross-layerig approaches is part of our ogoig work. ACKOWLEDGMETS The authors would like to thak Scalable etwork Techology for the Qualet simulator, Dr. Mark Wickert at UCCS, for the chael fadig simulator toolbox, ad Dr. Hueg-o Lee for his valuable commets ad suggestios. REFERECES [1] A. J. Goldsmith ad S. B. Wicker, Desig Challeges for Eergy- Costraied Ad Hoc Wireless etworks, IEEE Wireless Commuicatios, vol. 9, pp. 8-7, Aug. 00. [] R. F. H. Fisher ad J. B. Huber, A ew Loadig Algorithm for Discrete Multitoe 11

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