PHY and MAC Performance Evaluation of IEEE a WLAN over Fading Channels

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1 PHY and MAC Performance Evaluaion of IEEE a WLAN over Fading Channels Hiroyuki Yomo, Cong Huan Nguyen, Persefoni Kyrisi, Tien Duc Nguyen, Shyam S. Chakrabory, and Ramjee Prasad Absrac This paper presens an overview of physical (PHY) and medium access conrol (MAC) layers for IEEE a, which is a sandard of a broadband high-speed wireless local area nework (WLAN). IEEE a uses 5 GHz frequency band and is PHY is based on orhogonal frequency division muliplexing (OFDM). In his paper, we evaluae he OFDMbased PHY and MAC performance of IEEE a under various pracical wideband channel models, which are absraced from acual propagaion measuremens. The effecs of differen ransmission modes defined in PHY on he IEEE sysem performance are invesigaed. We also discuss he impac of link adapaion, which aemps o choose he bes-suied ransmission mode according o channel condiions, on he overall sysem performance. Index Terms OFDM, IEEE a, HYPERLAN/2, IFFT, convoluional coding, ime dispersive, frequency selecive, wideband, Rayleigh fading, MAC, Link Adapaion. I. INTRODUCTION Wireless local area neworks (WLANs) have gained a grea deal of aenion as a means for providing users wih wireless local access o he Inerne as well as ad hoc local conneciviy among mobile compuing devices. Two high-speed WLAN sandards have recenly emerged a unlicensed 5 GHz frequency band, IEEE a[1] and ETSI HIPERLAN/2[2], which boh use orhogonal frequency division muliplexing (OFDM) for heir physical layers (PHYs). OFDM is a ype of mulicarrier ransmission, where a single daa sream is ransmied over a number of lower rae subcarriers. The roos of OFDM can be raced back o some of he works done in fifies[3], sixies[4], [5], and in sevenies[6]. OFDM has received enormous aenion laely since i has been chosen as he physical layer for a variey of wireless broadband services, including no only WLANs bu also mobile mulimedia, digial video broadcasing (DVB) ec. One of he major reasons o use OFDM is o increase he robusness agains frequency selecive fading. In a single carrier sysem, a single fade or inerferer may cause he enire link o fail. However, in a muli-carrier sysem, such a fade would affec only a few of he subcarriers. A properly designed error correcion coding and inerleaving, possibly in conjuncion wih a reransmission scheme, can hen be used o miigae he fading effec. The IEEE a OFDM-based PHY defines 8 differen ransmission modes wih differen modulaions and coding The auhors are affiliaed wih Cener for TeleInFrasrucure (CTIF), Aalborg Universiy, Niels Jernes Vej 12, DK-9920 Aalborg, Denmark. S. S. Chakrabory is also associaed wih Academy of Finland and Helsinki Universiy of Technology, P.O.Box 2300, FIN HUT, Finland. raes. Exensive performance evaluaion and analysis wih pracical wireless channel models are required in order o undersand he behavior of he differen modes in real usage scenarios. A ypical performance measure in PHY layer is packe error rae (PER). However, PER shows only he reliabiliy of each ransmission mode, and i is difficul o see is efficiency because each packe includes overhead bis in addiion o he informaion bis. Furhermore, MAC mechanisms o resolve he conenion of users inroduce more overhead such as backoff ime, conrol frame, ec. Therefore, MAC-level evaluaion obaining goodpu, which shows how many informaion bis can be ransmied wihin a cerain ime inerval, is required in order o discuss he rade-off beween he reliabiliy and efficiency of he differen ransmission modes. The undersanding of MAC-level performance gives us a hin on how o appropriaely choose one of he modes according o he channel condiions for improving overall sysem performance. The purpose of he paper is o give an overview of PHY and MAC specificaions of IEEE a, and provide exensive PHY and MAC performance evaluaion under fading channel models which are absraced from pracical measuremens. We also discuss he applicaion of link adapaion mechanism which has been proposed for improving he IEEE a sysem performance. The paper is srucured as follows. Afer his inroducion, a general overview of IEEE a PHY is provided in Secion II, including a brief descripion of OFDM sysem. Secion III deals wih he descripion of IEEE MAC proocol. In Secion IV, we show simulaion model and resuls of IEEE a PHY, and discuss PER performance wih differen ransmission modes under differen environmens of wireless channels. Secion V deals wih he MAC-level simulaion of IEEE a where we provide goodpu performance. We also inroduce simple bu powerful link adapaion scheme, and discuss is impac on he goodpu performance. The paper ends wih conclusions and scope for fuure works in Secion VI. II. DESCRIPTION OF IEEE A OFDM PHY In his secion, we sar wih he general descripion of OFDM, which is employed in he IEEE a PHY, followed by an overview of IEEE a PHY specificaions. A. General descripion of OFDM The fundamenal principle of OFDM is he use of overlapping subcarriers o modulae parallel daa sreams. This makes

2 Binary daa Oupu Binary daa Fig. 1. Symbol mapping Symbol demapping S/P converer P/S converer OFDM Sysem. IFFT FFT P/S converer S/P converer CP exersion CP removal Carrier modulaion Carrier demodulaion OFDM much more bandwidh efficien han he convenional non-overlapping mulicarrier echnique. The subcarriers in OFDM sysem need o be orhogonal, so ha hey do no inerfere wih each oher. The fas fourier ransform (FFT) echnique is employed o derive a se of orhogonal subcarriers. By modulaing low rae daa sreams ono hese subcarriers, OFDM sysem can ensure fla fading condiion on each subcarrier. Fig. 1 is he block diagram of an OFDM sysem. Firs, he binary daa is mapped ino symbols using a specific modulaion scheme, such as QPSK or 16QAM. Then, he symbol sream is convered in N parallel symbol sreams by means of a serial o parallel (S/P) converer. Inverse fas fourier ransform (IFFT) is applied ono he symbol sreams, for OFDM modulaion. A parallel o serial (P/S) conversion is done furher, and a cyclic prefix (CP) exension, or guard inerval, is added o each OFDM symbol. The CP is insered o preven iner-symbol inerference (ISI) beween adjacen OFDM symbols, and iner-carrier inerference (ICI) beween adjacen subcarriers. Finally, he OFDM signal is ransmied afer carrier modulaion. The OFDM receiver chain begins wih he carrier demodulaor. Afer CP removal and S/P conversion, he FFT operaion is performed. A furher P/S conversion is needed o obain original symbol sream. The symbol de-mapping module is used o esimae he ransmied binary daa. For a deailed descripion of OFDM sysems, readers are referred o [7] and [8]. B. Descripion of IEEE a PHY The IEEE a PHY is an inerface beween he MAC layer and wireless media, specifically designed for OFDM modulaion. The IEEE a PHY uilizes he unlicensed 5 GHz band, o ransmi frames a daa rae up o 54 Mbps, enabling mulimedia ransmissions over WLAN neworks. The IEEE a PHY uses 52 sub-carriers ha can be modulaed wih BPSK, QPSK, 16QAM or 64QAM, depending upon he channel condiions. The forward error correcion (FEC) is implemened hrough convoluional coding wih he coding rae of 1/2, 2/3 or 3/4. As a resul, differen ransmission modes wih muliple daa raes (i.e. 6, 9, 12, 18, 24, 36, 48 and 54 Mbps) are suppored by he sysem as shown in Table I. The schemaic diagram of he IEEE a PHY is shown in Fig. 2. Firs, he daa for ransmission, as obained from he MAC layer, is scrambled o preven long runs of 1s and 0s in he inpu daa. The scrambled daa hen inpus o a convoluional encoder. The encoder consiss of 1/2 rae coder and subsequen puncuring process o obain 2/3 or 3/4 rae (if necessary). The coded daa is inerleaved in order o preven error burss from being inpu o he convoluional TABLE I RATE-DEPENDENT PARAMETER Daa Cod. Coded Coded Daa Mode rae Mod. rae bis per bis per bis per (Mbps) (R) subcarrier OFDM OFDM symbol symbol 1 6 BPSK 1= BPSK 3= QPSK 1= QPSK 3= QAM 1= QAM 3= QAM 2= QAM 3= Fig. 2. Daa from higher layer a PHY burs Scrambling OFDM modulaion IEEE a PHY Transmier. 1/2 rae convoluional code Pilo inserion Puncuring Inerleaving decoding process in he receiver. Afer inerleaving, pilo bis are insered. The daa sream is OFDM modulaed o creae IEEE a PHY burss and mapped ono 48 subcarriers, whereas, he pilo bis are mapped ono 4 subcarriers. A he receiver, inverse processes are used o recover he ransmied daa and deliver hem o he higher layer. We also noe ha, IEEE a PHY is very similar o ha of he HIPERLAN2 specificaion[2]. C. Inerleaving/Deinerleaving Scheme In a frequency selecive fading channel, he OFDM subcarriers generally have differen ampliudes. However, he deep fades in he frequency specrum may cause groups of subcarriers o be less reliable han ohers, hereby causing bi errors o occur in burss raher han being randomly scaered. Mos of FEC codes are no designed o deal wih error burss. As a resul, inerleaving is usually employed o randomize he burs channel errors, so ha he FEC codes could be more effecive. According o he IEEE a specificaions, all encoded daa bis shall be inerleaved by a block inerleaver wih a block size corresponding o he number of coded bis in a single OFDM symbol. The inerleaver is defined by a wosep permuaion: he firs permuaion ensures ha adjacen coded bis are mapped ono nonadjacen subcarriers; he second ensures ha adjacen coded bis are mapped alernaely ono less and more significan bis of he consellaion and, hereby, he probabiliy o have coniguous sream of error bis is decreased[1]. D. IEEE a PHY parameers The relevan iming-relaed parameers for IEEE a PHY are lised in Table II. The pilo subcarriers are uniformly disribued over he bandwidh of ineres. We assume ha

3 TABLE II TIMING-RELATED PARAMETERS IN IEEE A PHY LAYER Parameer Descripion Value N SD Number of daa subcarriers 48 N SP Number of pilo subcarriers 4 N ST Number of subcarriers, oal 52 (N SD + N SP ) F Subcarrier frequency spacing MHz (=20MHz/64) T FFT IFFT/FFT period 3.2μs (1= F ) T PREAMBLE PLCP preamble 16 μs duraion (T SHORT + T LONG ) T SIGNAL Duraion of he SIGNAL BPSK 4.0 μs (T GI + T FFT ) OFDM symbol T GI GI duraion 0.8μs (T FFT =4) T GI2 Training symbol GI duraion 1.6μs (T FFT =2) T SY M Symbol inerval 4μs (T GI + T FFT ) T SHORT Shor raining sequence duraion 8μs (10 Λ T FFT =4) T LONG Long raining 8μs sequence duraion (T GI2 +2Λ T FFT ) here is perfec iming and carrier synchronizaion (here is no frequency offse). This assumpion is idealisic because exacly hese wo facors limi he performance of OFDM sysems. Moreover we assume ha he channel esimaion performed wih he raining is perfec, i.e. he channel is perfecly known a he daa subcarriers in boh ampliude and phase. III. DESCRIPTION OF IEEE MAC A. General Descripion The IEEE sandard specifies a common MAC layer, which suppors he seamless operaion beween higher layer, e.g. logical link conrol (LLC) layer, and differen WLAN PHY layers, such as IEEE a OFDM-based PHY. The basic and mandaory mechanism in IEEE MAC is called disribued coordinaion funcion (DCF), which enables he saions (STAs) o auonomously share he medium hrough he usage of carrier sense muliple access wih collision avoidance (CSMA/CA). In his paper, we only focus on DCF. Before a STA sars ransmission, i senses he wireless medium for some period of ime o deermine if any oher STA is ransmiing. If he medium appears o be idle during he period, he ransmission may sar, oherwise he STA has o defer unil he end of he in-progress ransmission. Afer deferral, or prior o aemping o ransmi again immediaely afer a successful ransmission, he STA mus selec a random backoff inerval and wai unil he backoff couner reaches zero before ransmiing. The ransmission can sar only if he medium is sill idle a he end of backoff inerval. B. Carrier Sense Mechanism The DCF regulaes ha an IEEE STA mus perform boh physical and virual carrier sensing mechanisms. The physical mechanism is o lisen o he medium and o deec a carrier (i.e. ransmission aciviy) on he medium. The virual mechanism is based on he nework allocaion vecor (NAV), which always has he laes informaion possible on scheduled ransmission on he medium. The NAV is updaed when a STA receives a packe in which he informaion on he ransmission period is included, and i acs as a backup mechanism o avoid collision. If a carrier is no sensed by physical means (for example, due o shadowing effecs), he NAV can hopefully provide correc informaion on he medium aciviies. Thus, he STA should no aemp o ransmi daa unil boh physical and virual carrier sense mechanisms sense he medium being idle. More informaion abou seing of he NAV will be presened laer in he secion abou he RTS/CTS access mehod. C. Iner-Frame Spacing The CSMA/CA mechanism requires a specified gap beween coniguous frame ransmissions. The gap beween frames is called iner frame space (IFS). A STA mus ensure ha he medium has been idle for he specified IFS before aemping o ransmi. The IEEE defines four differen IFSs o provide differen prioriies for access o he wireless medium. They are lised here in he increasing order, saring wih he shores defined IFS: ffl Shor IFS (SIFS): This is he shores IFS. I is used when wo STAs have seized he medium and need o keep i for he duraion of he frame exchange. Using he smalles gap beween heir ransmissions, hese STAs can preven oher STAs from aemping o use he medium since hey are required o wai for he medium o be idle for a longer gap. Thus, i gives he prioriy o compleion of he frame exchange sequence in progress. For example, he SIFS is used beween a daa frame and he corresponding acknowledgemen (ACK) frame. ffl Poin Coordinaion Funcion (PCF) IFS (PIFS): This IFS is used only in STAs operaing under he opional PCF mode o gain prioriy access o he medium. A STA using he PCF is allowed o ransmi conenion-free raffic afer is carrier sense mechanism deermining ha he medium is idle for a PIFS. ffl DCF IFS (DIFS): The DIFS is used by STAs operaing under he DCF o ransmi daa frames. The STA under DCF is allowed o ransmi afer is carrier-sense mechanism deermines ha he medium is idle for a DIFS plus addiional backoff ime. ffl Exended IFS (EIFS): This IFS is used by STAs operaing under DCF whenever he PHY layer indicaes o he MAC layer ha a frame ransmission was sared bu did no resul in he correc recepion of a complee MAC frame wih correc frame check sequence (FCS) value. The EIFS is defined o provide enough ime for anoher STA o acknowledge wha was, o his STA, an incorrecly received frame before his STA commences ransmission[9]. D. Backoff procedure The backoff inerval in he DCF is referred o as collision avoidance mechanism, which aims a reducing he collision probabiliy beween muliple STAs accessing he medium. The high probabiliy of collision exiss when he medium becomes

4 idle following a busy condiion since muliple STAs could have been waiing for he medium o become available again o ransmi. The random backoff inerval for each STA could preven muliple STAs from accessing he medium a he same ime. The backoff mechanism is invoked for a STA in wo cases: (a) When he STA aemps o ransmi, bu finds he medium busy as indicaed by eiher he physical or virual carrier-sense mechanisms; and (b) Afer finishing ransmission of a daa packe (which can be eiher successful or failed). To begin he backoff procedure, a STA ses is backoff imer o a random backoff ime using he following equaion: BackoffT ime = Random() aslot ime (1) where Random() is a pseudo-random ineger drawn from a uniform disribuion over he inerval [0, CW]. The CW is referred o as conenion window, and i is an ineger wihin he range of acwmin and acwmax. aslotime is he sloime value depending on he characerisics of he PHY layer. If he medium is found o be inacive for a DIFS or EIFS, he STA sars o decrease is backoff imer. If no medium aciviy is indicaed for he duraion of a paricular backoff slo, hen he backoff procedure shall decremen is backoff ime by aslotime. The coun-down is suspended if aciviy is deeced on he medium, i.e. he backoff imer shall no be decremened for ha slo. In his case, he STA mus wai unil he channel is deermined o be idle for a DIFS period or EIFS, before resuming is backoff procedure. Only when he backoff imer reaches zero, he STA can sar is ransmission. The backoff procedure employed in IEEE is called a binary exponenial backoff algorihm. In his algorihm, if he las aemp o ransmi fails, he STA will assume ha a collision has happened due o oo many STAs accessing he medium a he same ime. Then, he STA doubles is conenion window, CW, o avoid collision in he nex ransmission. The iniial aemp always akes CW equal o acwmin o ransmi he packe. Then, he CW shall ake he nex value in he series every ime here is an unsuccessful aemp o ransmi ha frame, unil i reaches he value of acwmax[9]. The STA reses is CW o acwmin when i receives he acknowledgemen for he ransmied daa frame. E. Basic and RTS/CTS access mehods DCF provides wo differen access mechanisms: Basic access mehod and RTS/CTS access mehod. In he basic access mehod, a source STA sars o ransmi he daa packe direcly afer a DIFS or EIFS and backoff procedure (see Fig. 3). Upon successfully decoding he daa frame, he desinaion STA replies wih an ACK o acknowledge ha he daa packe has been received correcly. The SIFS is he ime inerval beween recepion of daa packe and ransmission of he corresponding ACK frame. If he daa packe is no received correcly by he desinaion, no ACK frame is ransmied o he source, and he source will have o reransmi he daa packe afer an ACK imeou. The basic access mode requires minimum overhead informaion for each daa frame ransmission. This provides good performance in he case of very few collisions happening in he nework. However, if he collisions are more likely Source Desinaion Fig. 3. Ohers DIFS Back Off Basic Access Scheme in DCF. Source Desinaion Fig. 4. Ohers DIFS Back Off RTS SIFS CTS RTS/CTS Scheme in DCF. DATA SIFS SIFS ACK Defer Access DATA SIFS NAV (RTS) NAV (CTS) Defer Access ACK DIFS DIFS Back Off Back Off o happen, he performance of basic access mode is quickly degraded especially when large packes are ransmied. This is because colliding packes wasefully consume he channel bandwidh for mos of he ime. The second access mechanism specified in DCF, he RTS/CTS access mehod, aims a reducing he impac of collision on sysem performance. Figure 4 illusraes he procedure of RTS/CTS access mechanism: before sending he daa frame, he source STA sars by launching a Reques- To-Send (RTS) frame o he desinaion. Upon successfully receiving he RTS frame, he desinaion STA shall reply wih a Clear-To-Send (CTS) frame o announce ha i is ready for receiving he daa frame, provided ha is NAV shows ha he medium is idle. In oher words, if he virual carrier-sensing mechanism a he desinaion STA indicaes he medium is no idle, ha STA shall no respond o he RTS frame. Only afer he compleion of exchanging RTS and CTS frame sequence, he daa packe and is corresponding ACK are ransmied. As a resul, his mehod is someimes referred o as handshaking access mehod. In combinaion wih he virual carrier sensing mechanism, he RTS/CTS access mehod is paricularly effecive agains hidden erminal problem. Hidden erminal problem happens when wo or more STAs wan o alk o he hird STA, bu boh senders are no in he range of he oher. In his case, collisions happen if one sender (A) sars ransmission o he desinaion (B) afer (physically) sensing he channel idle while he oher sender (C) is also ransmiing is packe o B. This hidden erminal

5 TABLE III IEEE MAC LAYER PARAMETERS Parameer Value aslot ime 9μs aairp ropagaiont ime << 1μs amacp rocessingt ime < 2μs ACKime 76μs SIFS 16μs DIF S 34μs (SIF S +2Λ aslot ime) EIFS 126μs (SIFS + ACKime + DIFS) acw min 15 acw max 1023 problem can be avoided hanks o he virual carrier sensing mechanism of IEEE sandard: C ses he duraion field in he RTS frame o B equal o he ime needed o send he CTS, daa, ACK frames plus hree SIFS in micro-second. Likewise, B ses he duraion field in he replied CTS frame equal o he duraion field of received RTS frame, minus one SIFS and CTS frame duraion. Any STA, which is no he source or desinaion, shall updae is NAV according o he duraion field conained in he mos recen and valid RTS or CTS. Then, hese STAs sar o coun-down heir NAV, and do no ransmi unil he NAV is couned o be zero. As a resul, A, which can lisen o he CTS frame from B, will defer is ransmission unil C and B finish heir conversaion. The disadvanage of RTS/CTS mechanism is he considerable overhead informaion required in each ransmission. Therefore, i is inefficien o use his access scheme for relaively shor daa frames. In IEEE , he use of RTS/CTS mechanism depends on he do11rtsthreshold aribue[9]. This aribue may be se on a per-sta basis, which allows STAs o be configured o use RTS/CTS eiher always, never or only on frames longer han a specified lengh. F. IEEE a MAC parameers Table III shows parameers and some requiremens for IEEE a MAC layer. In his able, he ACKime is he ime required o send an ACK frame a he lowes mandaory rae (i.e. 6 Mbps). The aairpropagaiontime is propagaion delay from he ransmiing STA o he receiving STA. In addiion, he amacprocessingtime is he nominal ime ha he MAC uses o process a frame and prepare a response o he frame[9]. IV. IEEE A PHY SIMULATION RESULTS A. Simulaion Model When we focus on he packe-based communicaion sysem wih convoluional coding like IEEE a, he mos indicaive performance measure for PHY is PER[10], [11]. Therefore, in his work, we evaluae he PER for IEEE PHY wih differen wireless channel models. References [12] and [13] provide a characerizaion of he radio propagaion channel for various indoor scenarios. These indoor channel models have been developed for HIPERLAN/2 physical layer simulaions. However, since he specrum for HIPERLAN/2 sysems is he same as ha used for IEEE sysems, he channels can be used for IEEE a link level simulaions as well. Environmen A Normalized power (db) Fig B C D E TABLE IV CHANNEL MODELS FOR COMPUTER SIMULATION Descripion Typical office environmen for NLOS condiion wih 50ns rms delay spread. Typical large open space and office environmen for NLOS condiion and 100ns rms delay spread. Typical large open space environmen for NLOS condiion, and 150ns rms delay spread. The same as environmen C, bu for LOS condiion. A 10dB spike a zero excess delay has been added, resuling in a rms delay of abou 140ns. Typical large open space environmen for NLOS condiion, and 250ns rms delay spread. The PDP of various environmens Environmen A Environmen B Environmen C Environmen D (firs ap Ricean) Environmen E Delay (ns) The PDP of various environmens. There are five models, labeled A hrough E, as shown in Table IV. Tap delay line models wih unequal ap spacing are used o characerize he differen delay spread siuaions. For shorer delays, denser ap spacing is used. The roo mean square (rms) delay spread varies beween 50 ns for he environmen A o 250 ns for he environmen E. Tha is, he environmen A has he shores, while he environmen E has he longes delay spread. The average power falls off exponenially wih delay. All he aps are assumed o have Rayleigh fading characerisics, excep for he firs ap of he environmen D, which has Rician saisics wih a K facor of 10. A classical (Jakes) Doppler specrum wih doppler spread of 5 Hz is assumed for all aps[14]. The power delay profiles (PDPs) of he above-menioned environmens are ploed in Fig. 5. They are based on values on Table VI hrough Table X, presened in he appendix. B. Simulaion Resuls and Discussions Fig. 6 and 7 show PER wih differen environmens for ransmission modes 1 and 8, respecively. In his work, he size of packe passed from MAC layer is fixed a 1000 byes. We also pu PER performance in Rayleigh (1-ap) and Rician (1-ap) fla fading channels in he figure. Firs observaion from hese figures is ha PER of he environmen A is lower han hose of he oher environmens

6 for lower SNR region. This is because he environmen A has he smalles rms delay spread, resuling in a smaller raio of OFDM signal bandwidh and channel coherence bandwidh. In his siuaion, he channel frequency response is relaively fla wihin he OFDM signal bandwidh. If he channel sae changes slowly during he packe ransmission, bi errors end o gaher wihin cerain packes, which leaves he oher packes error-free or wih few bi errors. On he oher hand, when here is a larger rms delay spread, he frequency response is more frequency selecive, and he condiions o have subcarriers affeced by he deep fade are equally shared by all he packes. Therefore, he bi errors spread over packes more randomly, even if he channel is changing slowly compared o packe duraion. This resuls in a worse PER performance han he one in he environmen A. The same reason applies o explain he fac ha he Rayleigh fla fading channel, which can cause bi errors o concenrae on some packes, performs much beer han he wideband channels. Second observaion from hese figures is ha he environmens B, C, D, E, which all have relaively larger rms delay spreads, have similar PER performance. For he environmen D where a Rician componen is presen, he PER is slighly lower for higher SNR. However, due o he exisence of he oher aps wih he Rayleigh componens, he performance is almos similar o he oher hree environmens, and i is quie differen from ha in a purely Rician channel. Nex, in Fig. 8, we compare PER for he differen ransmission modes. The Rae index in he figure corresponds o each ransmission mode in Table I. Here, he environmen E is used as he channel model. From his figure, we can see ha PER is lower as we use more reliable modulaion scheme or lower coding rae. In general, he PER performance is degraded when a higher daa rae is used. The only excepion is he ransmission mode 2. Our simulaion shows ha he mode 3 can perform equally o or even beer han he mode 2. Alhough QPSK in mode 3 has worse error performance han BPSK in mode 2, he use of 1/2 convoluional code helps mode 3 o have almos same PER wih mode 2 which employs 3/4 convoluional coding[16]. We confirmed ha his effec of differen modes on PER is similar among all he environmens. Fig. 8 shows ha more robus ransmission sraegy can lower PER. However, his reliabiliy is increased a he cos of he daa rae, and we need o evaluae and analyze he radeoff beween reliabiliy and efficiency. Therefore, in he nex secion, we analyze he MAC-level performance wih hese differen modes. PER Convoluional coded PER under various environmens (Pksize = 1000 byes, Rae Index = 1) Environmen A Environmen B Environmen C Environmen D Environmen E Rayleigh fla fading Ricean fla fading Averaged symbol SNR in db Fig. 6. The PER under differen environmens (Rae Index = 1). PER 10 0 Convoluional coded PER under various environmens (Pksize = 1000 byes, Rae Index = 8) Environmen A Environmen B Environmen C Environmen D Environmen E Rayleigh fla fading Ricean fla fading Averaged symbol SNR in db Fig. 7. The PER under differen environmens (Rae Index = 8) Convoluional coded PER a various daa raes (Environmen E, Pksize = 1000 byes) PER V. IEEE A MAC SIMULATION RESULTS A. Simulaion Model The main objecive of MAC simulaion is o examine he effec of differen ransmission modes on MAC-level performance. To his end, we assume a simple node configuraion where wo STAs locaed a disance d ransmi daa packes 10 2 Rae Index = 1 Rae Index = 2 Rae Index = 3 Rae Index = 4 Rae Index = 5 Rae Index = 6 Rae Index = 7 Rae Index = Averaged symbol SNR in db Fig. 8. The PER for differen daa raes wih environmen E.

7 TABLE V PARAMETERS FOR MAC LAYER SIMULATIONS Descripion Noaion Value Transmiing power P TX 17dBm Noise floor P NOISE -93dBm Min. SNR level SNR min 10dB Pah-loss exponen n 3.5 Channel models Env: A Size of PSDU P ksize 1000byes ACK imeou ACKimeou 108μs (2 Λ SIFS + ACKime) CTS imeou CT Simeou 108μs (2 Λ SIFS + ACKime) Goodpu (Mbps) Goodpu for Basic Access Scheme (Environmen A, Pksize = 1000 byes) Rae Index = 1 Rae Index = 2 Rae Index = 3 Rae Index = 4 Rae Index = 5 Rae Index = 6 Rae Index = 7 Rae Index = 8 6 wih each oher 1. We assume ha he raffic load is balanced and fully loaded, which means ha he offered load is much higher han he minimum hroughpu and here is always a leas one packe in he ransmission queue. For each ransmied frame, PHY link layer simulaion described in Secion IV is performed in order o decide if he frame is ransmied successfully. The parameers defined for he MAC simulaion are shown in Table V. We choose consan ransmied power, P TX, of 50 mw (or 17 dbm), which is equal o he maximum allowable ransmiing power for WLAN devices a 5GHz band regulaed by he Danish radio inerface specificaion [15]. We selec he noise floor o be -93 dbm, which is he same value used in [16]. The pah-loss exponen, n, is chosen o be 3.5, which represens he pah-loss characerisic in office building [17]. Since he hroughpu of WLAN devices is insignifican in region where he SNR of received signal is very low, we assume ha a packe is always discarded if he SNR is less han SNR min. As a resul, he usable range of received power is from -83 o 17 dbm. The IEEE menions he ACKimeou and CT Simeou, bu does no specify heir values [9]. In his simulaion, we define he ACKimeou and CT Simeou o be equal o he duraion for ransmiing an ACK frame (or CTS frame, which is idenical) a he minimum mandaory daa rae, plus wo SIFS. B. Simulaion Resuls and Discussions Figs. 9 and 10 show goodpu performance agains he disance d for differen ransmission modes wih he basic access and handshaking access schemes, respecively. Here, he goodpu is defined as he raio of oal number of informaion bis received by desinaions o oal simulaion ime. These figures show clearly ha he mode wih low daa rae, which canno provide high maximum goodpu, is able o achieve reliable connecion even a large disance. On he oher hand, he performance of he mode wih higher daa rae, which offers higher goodpu a close disance, is largely degraded a large disance. I is because he PER performance of he mode wih higher daa rae is severely degraded as SNR becomes 1 In his simple scenario where here are only wo saions, he packe loss occurs when (1) one saion receives a packe from he oher saion, bu he received SNR is no high enough o receive he packe correcly, and (2) he wo saions simulaneously sar he ransmiions by choosing he same backoff slo in he conenion period. We call he second case collision in his paper, alhough in he half-duplex sysem like IEEE , hese saions can no ry o receive any packe while hey are ransmiing Disance (m) Fig. 9. The Goodpu for differen daa raes (Basic Access Scheme, Environmen A). Goodpu (Mbps) Goodpu for Handshaking Access Scheme (Environmen A, Pksize = 1000 byes) Rae Index = 1 Rae Index = 2 Rae Index = 3 Rae Index = 4 Rae Index = 5 Rae Index = 6 Rae Index = 7 Rae Index = Disance (m) Fig. 10. The Goodpu for differen daa raes (Handshaking Scheme, Environmen A). lower, or equivalenly he disance becomes larger (see Fig. 7). Furhermore, we can observe ha he goodpu of mode 2 is always lower han mode 3. This is naural resul since, as we saw in PHY simulaion, PER of daa rae modes 2 and 3 are almos similar while mode 3 offers higher daa rae han mode 2. This is he coincide resul wih [16]. We can also noice ha he performance of basic access scheme is slighly beer han ha of handshaking access scheme. This is because, in his paricular configuraion where here are only wo STAs and no hidden erminal, he collision probabiliy is low, and RTS and CTS frames only resul in he addiional overhead. The mos imporan observaion from Figs. 9 and 10 is ha we can maximize he goodpu performance by adapively choosing he ransmission modes according o channel condiions. To achieve his link adapaion, we inroduce a simple bu powerful mechanism in he nex subsecion.

8 C. Applicaion of Link Adapaion Mechanism 1) Link Adapaion Mechanism: While IEEE a provides differen ransmission modes which can be used for link adapaion scheme, he acual link adapaion algorihm is lef open. As a resul, here are several link adapaion proposals for he IEEE a. For example, a bes PHY mode able is used o find he suiable daa rae according o packe size, SNR value and frame rery coun in [16]. This mehod is relaively complicaed, because i requires he esimaion of SNR of ransmission link. Besides, he bes PHY mode able migh no be valid for all ypes of channel models, which could induce negaive effecs o he performance of WLAN sysem operaing on differen ypes of channels. In his work, in order o examine he impac of link adapaion, we implemen anoher simpler, bu equally powerful, link adapaion mehod proposed in [18]. The mehod is similar o he auo-rae fallback (ARF) mehod used in Lucen s WaveLAN-II device [19]. The basic mechanism is as follows: 1) The ransmier mainains wo couners for each of is links, one for successful ransmission and one for failed ransmission. 2) The ransmier checks if he packe is ransmied successfully or no based on wheher i receives he MAClevel ACK. 3) If a packe is ransmied successfully, he success couner is increased by one, and he failure couner is rese o zero. 4) If he ransmission fails, hen he failure couner is incremened by one and he success couner is rese o zero. 5) If he success couner is greaer han a hreshold value S, hen he ransmier will sar he nex ransmission using he nex (i.e. higher) daa rae available. 6) If number of packe failed is greaer han he hreshold F, he ransmission rae is decreased by one. 7) All he couners are rese o zero afer he ransmission mode is changed. The mechanism only uses simple informaion obained from MAC, i.e., packe error monioring using ACK frame which is an original funcion implemened in he IEEE ) Simulaion Resuls and Discussions: Figs. 11 and 12 show goodpus of he link adapaion schemes wih basic access and handshaking access modes, respecively. The link adapaion mechanism employs only ransmission modes 1, 3, 5, 6, 7, and 8 since modes 2 and 4 are ofen below or close o hose of he oher modes. For comparison, goodpus of fixed modes, 1, 3, 5, 6, 7, and 8 are also displayed. The link adapaion scheme is evaluaed wih hree differen ses of parameers: (S =3, F =1), (S =10, F =1) and (S =3, F =2). Firs, from hese figures we can see ha he se of parameers (S =10, F =1) provides much lower goodpu han he oher ses a close disance. This is because he link adapaion scheme, wih a large value of S, canno reac fas enough o he improvemen of he link qualiy, i.e. he STA will mainain a low ransmission rae alhough he qualiy of he link allows he use of a higher rae [18]. However, a large disance, where Goodpu (Mbps) Goodpu for Link Adapaion wih Basic Access Scheme (Environmen A, Pksize = 1000 byes) Rae Index = 1 Rae Index = 3 Rae Index = 5 Rae Index = 6 Rae Index = 7 Rae Index = 8 Link Adp (S=3, F=1) Link Adp (S=10, F=1) Link Adp (S=3, F=2) Disance (m) Fig. 11. The Goodpu of Link Adapaion (Basic Access Scheme, Environmen A). Goodpu (Mbps) Goodpu for Link Adapaion wih Handshaking Access Scheme (Environmen A, Pksize = 1000 byes) Rae Index = 1 Rae Index = 3 Rae Index = 5 Rae Index = 6 Rae Index = 7 Rae Index = 8 Link Adp (S=3, F=1) Link Adp (S=10, F=1) Link Adp (S=3, F=2) Disance (m) Fig. 12. The Goodpu of Link Adapaion (Handshaking Scheme, Environmen A). only he lowes daa rae is possible, S =10leads o a slighly beer goodpu performance compared o hose of S =3.In his case, a large value of S can avoid ineffecive swiching o higher raes when he channel has no improved. Large value of S is also said o be effecive when he qualiy of he link is changing very slowly [18], for similar reason. In our simulaion, he channel changing rae is fixed a 5Hz and he value of S = 3 is proved o be a beer choice a his paricular rae. Second, we can observe he performance of differen values of F. The se of parameers (S = 3, F = 1) achieves he higher goodpu a disance larger han 10 meers, bu (S = 3, F = 2) is superior a closer disance. The value F = 1 dicaes he STA o reduce is ransmission rae whenever a ransmission failure occurs, regardless of he cause of failure. This scheme works well a large disance, because

9 mos of ransmission failures are due o he channel qualiy. A close disance, where he link qualiy is very good, collision becomes he main source of failure. Therefore, a larger value, F =2, is a beer choice in his case, because i reduces he probabiliy of falling back o lower daa rae incorrecly due o collision. Finally, we can see ha he link adapaion mechanism can achieve higher goodpu for mos of he disance d. Alhough more analyses are required o opimize parameers S and F according o channel condiions and collision probabiliy, we can conclude ha he link adapaion is effecive under he pracical wireless channel condiions. VI. CONCLUSIONS In his paper, we have sudied PHY (PER) and MAC (goodpu) performance of he OFDM-based WLAN sandard IEEE a under 5 pracical fading channel models obained from acual measuremens. We have invesigaed he effec of hese environmens on he PER, and shown ha he environmen A has relaively beer PER performance han he ohers since errors are more grouped (i.e., less random) han he oher environmens due o he small rms delay. The MAC-level performance has been also discussed by evaluaing goodpus of differen ransmission modes under fading environmen. The resuls have clearly shown ha i is necessary o inroduce link adapaion mechanism o choose he bes suied mode according o channel condiions. We have also shown ha he simple link adapaion mechanism using only MAC-level informaion (i.e., packe error monioring) can significanly improve overall sysem performance. However, he goodpu performance of link adapaion mechanism clearly depends on he choice of he parameers, and his indicaes he need owards an opimizaion aking accoun of channel condiions and collision probabiliy. This however demands some more exhausive sudy, and is lef for fuure works. ACKNOWLEDGEMENT This work is parially suppored by Aalborg Universiy of Denmark and Academy of Finland, Finland. The work is also suppored by CTIF research projec FACE Fuure Adapive Communicaion Environmen ha is an aciviy encompassing expers from four research groups, WING (Wireless Neworks Group), SMC (Speech and Mulimedia Communicaion), CSys (Cellular Sysems), and A&P (Anennas and Propagaion). REFERENCES [1] IEEE Sd a Wireless LAN medium access conrol (MAC) and physical (PHY) specificaions: High-speed Physical Layer in 5 GHz Band, [2] ETSI Broadband Radio Access Neworks (BRAN): HIPERLAN Type2 [3] Mosier R.R. and Clabaugh R.G., A Bandwidh Efficien Binary Transmission Sysem, IEEE Trans. Comm., Vol. 76, pp , Jan [4] Chang R. W., Synhesis of Band Limied Orhogonal Signals for Mulichannel Daa Transmission, Bell Sys. Tech. J., Vol. 45, pp , Dec [5] Salzberg, B. R., Performance of an efficien parallel daa ransmission sysem, IEEE Trans. Comm., Vol. COM-15, pp , Dec [6] Orhogonal Frequency Division Muliplexing, U.S. Paen No. 3, 488, 4555, filed November 14, 1966, issued Jan. 6, [7] R. van Nee and R. Prasad, OFDM Wireless Mulimedia Communicaions, Arech House Publicaions, [8] J. Terry and J. Heiskala, OFDM Wireless LANs: a Theoreical and Pracical Guide, SAMS, [9] IEEE Sd , Wireless LAN medium access conrol (MAC) and physical (PHY) specificaions, [10] J. Lassing, T. Oosson, E. Srom, Packe Error Raes of Terminaed and Tailbiing Convoluional Codes. In T Wysocki, M Darnell, B Honary, ediors, Advanced Signal Processing for Communicaion Sysems, The Kluwer Inernaional Series in Engineering and Compuer Science, Vol.703, Boson, Sep [11] D. J. Cosello and O. Y. Takeshia, On he Packe Error Rae of Convoluional Codes, ITW 1999, Mesove, Greece, pp. 29, [12] J. Medbo and P. Schramm, Channel models for HIPERLAN/2, ETSI/BRAN documen no. 3ERI085B. [13] J. Medbo and J-E. Berg, Measured radiowave propagaion characerisics a 5 GHz for ypical HIPERLAN/2 scenarios, ETSI/BRAN documen no. 3ERI084A. [14] W. C. Jakes, Jr., Microwave Mobile Communicaions, Wiley, New York, [15] CEPT/ECC Working Group: ECC Guidance Documen on 5 GHz Wireless LANs. Technical repor, The Radio and Telecommunicaions Terminal Equipmen Compliance Associaion. [16] D. Qiao, S. Choi and K.G. Shin, Goodpu analysis and link adapaion for IEEE a wireless LANs, IEEE Transacions on Mobile Compuing, vol. 1, Issue: 4, pp Oc.-Dec [17] T. S. Rappapor, Wireless Communicaions: Principles and Pracice, Prenice Hall PTR. [18] P. Chevilla, J. Jelio, and H. L. Truong, A Dynamic Link Adapaion Algorihm for IEEE a Wireless LANs, Proceedings of ICC 2003, pp , May, [19] A. Kamerman and L. Monean, WaveLAN-II: A High-Performance Wireless LAN for he Unlicensed Band, Bell Labs Technical Journal, pp Summer APPENDIX Channel Models for compuer simulaion TABLE VI MODEL A. CORRESPONDS TO A TYPICAL OFFICE ENVIRONMENT FOR NLOS CONDITIONS AND 50NS AVERAGE RMS DELAY SPREAD Tap Delay Average Rician Doppler Number (ns) Relaive K Specrum Power (db) Classic Classic Classic Classic Classic Classic Classic Classic Classic Classic Classic Classic Classic Classic Classic Classic Classic Classic

10 TABLE VII MODEL B. CORRESPONDS TO TYPICAL LARGE OPEN SPACE AND OFFICE ENVIRONMENTS FOR NLOS CONDITIONS AND 100NS AVERAGE RMS DELAY SPREAD Tap Delay Average Rician Doppler Number (ns) Relaive K Specrum Power (db) Classic Classic Classic Classic Classic Classic Classic Classic Classic Classic Classic Classic Classic Classic Classic Classic Classic Classic TABLE IX MODEL D. SAME AS MODEL C BUT FOR LOS CONDITIONS.A10DB SPIKE AT ZERO DELAY HAS BEEN ADDED RESULTING IN A RMS DELAY SPREAD OF ABOUT 140NS Tap Delay Average Rician Doppler Number (ns) Relaive K Specrum Power (db) Classic+spike Classic Classic Classic Classic Classic Classic Classic Classic Classic Classic Classic Classic Classic Classic Classic Classic Classic TABLE VIII MODEL C. CORRESPONDS TO A TYPICAL LARGE OPEN SPACE ENVIRONMENT FOR NLOS CONDITIONS AND 150NS AVERAGE RMS DELAY SPREAD Tap Delay Average Rician Doppler Number (ns) Relaive K Specrum Power (db) Classic Classic Classic Classic Classic Classic Classic Classic Classic Classic Classic Classic Classic Classic Classic Classic Classic Classic TABLE X MODEL E. CORRESPONDS TO A TYPICAL LARGE OPEN SPACE ENVIRONMENT FOR NLOS CONDITIONS AND 250NS AVERAGE RMS DELAY SPREAD Tap Delay Average Rician Doppler Number (ns) Relaive K Specrum Power (db) Classic Classic Classic Classic Classic Classic Classic Classic Classic Classic Classic Classic Classic Classic Classic Classic Classic Classic