DCF/DSDMA: Enhanced DCF with SDMA Downlink Transmissions for WLANs

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1 DCF/DSDMA: Enhanced DCF wih SDMA Downlink Transmissions for WLANs Ruizhi Liao, oris ellala, Crisina Cano and Miquel Oliver NeTS Research Group Deparmen of Informaion and Communicaion Technologies Universia Pompeu Fabra Roc orona 8, 88, arcelona, Spain {ruizhi.liao, boris.bellala, crisina.cano, Absrac Muli-user MIMO is a promising echnology o enhance he performance of WLANs. However, he MAC proocol of he IEEE 8. sandard (he DCF MAC proocol) is no able o solve he new coordinaion and synchronizaion issues ha are needed o enable muliple simulaneous ransmissions. This paper presens an enhanced MAC proocol for Downlink SDMA in WLANs where only he Access Poin is equipped wih muliple anennas. Compared o he radiional WLANs, resuls show ha he proposed proocol is able o exploi he benefi of muliple anennas a he Access Poin, providing significan performance gains in erms of hroughpu and delay as well as he mos suiable number of acive users ha a WLAN can suppor. Index Terms MU-MIMO, MAC, SDMA, downlink, WLANs I. INTRODUCTION In he las decade, IEEE 8. Wireless Local Area Neworks (WLANs) have been widely deployed due o heir meris, for example, low cos and simpliciy. However, he hroughpu of a WLAN is, someimes, less han half of he Physical layer (PHY) raw daa rae because of he collisions, iner-frame spaces and proocol overheads [][], which can no be miigaed by increasing he ransmission rae only. Muliple-Inpu Muliple-Oupu echnology (MIMO), in boh Single-User MIMO (SU-MIMO) and Muliple-User MIMO (MU-MIMO) configuraions, provides a new direcion o improve he performance of WLANs by exploiing he spaial dimension ha becomes available due o he presence of muliple anennas. SU-MIMO is able o provide a significan improvemen when mos of he raffic load is direced o only a few number of muliple-anenna Mobile (MNs) by using he muliple spaial sreams in poin-o-poin communicaions. The IEEE 8.n-9 [], which is a recen amendmen o he IEEE 8.-7 [], is able o improve he sysem hroughpu compared o he wo previous sandards (he 8.a and 8.g) by inroducing SU-MIMO. For insance, he use of four spaial sreams, ogeher wih a channel widh of MHz, achieves a significan improvemen in he maximum raw daa rae, which is increased from 5 Mbi/s o 6 Mbi/s. On he oher hand, MU-MIMO is able o enhance he sysem performance in scenarios wih a high number of acive MNs, and among where he raffic is uniformly disribued. Addiionally, MU-MIMO only requires mulipleanennas a he AP. In he downlink, he AP can ransmi muliple frames o differen MNs simulaneously, increasing he sysem hroughpu and reducing he AP boleneck effec; In he uplink, MNs can communicae wih he AP a he same ime o reduce collisions. In his paper, boh SU-MIMO and MU-MIMO are used o refer o he PHY layer echniques, which allow o ake benefis of he spaial dimension of he channel. In comparison, Space Division Muliple Access (SDMA) is used o refer o a channel access scheme ha uses he MIMO echniques o muliplex differen frames (daa sreams) over ha space dimension. There are wo challenges ha need o be ackled before SDMA is applied o WLANs: firsly, in he downlink, he AP needs he Channel Sae Informaion (CSI) feedback from MNs for he frame/desinaion selecion and precoding scheme, which can be solved by acquiring he CSI embedded in he Clear-o-Send () message or by direcly esimaing i a he AP from he frames received; secondly, in he uplink, MNs need o be coordinaed and synchronized, which are much more complicaed as MNs access he channel in a disribued and asynchronous way, hus requiring more signalling and message exchanges []. Addiionally, he raffic characerisics from Inerne, where he downlink raffic is usually much higher han he uplink, make less urgen o exploi he benefis of SDMA in he uplink. Therefore, in his paper, a SDMA-enhanced MAC proocol focusing only on he downlink ransmission is proposed. In he uplink, MNs will operae as he convenional Disribued Coordinaion Funcion (DCF) mechanism, alhough wih some modified parameers such as he EIFS and he Collision Timer. The res of he paper is organized as follows: Secion II gives a brief review of he relaed work, hen in Secion III he scenario and sysem parameers of he simulaion are inroduced. Afer ha, Secion IV presens he proposed SDMA MAC proocol while Secion V evaluaes he performance of he proposed approach. Finally, Secion VI concludes he paper. R. Liao,. ellala, C. Cano, M. Oliver; DCF/DSDMA: Enhanced DCF wih SDMA Downlink Transmissions for WLANs. CFIC, Riga, Lavia (hp://bcfic.org/)

2 II. RELATED WORK The MU-MIMO echnology has became an imporan research opic due o is poenials o improve he performance for he nex generaion wireless sysems wihou uilizing addiional frequency bandwidh or ransmiing power. As a resul, exensive research effors have been seen in differen areas such as signal processing [5], [6], coding/precoding schemes [7], [8], queueing heory [], [], and, he one in which we are ineresed, medium access conrol for WLANs [], [], [9], [], [], []. In downlink SDMA WLANs, he channel access, scheduling and CSI acquisiion are he hree mos imporan issues ha need o be addressed in he design of he MAC proocol. A DCF scheme wih he necessary modificaions for he MIMO suppor is presened in [5]. Reques-o-Send/Clear-o-Send (/) frames are modified o exchange he anenna selecion informaion. Similar works could be found in [], []. In [], a muliuser downlink ransmission scheme is presened wihou - exchanges, bu how o synchronize before he ransmission is no specified. In [], he auhors propose a dual-mode responding mechanism o achieve he channel esimaion and nodes synchronizaion in he uplink. Moreover, an analyical model is designed o adap he sysem parameers o he channel condiion and nework load. In [], he auhors discuss four MAC schemes for muliuser downlink ransmissions in IEEE 8. infrasrucure-based WLANs: he firs is he MU-asic scheme ha is used as a reference for ohers; he second is he MU-Deerminisic scheme ha ensures collision-free ransmissions; he hird is he MU-Probabilisic scheme ha divides he conenion window ino slos and nodes randomly choose a slo for he ransmission, which leads o he exisence of collisions; he fourh is he MU-Threshold Selecive scheme ha is similar o he hird one. The difference is ha a SNIR hreshold is inroduced o allow only hose nodes who have good channel condiions o ake par in he conenion phase. The auhors presen a MIMO-aware MAC scheme in [6] o schedule simulaneous ransmissions in a single collision domain, which is achieved by adjusing anenna weighs o lisen or ignore a paricular ransmission. An overview of he packe scheduling algorihms for single and muliple anenna wireless sysems is given in [7], which advocaes he need for a cross-layer consideraion in he design of scheduling algorihms. In [7] a wo-sage CSI feedback scheme is designed, which includes a firs sage dedicaed o he scheduling and a second one for he precoding o maximize he sum rae under a fixed feedback consrain. In [8], MNs obain he CSI via he explici downlink raining, and he uplink feedback is used o provide he base saion wih he CSI from he MNs. III. SCENARIO A single-hop IEEE 8. WLAN consising of one Access Poin (AP) and M MNs is considered. The AP has an array of N anennas, while each MN has a single anenna, which is a reasonable assumpion considering MNs power consumpion AP (N=) MN (N=) Fig.. The considered WLAN scenario wih N = anennas a he AP and single-anenna MNs Fig.. 5 q() Q Space bach SDMA Transmier The SDMA ransmission queue and he Scheduler and cos. A specific scenario for an AP wih N = anennas is shown in Figure. The channel is assumed frame-fla, wih he independen fading from frame o frame. Wihou loss of generaliy, i is assumed ha he insananeous channel marix is always ideally condiioned (i.e. he channel realizaions are orhogonal), and hence, he beamforming a he AP is able o compleely suppress he muli-user inerference wihou any power penaly. Moreover, a single ransmission rae is considered. I is assumed ha all MNs always can receive error-free frames a ha rae. These assumpions lead o N independen downlink channels, which could allow up o N simulaneous ransmissions from he AP o he MNs. In he uplink, as no enhancemen is considered in his paper, only one MN is allowed o access he channel each ime. Frames ha arrive o he AP follow he Poisson process wih a rae λ. The raffic load of he downlink is en imes higher han ha of he uplink, which is reasonable because mos of he downlink raffic is a response (i.e. an online video) o he requess (i.e. a hp reques) from MNs. Through using he spaial muliplexing provided by he MIMO echnology, he AP assembles hose frames queued a he MAC layer ino a Space-bach (a group of frames ransmied simulaneously) according o he desinaion address, as illusraed in Figure, where Q is he buffer size and q() he insananeous queue occupaion. The AP selecs sequenially all frames ha saisfy he condiion of being N

3 Parameers TALE I SYSTEM PARAMETERS Values AP andwidh per MN Kbps MN andwidh Kbps Daa Rae Mbps Phy/asic Rae Mbps Queue Lengh Frames Frame Lengh, 8 bis Preamble Lengh bis MAC Header/// Lengh 6 bis Slo Time µs µs 5 µs CWmin CWmax Rery Limi 5 AP Anennas,, direced o differen desinaions. Noe ha, he firs frame waiing in he queue is always seleced, and hen, up o N frames. Frames in ransmission are no dequeued unil hey are compleely ransmied. The daa rae is fixed a Mbps, while he physical and basic raes are fixed a Mbps. The relevan parameers considered are lised in Table I. IV. DCF/DSDMA: ENHANCED DCF WITH SDMA DOWNLINK TRANSMISSIONS IEEE 8. DCF [] relies on Carrier Sense Muliple Access wih Collision Avoidance (CSMA/CA) and he opional / mechanism o share he medium beween wireless nodes. The proposed proocol, which is called enhanced DCF wih SDMA Downlink ransmissions (DCF/DSDMA) exends he DCF mechanism, wih he / opion enabled, o suppor muliple simulaneous downlink ransmissions. The / mechanism is adoped for wo reasons. Firsly, he / mechanism allows he AP o esimae he CSI via he preamble of he replied s from he MNs. The channel informaion will be used for he precoding scheme a he AP o cancel he muliuser inerference. Secondly, he DCF/DSDMA needs o exend he sandard o he Muli-User (MU- ) frame, as illusraed in Figure, as he AP has o specify all receiving addresses of he nex Space-bach. For a MU- frame, he ype and sub-ype fields of he frame are (ha denoes a conrol frame) and (MU- idenifier, one of he non-assigned combinaions in he sandard []), hese values allow he receiver o idenify he recepion of a MU- and process i accordingly. The DCF/DSDMA conducs some minor changes compared o he convenional MAC proocol operaions, as illusraed in Figures (a), (b), 5(a) and 5(b). The descripion of he Noe ha if a single frame is included in he Space-bach, he MU- is equivalen o a sandard. byes byes n * 6 byes 6 byes byes Frame Conrol Duraion Fig.. n * Receiver Addresses Transmier Address MU- Frame Srucure Frame Check Sequence proocol is divided ino wo pars: successful ransmissions and recovering from collisions. Noe ha, iniially in all figures he channel is assumed busy (), and he random ack-off ime is denoed as O. A. DCF/DSDMA: Successful Transmissions Firs of all, any node, including he AP, which has frames o send operaes like he convenional IEEE 8. DCF wih he / mechanism: when he medium has been idle for a DCF Iner Frame Space () ime, afer a random backoff ime i sars o send an or an MU- based on he number of anenna and he frames in he queue, as previously depiced in Figure. If an is sen ou, he DCF/DSDMA operaes exacly he same as he convenional one, as shown in Figure (a). If a MU- is sen ou, as illusraed in Figure (b), he MNs lised in he receiving field will send a back sequenially (following he same order of heir addresses included in he MU-) o confirm ha hey are ready o receive a frame from he AP. Those MNs who are no included in he MU- will se heir Nework Allocaion Vecor (NAV) o defer heir ransmission aciviies. The NAV is equal o he duraion of he whole ransmission in order o reserve he medium. On receiving all he messages (if no all, he AP is going o send frames in parallel only o hose who successfully replied wih a ). Lasly, afer he ransmission of all frames is compleed and a Shor Iner Frame Space () ime, MNs sar o send back heir respecive Acknowledgemen () sequenially in he same order of he.. DCF/DSDMA: Recovering from collisions If wo (or more) nodes unluckily pick he same O from heir respecive Conenion Window (CW), which is a MU- and a or wo are sen ou a he same ime, a collision will happen, as illusraed in Figures 5(a) and 5(b), where he dashed lines denoe ha he frames should be here if he ransmission was successful. In sandard DCF, on sending he, he AP and MNs will se a imer equal o Equaion () o receive he expeced, where CT S x denoes he ransmission duraion of a. If he is no received before he Collision imer expires, he sending nodes assume ha collisions or errors have happened during he ransmission, and hey can sar o compee for he channel again (o rery he ransmission of he on-going frame if he maximum rery limi has no been reached, or o ransmi he nex frame in he queue, if any). For he non-ransmiing nodes, none of he can be received correcly, so afer he collision ime, which is he ransmission ime, he receiving nodes wai for an Exended Iner frame space (EIFS)

4 AP O O > AP O O (a) Successful ransmission of a MN AP O MU AP > AP > O O O (b) Successful ransmisssion of he AP Fig.. DCF/DSDMA Successful Transmissions ime, shown in Equaion, and hen hey can sar compeing for he channel again. CT S imer = SIF S + CT S x () longer ime when he collisions are beween MNs or when he collisions are beween he AP and MNs bu he AP is sending a Space-bach wih less han N frames. MU CT S imer = N (SIF S + CT S x ) () EIF S = SIF S + CT S x + DIF S () The above menioned procedure has o be modified due o a longer duraion of he MU- and he longer period for receiving he expeced s. Oherwise, some issues such as unfairness beween he AP and MNs (i.e. he MNs iniiae he channel access procedure before he AP) and hiddenerminal problems could appear. In order o preven hese issues, he imer of all nodes has o be se o he new MU CT S imer (as shown in Equaion ) and he EIFS value o he new MU EIFS (Equaion ) value. oh parameers have o consider he wors case o make sure ha all nodes sar compeing for he channel a he same ime. I is imporan o noe ha in all siuaions hese new values have o be considered for all nodes, even in he scenario when here are only collisions beween MNs, as illusraed in Figure 5(b). This is because colliding MNs can no know if hey are colliding wih he AP or anoher MN. Obviously, hese required changes inroduce some overheads such as waiing a MU EIF S = N (SIF S + CT S x ) + DIF S () C. Oher consideraions The AP will only send frames o hose MNs whose channel condiion mees some cerain qualiy and wih he compaible channel signaures. As illusraed in Figure 6, he channel beween he AP and Node does no reach he required qualiy, and herefore, he AP will send a frame o Node only. A noiceable hing is ha Node has o wai for an exra SIF S + ime before i can send is own even if i is he only receiver. This is because he ransmission duraion and he sequence of sending and are decided in he MU- and all nodes se heir NAV imer based on ha. V. PERFORMANCE EVALUATION The parameers considered o evaluae he proposed DCF/DSDMA are lised in Table I. The Componen Oriened

5 AP O MU O O O... MU EIFS (a) Collision beween he AP and a MN AP O O O O... MU EIFS (b) Collision beween wo MNs Fig. 5. DCF/DSDMA: Recovering from collisions AP O MU AP > O O G O Fig. 6. bad. Unfeasible ransmission due o he channel condiions beween he AP and he -h MN. The (-G) in he means good and he (-) means Simulaion Toolki (COST) [9] libraries have been used o implemen and simulae he considered sysem. The heoreical maximum hroughpu of he AP using N anennas is shown in Equaion 5, where MU RT S x (N) denoes he ransmission ime of he MU frame wih N receiving addresses. The subscrip x means he ransmission duraion of he conrol or daa frame. Noe ha he assumpions made for compuing he heoreical maximum hroughpu are: ) only he AP is ransmiing and ) he AP is sauraed. For insance, considering a frame lengh L equal o bis, he maximum achievable hroughpu following he Equaion 5 are S max () =.8 Mbis/s, S max () =. Mbis/s, and S max () = 5.66 Mbis/s, for he AP wih N =, N = and N = aennas respecively. Figures 7(a) and 7(b) show he hroughpu of he AP and MNs. As we can see from Figure 7(a), he hroughpu of he AP wih one anenna (AP-N= in he legend) increases proporionally wih he number of MNs unil i sauraes (around.5 Mbis/s). Given he considered downlink /uplink raffic disribuion, he AP becomes sauraed mainly due o is own raffic load, alhough collisions wih MNs can no be negleced, which is why he hroughpu is decreasing as he number of MNs goes higher afer he sauraion. In Figure 7(a), he considerable gain is observed in he AP

6 N L S max(n) = (DIF S + (Slo CW min/) + MU RT S x(n) + N (CT S x + x) + F RAME x + ( N + ) SIF S) (5) hroughpu, from.5 Mbis/s wih N = anenna o.8 Mbis/s wih anennas (AP-N= in he legend) and 5. Mbis/s wih anennas (AP-N= in he legend). A noiceable oucome is ha he gain is no linear wih he increase of he number of anennas, he reason of ha is wo-fold: ) he higher service ime for each frame due o he backoff inerrupions and he higher collisions as he sysem ges more MNs and ) he presence of a finie queue as shown in [] can no ake full benefis of employing more anennas a he AP. These reasons also jusify he difference beween he maximum hroughpu compued in Equaion 5 increasing wih N. In Figure 7(b), significan gains are achieved if a longer frame lengh (L = 8 bis) is used. For example, he maximum hroughpu of he AP wih N = anennas increases from.8 Mbis/s in Figure 7(a) o 6.9 Mbis/s in Figure 7(b) and he number of MNs ha he wireless sysem can suppor goes from o nodes. The main reason is ha each Space-bach now includes more bis as he frame lengh is longer, which makes he overhead of he conrol messages (i.e. MU RT S x (N)) less significan and he WLAN works more efficienly. Noe ha in he considered scenario he MNs never become sauraed as he aggregae hroughpu of MNs (Figures 7(a) and 7(b)) increases proporionally wih he number of nodes regardless he number of he anennas a he AP. In Figures 8(a) and 8(b) he delay of he AP is shown. Obviously he average delay decreases significanly by employing muliple anennas. A noiceable aspec here is ha he average delay increases significanly a a cerain region before he AP becomes sauraed. I is called he urmoil region in his paper. For example, he urmoil region for he AP wih one anenna in Figure 8(a) is from o 5 nodes and for he AP wih one anenna in Figure 8(b) is from 5 o 5 nodes, which could be useful for finding ou wha is he mos suiable number ha a wireless nework can suppor. Finally, i is ineresing o observe he average Space-bach size in Figures 9(a) and 9(b) increases as he number of MNs increases. Noe ha i is load-aware as he size of he Spacebach increases wih he number of MNs. VI. CONCLUSIONS In his work he DCF/DSDMA proocol has been presened. The resuls show ha considerable performance gains, in erms of a higher hroughpu and lower delays, are achieved by employing muliple anennas a he AP. However, he resuls also show ha he achieved gain is neiher linear wih he number of anennas nor he frame lengh due o he exra overheads of he DCF/DSDMA, he own behavior of he DCF and he presence of a finie queue lengh. Throughpu (bps) [L= Kbis] Throughpu (bps) [L=8 Kbis] 6 x 6 5 AP N= AP N= MN AP N= MN AP N= MN (a) Throughpu using a frame lengh of bis. 9 x AP N= AP N= MN AP N= MN AP N= MN (b) Throughpu using a frame lengh of 8 bis. Fig. 7. Throughpu agains he number of nodes wih differen ransmiing anennas N Nex seps will focus on he consideraion of SDMA ransmissions in he uplink, he use of a realisic channel model, including he impac of he beamforming a he AP for he downlink ransmissions, he pos-processing for he uplink ones, as well as he presence of channel errors and differen ransmission raes. Anoher imporan issue o be addressed is he coexisence beween legacy saions using he convenional DCF and hose using he DCF/DSDMA proocol. NOWLEDGMENTS This work has been parially suppored by he Spanish Governmen under projecs TEC8-655/TEC (GEPETO, Plan Nacional I+D), CSD8- (COMONSENS, Consolider-Ingenio Program) and by he Caalan Governmen (SGR9#67).

7 Delay (seconds) [L= Kbis] AP N= AP N= ach Size (frames) [L= Kbis] AP N= AP N= (a) Delay using a frame lengh of bis (a) Average Space-bach using a frame lengh of bis. Delay (seconds) [L=8 Kbis] AP N= AP N= ach Size (frames) [L=8 Kbis] AP N= AP N= (b) Delay using a frame lengh of 8 bis. Fig. 8. Average Delay agains he number of nodes wih differen ransmiing anennas N (b) Average Space-bach using a frame lengh of 8 bis. Fig. 9. Average Space-bach agains he number of nodes wih differen ransmiing anennas N REFERENCES [] L.X. Cai, H. Shan, W. Zhuang, X. Shen, J.W. Mark and Z. Wang. A Disribued Muli-User MIMO MAC Proocol for Wireless Local Area Neworks. IEEE GLOECOM, pp. -5, 8. [] S. Zhou and Z. Niu. Disribued medium access conrol wih SDMA suppor for WLANs. IEICE Transacions on Communicaions, pp ,. [] IEEE 8.n-9, Amendmen 5: Enhancemens for Higher Throughpu. Sandard, IEEE, 9. [] IEEE 8.-7, IEEE 8.: Wireless LAN Medium Access Conrol (MAC) and Physical Layer (PHY) Specificaions. Sandard, IEEE, 7. [5] L. Tong, V. Naware, and P. Venkiasubramaniam. Signal processing in random access. IEEE Signal Processing Magazine, pp. 9-9,. [6] Q.H. Spencer, C.. Peel, A.L. Swindlehurs and M. Haard. An Inroducion o he Muli-User MIMO Downlink. IEEE Communicaions Magazine, pp. 6-67,. [7] R. Zakhour and D. Gesber. A Two-Sage Approach o Feedback Design in Muli-User MIMO Channels wih Limied Channel Sae Informaion. IEEE 8h Inernaional Symposium on PIMRC, pp. -5, 7. [8] N. Jindal. MIMO roadcas Channels wih Finie Rae Feedback. IEEE Transacions on Informaion Theory, pp , 6. [9] P.X. Zheng, Y.J. Zhang and S.C. Liew. Mulipacke Recepion in Wireless Local Area Neworks. IEEE Inernaional Conference on Communicaions, pp , 6. [] J. Mirkovic, J. Zhao and D. Deneneer. A MAC Proocol wih Muli-User MIMO Suppor for Ad-Hoc WLANs. IEEE 8h Inernaional Symposium on PIMRC, pp. -5, 7. [] E. Karsakli, N. Zorba, L. Alonso and C. Verikoukis. Muliuser MAC Proocols for 8.n Wireless Neworks. IEEE Inernaional Conference on Communicaions, pp. -5, 9. [] Y.J. Choi, N.H. Lee and S. ahk. Exploiing Muliuser MIMO in he IEEE 8.Wireless LAN Sysems. IEEE Inernaional Conference on Communicaions, pp , 9. []. ellala and M. Oliver. A space-ime bach-service queueing model for muli-user MIMO communicaion sysems. h ACM Inernaional Conference on Modeling, Analysis and Simulaion of Wireless and Mobile Sysems, pp. 57-6, 9. [] R. Liao,. ellala, M. Oliver and N. Garcia. A Queueing Model for SDMA Downlink Transmissions. Muliple Access Communicaions (MACOM), pp. 7-78,. [5] J. Mirkovic, G. Orfanos, H.J. Reumerman and D. Deneneer. A MAC Proocol for MIMO ased IEEE 8. Wireless Local Area Neworks. IEEE Conference on Wireless Communicaions and Neworking, pp. -6, 7. [6] D.J. Dechene, K.A. Meerja, A. Shami and S. Primak. A Novel MIMO- Aware Disribued Media Access Conrol Scheme for IEEE 8. Wireless Local Area Neworks. IEEE Conference on Local Compuer Neworks, pp. 5-, 7. [7] C. Anon-Haro, P. Svedman, M. engsson, A. Alexiou, and A. Gameiro. Cross-Layer Scheduling for Muli-User MIMO Sysems. IEEE Communicaions Magazine, pp. 9-5, 6. [8] M. Kobayashi, G. Caire and N. Jindal. How Much Training and Feedback are needed in MIMO roadcas Channels?. IEEE Communicaions Magazine, pp , 8. [9] G. Chen. Componen-oriened simulaion oolki. hp://