Performance characterization of CSMA/CA adapted multi-user MIMO aware MAC in WLANs

Transcription

1 Thp et l. EURASIP Journl on Wireless Communitions n Networking, :4 RESEARCH Open Aess Performne hrteriztion of CSMA/CA pte multi-user MIMO wre MAC in WLANs Anup Thp, Suoh Pusini n Seokjoo Shin * Astrt To relize the multi-user multiple input multiple output (MIMO) vntge over WLANs, it requires signifint hnges in the MAC protool. Either the ominnt MAC protool rrier sense multiple ess/ollision voine (CSMA/CA) nees to e reple y novel multi-user MIMO wre MAC protool or it shoul e upgre into multi-user MIMO wre CSMA/CA. Nevertheless, the simplest pproh woul e upgring the CSMA/CA. Simple moifitions in the ontrol pkets formt n/or the hnnel ess mehnism n upgre CSMA/CA into simple, yet prtile, multi-user MIMO wre MAC protool. By utilizing onvenient hnges, severl moifition pprohes n e provisione for this purpose. Hene, it is importnt to unerstn their performne enefits n tre-offs. In this rtile, we isuss some of suh moifition pprohes tht est represent the possile moifitions. We provie their etil performne nlysis se on nlytil moeling n erive expressions in terms of throughput n ely. We lso erive expressions for hievle performne n present their performne limits too. Keywors: MIMO wre MAC, multi-user MIMO wre CSMA/CA, multi-user sptil multiplexing, WLAN. Introution Multiple input multiple output (MIMO) is rio ommunition tehnology tht uses multiple ntenn elements t oth the trnsmitting n the reeiving ens either to oost up hnnel pity or to ttin trnsmission reliility. Wireless networks eploye with the MIMO system n utilize these fetures y employing sptil multiplexing n/or sptil iversity [,]. Sptil multiplexing is MIMO trnsmission tehnique tht trnsmits multiple inepenent t strems onurrently from multiple ntenn elements so tht eh ntenn element n e logilly trete s seprte hnnel. Wheres, sptil iversity is MIMO trnsmission tehnique tht trnsmits the sme t strem from multiple ntenn elements so tht they oul e proesse for orretly eoing the esire informtion. Reently, the MIMO system hs gine inrese interest. Most of the existing wireless networks re pying onsierle ttention towr MIMO implementtion. They re expeting to meet their ever inresing * Corresponene: sjshin@hosun..kr Deprtment of Computer Engineering, Chosun University, Gwngju, Repuli of Kore pity emn (mostly from higher t rte servies like vieo teleonferening, multimei streming, et.) y exploiting MIMO offere spetrl effiieny t the physil lyer (PHY) [,4]. However, from network point of view, only n inrese pity in one speifi lyer is not suffiient to improve n overll network performne. Moreover, eh lyer must e wre of the hnges tht hve ourre in the onjugte lyers n their pplie protools must e smrt enough to relize the resulting effets positively [5]. Hene, even though the MIMO implementtion n inrese the PHY pity, suh inepenently enhne pity nnot e trnslte esily into MAC lyer pity gin unless n pplie MAC protool is lso MIMO wre. Simply, MIMO wre MAC protool n e viewe s protool tht possesses the pility to pply some speil mesures t the MAC lyer, sujet to mximizing the use of MIMO pity t the PHY. Suh mesures re ruil to ress importnt MAC lyer s issues like MIMO funtionlities informtion exhnge, sheuling of the MIMO enhne nwith, time synhroniztion, n the error free ontrol pkets trnsmission. In ition, it is lso eqully importnt to ensure kwr omptiility when pplying suh Thp et l; liensee Springer. This is n Open Aess rtile istriute uner the terms of the Cretive Commons Attriution Liense ( whih permits unrestrite use, istriution, n reproution in ny meium, provie the originl work is properly ite.

2 Thp et l. EURASIP Journl on Wireless Communitions n Networking, :4 Pge of mesures to filitte oexistene of legy evies with only single input single output pility. Applying suh mesures is reltively esier in networks with entrlize ontrol rhiteture like ellulr networks where highly sophistite entrlize ministrtion unit n govern the meium ess proeure n tke ontrol over resoure llotion n utiliztion [6]. However, pplying suh mesures is more hllenging in se of istriute wireless networks like WLANs [7], where meium ess is ontrolle y n synhronous rnom ess mehnism known s rrier sense multiple ess/ollision voine (CSMA/CA). Relizing the vntges of the MIMO system over existing WLANs requires signifint hnges in its MAC protool. Either its ominnt MAC protool CSMA/CA nees to e reple y novel MIMO wre MAC protool or it shoul e upgre into MIMO wre CSMA/CA. Nevertheless, the simplest pproh woul e upgring the wiely eploye MAC protool. An ppropritely moifie ontrol pkets exhnge provisione with n equtely rrie out hnnel ess mehnism se on CSMA/CA request to sen/ler to sen (RTS/CTS) ess sheme n upgre it into simple yet prtile MIMO wre MAC protool. Some of the prior reserhes [8-] vise suh moifitions n emonstrte enhne performne too. With proper moifition hnling, oth the single user sptil multiplexing se MIMO (SU-MIMO) n the multiuser sptil multiplexing se MIMO (MU- MIMO) trnsmissions n e supporte with MIMO wre CSMA/CA. Here, SU-MIMO refers to point-topoint MIMO ommunition where trnsmitter trnsmits multiple inepenent t strems estine for single reeiver. Wheres, MU-MIMO refers to point-tomultipoint ommunition where trnsmitter trnsmits multiple inepenent t strems eh estine for ifferent reeiver. As SU-MIMO is point-to-point ommunition, in generl, it n e oneive tht SU-MIMO wre CSMA/CA follows the sme hnnel ess mehnism s tht of legy CSMA/CA with exhnge of slightly moifie ontrol pkets only. Thus, it n e envisione tht throughput inreses pproximtely in the sme fol oring to the numer of ntenn elements in use; leving the ely onstnt. But the sme oes not pply for MU-MIMO. As MU-MIMO is point-tomultipoint ommunition, it nees to exhnge higher numer of the extene ontrol pkets uring negotition with multiple reeivers. If ontrol pkets re trnsmitte serilly, one fter one, to voi risk of ontrol pkets orruption n to sve ost n omplexity from signl proessing in MU-MIMO, it les to hevy overhe in time n ultimtely ereses the network performne. If the ontrol pkets re trnsmitte simultneously to erese overhe s effet, it les to higher ost n omplexity in signl proessing n my lso inrese the risk of ontrol pkets orruption. Hene, MU- MIMO fils to give similr performne to tht of SU- MIMO while mintining the sme level of network ost n omplexity. Nevertheless, noteworthy point is tht though SU- MIMO seems to e esirle, it is not lwys pplile. Owing to vrious network hrteristis like vrile hnnel lo, onstrint of kwr omptiility, n ely sensitivity, SU-MIMO nnot lwys leverge linerly enhne performne [8-]. For exmple, unless ll the queues of orresponing ntenn elements hve enough pkets to sen, its not worth pplying SU- MIMO. On the other hn, SU-MIMO implementtion is worthwhile only when ntenn elements re evenly istriute in trnsmitter n reeiver. Similrly, PHY hrteristis like hnnel rnk loss n ntenn orreltion effets lso ply n verse role in SU-MIMO performne []. Hene, in mny ses, SU-MIMO n prevent from fully utilizing the ville MIMO pity. In suh senrios, MU-MIMO woul e preferle. However, lthough its high prtil importne hs een shown oth theoretilly n prtilly [-4], MU-MIMO hs not een stnrize yet in WLANs stnr. While SU-MIMO hs lrey een stnrize in IEEE 8.n [5]. IEEE 8.n hs lso provisione moifie CSMA/ CA s its MIMO wre MAC protool. A ontrol frme lle ontrol wrpper frme hs een efine for this purpose suh tht the ontrol pkets re wrppe within the ontrol wrpper frme n then exhnge etween the trnsmitter n the reeiver [6]. On the other hn, s few of the unresolve mtters relte to MAC lyer issues re still uner onsiertion, MU- MIMO is yet to e stnrize. For instne, issues relte to hnnel ess proeure, sheuling mehnism, hnnel stte feek tehniques, et., re still uner ontempltion. Even so, euse of its superiority in vrious network onitions, MU-MIMO n e expete to eome one of the si essentils of the future wireless networks n their stnrs. For exmple, IEEE 8. Tsk Group is now working to exten IEEE 8.n like pilities in the 5 GHz spetrum with wier hnnels, etter moultion shemes, n MU-MIMO inlusion [7,8]. As mentione erlier, moifition in CSMA/CA is simplest pproh towr MU-MIMO wre MAC protool. The moifition in CSMA/CA is require to omplish hnnel stte informtion (CSI) of ll the intene reeivers t the trnsmitter suh tht trnsmitter n know out the interferene sitution of its

3 Thp et l. EURASIP Journl on Wireless Communitions n Networking, :4 Pge of reeivers n pply the interferene limite preoing, lso known s interferene limite t preproessing, prior to the t trnsmission in suh wy tht ousers interferene n e mitigte t the reeiver [9-]. Bsilly, CSI n e omplishe from three ifferent wys: perfet feek with full hnnel informtion, prtil feek with limite hnnel informtion, n fully lin feek with no hnnel informtion. Oviously, se on these mehnisms, severl moifition shemes in CSMA/CA n e provisione to support MU-MIMO. Hene, it is importnt to unerstn their performne enefits n tre-offs. Similrly, s CSMA/CA is often ritiize for its oune performne (ourrene of throughput limit n ely limit euse of the effets of inispensle overhe ssoite with its funmentl opertion) [], unerstning their hievle performne, i.e. performne tht n e hieve on the est se senrio, n their performne limits re lso importnt. Therefore, the performne hrteriztion (stuy, nlysis, n omprison) of the moifition pprohes fter employing ove mentione feek mehnisms is the mtter of interest in this rtile. In this rtile, we investigte three si types of moifition pprohes tht est represent the possile moifitions, nme s: () CSI feek from serilly trnsmitte CTS pkets, () CSI preition from serilly trnsmitte CTS pkets, n () CSI preition from simultneously trnsmitte CTS pkets (etil in Setion ). Along with the isussion on these pprohes, we provie their etile performne nlysis, se on the nlytil moeling n erive expressions, in terms of throughput n ely. Similrly, we lso erive expressions for hievle performne n therey present their performne limits too.. Relte works MIMO wre CSMA/CA is simple pproh towr MIMO ptility in WLANs. As mentione erlier, there hs een some prior reserh [8-,] etiling some moifitions in the CSMA/CA to mke it MIMO wre CSMA/CA. Even though they hve signifintly ifferent moifition pprohes, ontrol pkets formts, n hnnel ess mehnisms n lthough they hve een propose s new MIMO wre MAC protools, it will not e n unersttement to mention thtsillytheyrelyonthecsma/casemac uner RTS/CTS ess mehnism. In [8], istriute MU-MIMO MAC protool using lekge se preoing sheme from [4] hs een propose. It hs use moifie RTS n CTS ontrol pkets exhnge with n oringly moifie hnnel ess mehnism to hve negotition out the ntenn weights etween trnsmitter n reeivers. Along with simultion results, they [8] presente n nlytil moel to stuy the performne of the propose MAC protool. Performnes were nlyze in terms of mximum numer of users tht n e supporte in the stle network n the orresponing network throughput, onsiering symmetril trnsmission rtes of uplink n ownlink, in terms of trffi intensity n trffi rrivl rte, respetively [8]. However, in [8], ely nlysis hs not een overe. In [9], MIMO-DCF MAC, using moifie ontrol pkets n hnnel ess mehnism to exhnge the ntenn seletion informtion for oth the SU-MIMO n the MU-MIMO in A-Ho WLANs, hs een propose. In generl, [9] is se on the ntenn numer seletion y the reeiver fter reeiving the propose ntenn it mp in n extene RTS pket from trnsmitter. The rtile presente the simultion results in terms of rrie lo versus offere lo n pket loss rtio onsiering hot-spot senrio with ownlink onnetions from ess point (AP) to few numers of rnomly lote noes. Similrly in [], MU-MIMO MAC terme s multiple RTS hnshke MAC (MRH-MAC) with moifie hnnel ess mehnism hs een presente. In [], sme tive pir of noes hnshke multiple times with exhnge of RTS-CTS pkets in orer to hoose the most suitle trnsmitting ntenns for t trnsmission. In [] lso, threshol-seletive multiuser ownlink MAC hs een presente. In this sheme, signl-to-noise rtio (SNIR) threshol is efine y the AP n is onsiere known to the users. The trnsmission sequene is ivie into ontention phse, t phse, n ACK phse. When RTS frme is trnsmitte, multiple users n prtiipte in the ontention phse if their mximum SNIR exees the preefine threshol. Depening upon the outome of the ontention phse inepenent t strems re trnsmitte to the suessful users. IEEE 8. is lso in the proess of olleting speifi proposls n its rtifition for MU-MIMO inlusion. In prtiulr, the reently ville menment [8] hs propose some moifitions on physil lyer onvergene protool (PLCP) heer n ontrol pkets formt. PLCP heer will inite the moe of trnsmission (SU-MIMO or MU-MIMO) while ontrol pket will inite the group of reeivers selete for MU-MIMO trnsmission y ssigning ommon group ientity. As mjor moifition is require t the MAC lyer to smooth operting rules in wien hnnels uring vrile network onition, IEEE 8. is on the proess to moify the ontrol pkets formt on suh wy tht it oul inite trffi types, pket length, supporte nwith, n ping sequenes. The very high throughput (VHT) ontrol fiel will e present in

4 Thp et l. EURASIP Journl on Wireless Communitions n Networking, :4 Pge 4 of ontrol wrpper frme n expliit souning n ompresse mtrix feek will e use.. MIMO wre CSMA/CA for MU-MIMO In CSMA/CA, noe with pket to sen first monitors the hnnel tivity. If the hnnel is foun to e ile for n intervl tht exees the istriute inter frme spe (DIFS), the noe ontinues its trnsmission. Otherwise, the noe wits until the hnnel eomes ile for the DIFS perio n then omputes rnom koff time for whih it will efer its trnsmission. The efer time is prout of the selete koff vlue n slot urtion. After the meium eomes ile for DIFS perio, noes erement their koff timer until the hnnel eomes usy gin or the timer rehes zero. If the timer hs not rehe zero n the meium eomes usy, the noe freezes its timer. When the timer is finlly eremente to zero, the noe trnsmits its pket. If two or more noes erement to zero t the sme time, ollision ours. In CSMA/CA RTS/CTS ess mehnism, when noe monitors the hnnel tivity n fins it ile for more thn the DIFS, noe sens speil reservtion pket lle RTS, n the intene reeiving noe will respon with CTS fter short inter frme spe (). Other noes whih overher RTS n CTS upte their network llotion vetor (NAV) oringly. The trnsmitting noe is llowe to trnsmit its pket only if the CTS pket is reeive orretly. MIMO wre CSMA/CA is n extene version of the RTS/CTS mehnism. Although the min purpose of the RTS/CTS mehnism is to reserve hnnel for urtion of pket trnsmission with exhnge of hnnel reservtion prmeters, it n lso serve to exhnge informtion relte to MIMO funtionlities fter pplying frme extension. The extene version of the ontrol pkets ppen new fiel or heer eite for mnging the MIMO funtionlities while keeping the rest of the fiels unhnge. In MU-MIMO, trnsmitting noe trnsmits X inepenent prllel t strems from X trnsmit ntenn elements to K noes (X K), K X y pplying interferene limite preoing. Hene, in MU-MIMO wre CSMA/CA, when the trnsmitting noe hs pkets to sen it first quires the hnnel using the CSMA/CA stnr rule. After quiring the hnnel, it trnsmits n extene RTS (M-RTS) pket, s shown in Figure, expliitly inluing the informtion out K reeivers resses, serilly. All other fiels ontin the regulr informtion s they o in legy RTS pket [7]. After time intervl, long with other regulr informtion, reeiving noes whih re rey to reeive reply with iniviul extene CTS (M-CTS) pket ontining informtion tht oul e proesse to hieve CSI. The M-CTS n extene knowlegement (M-ACK) pket exhnge mehnisms n the frme formts re ifferent for ifferent moifition pprohes. For our investigte pprohes, it is isusse in etil elow... CSI feek from serilly trnsmitte CTS pkets (CSIF-STCP) In this moifition pproh, RTS/CTS hnshke n e moifie to llow their reeiver to feek CSI orresponing to reeive signl using M-CTS pket, s shown in Figure. All the reeivers estimte their hnnel from reeive M-RTS pket n, long with other regulr informtion, feek tht vlue to trnsmitter y sening iniviul M-CTS pket fter eh time intervl, s shown in Figure for ( MU- MIMO), oring to their seril orer ssigne in M- RTS pket. Bse on the informtion reeive from M- CTS pkets, the trnsmitting noe selets the est ntenn element orresponing to eh reeiver noe n then pplies pproprite preoing. Similrly fter eh time intervl, reeiver noes suessfully reeiving the t strem knowlege the reeption vi M-ACK, serilly. Therefore, this metho n e onsiere s the perfet CSI feek metho. This is the simplest n the most effetive metho espite the introue overhe resulting from trnsmission of multiple extene M-CTS n M-ACK pkets serilly. This mehnism, however, reues the ost n omplexity in signl proessing n lso minimizes the risk of ontrol pkets orruption... CSI preition from serilly trnsmitte CTS pkets (CSIP-STCP) In this moifition pproh, ifferent from the CSIF- STCP mehnism, the M-CTS pket oes not expliitly ontin the CSI, inste reeivers n sen M-CTS pket in the sme orer s in CSIF-STCP, i.e. serilly fter eh time intervl, ut with preefine pilot symols inlue in the PHY heer. From the enlose pilot symol, with pproprite signl proessing, the trnsmitter noe n preit the CSI orresponing to the respetive reeiver noe se on reiproity priniple, i.e. in the ssumption of sme hnnel hrteristis in uplink n ownlink in ontiguous trnsmission with TDMA. This metho n e onsiere s semi lin hnnel stte estimtion metho s limite informtion is provie y preefine pilot symols. After preiting CSIs, the trnsmitter n pply pproprite preoing n then sens the t strems. M-ACK pkets re lso trnsmitte in the sme wy s in CSIF-STCP, i.e. serilly. Hene, s whole, this mehnism reues the overhe tht results from feek its in spite of moerte rise in the preition uren. Even so, sine M-CTS pkets re trnsmitte serilly, there

5 Thp et l. EURASIP Journl on Wireless Communitions n Networking, :4 Pge 5 of Frme Control Durtion K*Reeiver Aress Trnsmitter Aress Frme Chek ytes ytes K x 6 ytes 6 ytes 4 ytes Figure M-RTS ontrol pket formt. M-RTS Frme is less hne of pkets eing orrupte n in most of the ses preition ws foun to work quite well... CSI preition from simultneously trnsmitte CTS pkets (CSIP-SmTCP) In this moifition pproh, ifferent from CSIF-STCP n CSIP-STCP, M-CTS pkets re not trnsmitte serilly. Inste, they re trnsmitte simultneously fter time intervl y ll the reeiver noes inluing the preefine pilot symol in the PHY heer s in CSIP- STCP. This metho n e onsiere s full lin hnnel stte estimtion metho espite the inlusion of the preefine pilot symol. As the reeiver noes trnsmit in sme time n frequeny omin, eoing the informtion ompletely omes s lin. Nevertheless, employing ville ntenn elements n the pproprite signl proessing, the trnsmitter noe n preit the CSI of ll the reeiver noes n n pply pproprite preoing. The M-ACK pkets re lso trnsmitte in the sme wy. The M-CTS frme formt n the ess mehnism for this pproh hve een shown in Figures 4 n 5, respetively. This mehnism reues the overhe tht oul result from trnsmission of feek its s in CSIF-STCP n overhe tht oul result from serilly trnsmitte M-CTS pkets s in CSIF-STCP n CSIP-STCP. However, this mehnism s higher ost n omplexity in signl proessing n my lso rise the risk of ontrol pkets orruption. 4. Numeril nlysis 4.. Mthemtil nlysis for hievle performne Ahievle mximum performne of system is the performne tht the system n eliver in the est se senrio. In orer to emulte the est se in wireless network, we ie y the following ssumptions: there is only one tive trnsmitting noe whih lwys hs pkets to sen, n the hnnel is error free. Consiering the forementione ssumptions, we nlyze the hievle mximum performne of our investigte pprohes in terms of throughput n ely. Herefter, we represent CSIF - STCP, CSIP - STCP, n CSIP - SmTCP s M,M, n M, respetively Ahievle mximum throughput Throughput n e efine s the rte of suessful trnsmission of the t pkets in the hnnel. Thus, mximum hievle throughput, S mx,forthemu- MIMO n e expresse s K S mx j= = E[P], () T s where E[P] isthepylosizeinitsnt s is the time for suessfully trnsmitting those its. T s for ll the three moifitions pprohes, T s,m, T s,m,n T s,m, re ifferent from eh other euse of the ifferenes in M-RTS, M-CTS, n M-ACK pket formts n/or exhnge mehnisms. However, it is importnt to note tht mthemtil expressions for M n M remin sme s the hnges only our in frme formts ut not in the exhnge mehnisms. T s,m /M =W σ + T DIFS + T M RTS +KT + KT M CTS + T HDR + T E[P] + KT M ACK, () Frme Control Durtion Reeiver Aress CSI Frme Chek ytes ytes 6 ytes X x K ytes 4 ytes Figure M-CTS ontrol pket formt for CSIF-STCP. M-CTS Frme I

6 Thp et l. EURASIP Journl on Wireless Communitions n Networking, :4 Pge 6 of DIFS Sener Reeiver Reeiver M RTS M CTS M CTS DATA DATA M ACK M ACK B k o f f Other STAs NAV NAV Time Figure Chnnel ess mehnism in CSIF-STCP n CSIP-STCP for MU-MIMO. T s,m =W σ + T DIFS + T M RTS +T + T M CTS + T HDR + T E[P] + T M ACK, where W is the verge koff vlue, s is the slot time, n T ( ) inites the totl time require for sening respetive pket. The heer, HDR, onsists of oth the physil n the MAC heers. By repling T s in () with T s,m, T s,m,nt s,m, the mximum hievle throughput for ll the three moifition pprohes, SM mx, SM mx, n S mx M, n e otine Ahievle minimum ely Aess ely n e efine s the time intervl from the moment noe is rey to ess the meium to the moment the trnsmission is suessfully finishe. Thus, the hievle minimum ely for the investigte pprohes, D min M, D min M,nD min M, n e expresse s (4) n (5). Note tht mthemtil expressions for M n M remin sme here s well. () D min M /M =W σ + T DIFS + T M-RTS + KT + KT M-CTS + T HDR + T E[P], (4) D min M =W σ + T DIFS + T M-RTS +T + T M-CTS + T HDR + T E[P]. 4.. Mthemtil nlysis for verge performne The rrie numeril nlysis follows moulr pproh. First, we nlyze the ehvior of single tggenoeyformultingsingleimensionlmrkov moel s in [5]. With the i of the formulte moel, the proility τ tht the noe strts to trnsmit in rnomly hosen slot time is lulte. Seon, we express the verge throughput n verge pket ely s funtion of τ. The ssumptions me for the nlysis re s follows: () the numer of noes in the (5) Frme Control Durtion Reeiver Aress Frme Chek ytes ytes 6 ytes 4 ytes M-CTS Frme II Figure 4 M-CTS ontrol pket formt for CSIP-STCP n CSIP-SmTCP.

7 Thp et l. EURASIP Journl on Wireless Communitions n Networking, :4 Pge 7 of Sener K Reeivers Other STAs DIFS M-RTS M-CTSs DATA DATA DATA K NAV NAV M-ACKs B k o f f Time Figure 5 Chnnel ess mehnism for CSIP-SmTCP. network is finite (sy n), () the noes lwys hve pkets to trnsmit, n () the hnnel is iel. For simpliity n for mintining esy reility of this rtile, we use the sme nottions s presente in [5] wherever pplile. The proility tht noe trnsmits in rnomly hosen slot while employing efult ontention resolution lgorithm, inry exponentil koff (BEB), n e erive s in [5] n n e expresse s τ = + p p R+, R p i E[ i ] i= where p is the ollision proility of the trnsmitte pket, n E[ i ] is the verge koff time in ontention stge i, i R. R is the mximum llowe retrnsmission stge. E[ i ]forstgei is W i,wherew i is the mximum ontention winow size in ontention stge i. In the sttionry stte, noe trnsmits pket with proility τ. Hene, the ollision proility, p, i.e. proility of trnsmission of other noes t sme ritrry time slot, n e expresse s (6) p = ( τ) n. (7) Equtions 6 n 7 represent nonliner systems with two unknowns, τ n p, whih n e solve using numeril methos to get unique solution. When τ n p re otine, performne metris like throughput n ely n e erive onsiering other system prmeters Averge throughput Throughput is one of the most importnt initors to evlute network performne. Throughput n e efine s the rte of suessful trnsmission of the t pkets over the hnnel. Thus, throughput for MU- MIMO, S, n e relte s K P s P tr E[P] j= S =, ( P tr )T i + P s P tr T s + ( P s )P tr T s where P tr is the proility tht there is t lest one trnsmitting noe tive in the onsiere slot time, n P s is the proility tht the trnsmission is suessful. P tr n P s n e otine esily when τ n p re known. T s n T rethevergetimethehnnelis sense to e usy euse of suessful trnsmission or ollision, respetively, while T i is the urtion of n empty slot time. T s n T for our investigte pprohes n e erive s follows: T s,m /M =T DIFS + T M-RTS +KT + KT M-CTS + T HDR + T E[P] + KT M-ACK, T s,m =T DIFS + T M-RTS +T + T M-CTS + T HDR + T E[P] + T M-ACK, (8) (9) () T,M /M /M = T DIFS + T M-RTS. ()

8 Thp et l. EURASIP Journl on Wireless Communitions n Networking, :4 Pge 8 of 4... Averge ely Pket ely is efine to e the time intervl from the time pket is t the he of its MAC queue rey to e trnsmitte until the ACK for tht pket is reeive. Averge pket ely, D, n e erive y following the moel in [5], n for the MU-MIMO it n e expresse s n D = S/E[P] E[slot]( B ) p R+ p R+ R i= ( + E[() i ]), where S is the throughput with single ntenn element while E[slot] =( P tr )T i + P s P tr T s + ( P s )P tr T s. Here, T s is the verge of the suessful trnsmission times with respetive ntenn elements n B = W. 5. Performne evlution We evlute the performne numerilly se on the ove presente mthemtil expressions tking into onsiertion ll the prmeters presente in Tle. The selete prmeters hve een opte in suh wy tht they oul insure the interoperility etween MIMO pte n MIMO less WLANs. The MAC heer n PHY heer prmeters re opte from IEEE 8.n mixe moe trnsmission [5]. Slight moifition in heers hs een pplie to omplish mximum 4 numers of MU-MIMO reeivers []. Rests of the prmeters re opte from IEEE 8.g. Extene RTS n CTS frmes re use s esrie erlier. Ahievle mximum throughput n hievle minimum ely with respet to E[P] for ifferent X K onfigurtion n for ifferent hnnel t rte (DR) re presente in Figures 6,, n 6 n 7,, n 7, respetively, for M,M, n M. It is importnt to note tht the hievle throughput inreses with the numer of ntenn elements n DR, n the hievle minimum ely ereses with n inrese in DR ut inreses with ntenn elements. However, it is evient tht from PHY point of view hievle throughput Tle System prmeters Prmeters Vlue MAC heer 7 its PHY heer 4 μs ACK pket its DIFS 5 μs μs Slot time μs Bsi t rte 6 Mps Aville ntenn,, 4 Minimum ontention winow (W) 6 Mximum retry limit (R) 6 Ahievle Mximum Throughput (Mps) Ahievle Mximum Throughput (Mps) Ahievle Mximum Throughput (Mps) TUL, 4x4 MU-MIMO 4x4 MU-MIMO x MU-MIMO IEEE 8. e Mps DR 54 Mps DR 44 Mps DR 6 Mps DR Mps DR Pylo Size (Bytes ) () M (CSIF-STCP) TUL, 4x4 MU-MIMO 4x4 MU-MIMO x MU-MIMO IEEE 8. e Mps DR 54 Mps DR 44 Mps DR 6 Mps DR Mps DR shoul inrese with n inrese in ntenn elements n DR, s the hnnel pity inreses with them. Similrly, the hievle minimum ely shoul erese with n inrese in DR n shoul show no e Pylo Size (Bytes ) () M (CSIP-STCP) TUL, 4x4 MU-MIMO 4x4 MU-MIMO x MU-MIMO IEEE 8. Mps DR 54 Mps DR 44 Mps DR 6 Mps DR e Mps DR e Pylo Size (Bytes ) () M (CSIP-SmTCP) e Figure 6 Ahievle mximum throughput of CSMA/CA pte MU-MIMO wre MAC protools for WLANs. () M (CSIF-STCP), () M (CSIP-STCP), () M (CSIP-SmTCP).

9 Thp et l. EURASIP Journl on Wireless Communitions n Networking, :4 Pge 9 of Ahievle Minimum Dely (S) x DLL, 4x4 MU-MIMO 4x4 MU-MIMO.5 x MU-MIMO IEEE 8. Mps DR 54 Mps DR 44 Mps DR.5 6 Mps DR e Mps DR Ahievle Minimum Dely (S).5 Ahievle Minimum Dely (S) Pylo Size (Bytes) () M (CSIF-STCP) x DLL, 4x4 MU-MIMO 4x4 MU-MIMO.5 x MU-MIMO IEEE 8. Mps DR 54 Mps DR 44 Mps DR 6 Mps DR.5 e Mps DR Pylo Size (Bytes ) () M (CSIP-STCP) x DLL, 4x4 MU-MIMO 4x4 MU-MIMO.5 x MU-MIMO IEEE 8. Mps DR 54 Mps DR 44 Mps DR 6 Mps DR.5 e Mps DR inition of hnges on ntenn elements vrition, s simultneous trnsmission with MIMO mens onurrent trnsmissions on sme time n frequeny omin. In these results, S mx M < S mx M e e Pylo Size (Bytes ) () M (CSIP-SmTCP) e Figure 7 Ahievle minimum ely of CSMA/CA pte MU- MIMO wre MAC protools for WLANs. () M (CSIF-STCP), () M (CSIP-STCP), () M (CSIP-SmTCP). < S mx M n D min M > D min M > D min M. These results show the effets of overhes ssoite with eh of the moifition pprohes. As mentione erlier, in orer to solve the importnt MAC lyer issues like MIMO funtionlities informtion exhnge n error free ontrol pkets reeption, MAC protool nees to exhnge ifferent extene ontrol pkets with ost of itionl overhe. Similrly, when the numer of ntenn elements inreses more ontrol pkets exhnge is require to ssoite eh of the elements gin in ost of itionl overhe. The results revel tht in the investigte pprohes M hs higher overhe ompre to M n M. Similrly, M hs higher overhe ompre to M. However, the resulting effets oserve here re not only from the overhe ssoite with extene ontrol pkets ut lso from si CSMA/ CA opertion n its requirement of ontrol pkets exhnge in lower trnsmission rte. Aprt from this, the results lso show tht the throughput oes not inrese linerly in M n M while in M it inreses more or less linerly with ntenn elements ut not with DR. Note tht in ll these ses there is no liner throughput-ely gin with respet to DR. Even for the infinite DR, the throughput ouns to throughput upper limit n ely ouns to ely lower limit. It n lso e oserve tht for M, in spite of our ssumption of no itionl overhe uring the moifition, the performne goes towr ouning euse of overhe relte to si CSMA/CA opertion n its requirement of ontrol pkets trnsmission in lower trnsmission rte. Averge throughput with respet to n for ifferent X K onfigurtion n with ifferent DR for M,M,n M re presente in Figure 8,, n 8, respetively. It n e seen tht throughput inreses with ntenn elementsndr.theresultslsoshows M < S M < S M. These results gin epit the overhe s effet n effets relte to si CSMA/CA opertion n its requirements s mentione ove. In ition, it n e oserve tht throughput inreses in the eginning when n strts to inrese ut fter rehing ertin threshol the throughput strts to erese. This is euse when there re only fewer numer of noes there will e higher proility of the slots remining ile euse of witing time ssoite with koff lgorithm. But, initilly when the numer of noes strts to rise, the throughput inreses s the proility of slots remining ile gets reue. However, when n inreses further the proility of ollision lso inreses whih ultimtely reues the throughput. Besies these oservtions, the throughput oes not inrese linerly in M n M while in M it inreses more or less linerly with ntenn elements like in the

10 Thp et l. EURASIP Journl on Wireless Communitions n Networking, :4 Pge of Averge Throughput (Mps) x4 MU-MIMO x MU-MIMO IEEE 8. Mps DR 54 Mps DR 44 Mps DR 6 Mps DR Averge Dely (S) x4 MU-MIMO x MU-MIMO IEEE 8. Mps DR 54 Mps DR 44 Mps DR 6 Mps DR Numer of Noes (n) () M (CSIF-STCP) Numer of Noes (n) () M (CSIF-STCP) Averge Throughput (Mps) x4 MU-MIMO x MU-MIMO IEEE 8. Mps DR 54 Mps DR 44 Mps DR 6 Mps DR Averge Dely (S) x4 MU-MIMO x MU-MIMO IEEE 8. Mps DR 54 Mps DR 44 Mps DR 6 Mps DR Numer of Noes (n) Numer of Noes (n) () M (CSIP-STCP) () M (CSIP-STCP) Averge Throughput (Mps) x4 MU-MIMO x MU-MIMO IEEE 8. Mps DR 54 Mps DR 44 Mps DR 6 Mps DR Numer of Noes (n) () M (CSIP-SmTCP) Figure 8 Averge throughput of CSMA/CA pte MU-MIMO wre MAC protools for WLANs. () M (CSIF-STCP), () M (CSIP-STCP), () M (CSIP-SmTCP) Averge Dely (S) x4 MU-MIMO x MU-MIMO IEEE 8. Mps DR 54 Mps DR 44 Mps DR 6 Mps DR Numer of Noes (n) () M (CSIP-SmTCP) Figure 9 Averge ely of CSMA/CA pte MU-MIMO wre MAC protools for WLANs. () M (CSIF-STCP), () M (CSIP-STCP), () M (CSIP-SmTCP). previous results. Figure 9,, n 9 shows the verge ely results for M,M,nM, respetively. It n e seen tht the ely inreses with ntenn elements ut in opposite ereses with DR. However, in these results s well, D M < D M < D M euse of overhe s effet n si CSMA/CA opertion s effet s mentione ove. Moreover, it n lso e remrke tht the ely inreses with n in ll the ses s the ition in

11 Thp et l. EURASIP Journl on Wireless Communitions n Networking, :4 Pge of numer of noes uses the higher proility of ollision n les to high witing time. Similr to the previous results, there is no liner throughput-ely gin in these results s well. Asfrswehveisusse,themjorftortht ouns throughput n ely is the overhe ssoite per suessful t trnsmission when pting onventionl CSMA/CA. Clerly, from our results, the overhe s effet n e reue t the ost of omplexity. Hene, performne n omplexity n e flexily tre off ginst eh other. Aprt from this, in MIMO wre CSMA/CA, long with the moifitions in ontrol pket formts n/or hnnel ess mehnism, other shemes to reue overhes like frme ggregtion, lok knowlegement, et., [5] shoul lso e investigte prllelly to etter utilize MIMO pity. 6. Conlusion We hrterize the performne of CSMA/CA pte MU-MIMO wre MAC in wiely eploye IEEE 8. WLANs. Along with the isussion on moifition pprohes tht est represent the possile wys tht oul e rrie out to upgre onventionl CSMA/CA into MU-MIMO wre CSMA/CA, we provie their etil performne nlysis, se on the nlytil moeling n erive expressions, in terms of throughput n ely. Thus, on the one hn, fter presenting the importne of MU-MIMO wre MAC protool, we presente the isussion on moifition pprohes n the nlytil moel to unerstn their performne, while on the other hn, we lso showe the limittions of suh protools euse of the effets of inispensle overhe ssoite. Ennotes To eoe simultneously trnsmitte signls, it emns high omputtionl omplexity with sophistite hrwre filters. Size of the RTS Reeiver Aress fiel is inrese y K - times in M-RTS. Aknowlegements This stuy ws supporte y the Bsi Siene Reserh Progrm through the Ntionl Reserh Fountion of Kore (NRF) grnt fune y the Kore government (MEST) (9-756). Competing interests The uthors elre tht they hve no ompeting interests. 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