Induced Cooperative Multi-user Diversity Relaying for Multi-hop Cellular Networks

Transcription

1 Iduced Cooperative Multi-user Diversity Relayig for Multi-hop Cellular Networks Keiva Navaie ad Halim Yaikomeroglu Broadbad Commuicatios ad Wireless Systems (BCWS) Cetre Departmet of Systems ad Computer Egieerig, Carleto Uiversity {keiva, Abstract I this paper we study the multi-user diversity gai i the dowlik of sigle-hop ad multi-hop ifrastructure-based etworks. We propose a base-statio coordiated cooperative relayig method, Cooperative Iduced Multi-user Diversity Relayig (CIMDR), to overcome the fudametal limitatios o the average achieved et throughput per-user. I the proposed method, multi-user diversity is iduced ad the exploited i a 2-hop relayig scheme to improve per user achieved data throughput. We show that by usig the proposed method, the throughput per-user ad the packet-drop-ratio are sigificatly improved. I. INRODUCION I wireless cellular etworks with multiple users, multiuser diversity is achieved as a result of havig idepedet time-varyig wireless chaels betwee the base-statio ad differet users i the coverage area. he multi-user diversity gai arises from the fact that, i a system with may users whose chaels vary idepedetly, there is likely to be a user with a very good chael at ay time. System throughput is the maximized by allocatig the shared radio resource at ay time to the user that ca best exploit it [], [2], [3]. his approach has bee employed i high-speed dowlik stadards for the third geeratio (3G) cellular wireless commuicatios stadards, HSDPA ad xev-dv. For Beyod-3G wireless commuicatio systems, multi-hop deploymet has bee widely cosidered as a augmetig optio to sigle-hop trasmissio [4], [5]. I multi-hop cellular etworks, the data-uits are trasmitted to the destiatio through relays. Such deploymet cocept deems to be a promisig combiatio of the dyamics of mobile ad-hoc etworks ad the reliability of cellular etworks. o exploit the multi-user diversity i a multi-hop etwork, a relayig method is proposed i [6], i which multi-user diversity is exploited i each hop by selectig the ext relay based o the istataeous chael quality. However, the trasmissio to oly oe relay reduces the opportuity of fidig a good chael i the ext hop. I this paper, we study the multi-user diversity gai i sigle-hop ad multi-hop cellular etworks. We obtai a upper-boud o the achievable average throughput per-user i a cellular data etwork i which multi-user diversity schedulig [3] is utilized. he derived upper-boud is a fuctio of the umber of users i the cell coverage area, base-statio maximum trasmit power, maximum supported trasmit bit-rate, ad average maximum chael gai. o improve achievable average throughput per-user, this paper proposes a base-statio coordiated cooperative relayig scheme, amely Cooperative Iduced Multi-User Diversity Relayig (CIMDR), which uses the broadcast feature of the wireless chael to iduce multiuser diversity through a two-pase relayig process. We show that by usig the proposed method, the throughput per-user ad the packet-drop-ratio are sigificatly improved. II. SYSEM MODEL We cosider a ifrastructure-based wireless etwork i which base-statios with a maximum trasmit power level of P max are located at the ceter of their coverage area. A access-poit trasmits a sigallig chael that ca be received by all users i the coverage area. I our modellig, there are mobile users, idexed by i, distributed uiformly i the coverage area. Each packet has a large delay tolerace ad icludes the idetity (e.g., physical address) of the destiatio user. he wireless chael gai betwee the access-poit ad ith user at time t is give by the process {g i (t)} which is assumed to be statioary ad ergodic. Moreover, for differet users i the coverage area, the correspodig chael processes are assumed to be idepedet ad idetically distributed (i.i.d.). At ay time t, a resource allocatio policy, P, coordiates the data trasmissios from the access-poit to relays, or relays to destiatio users. For a resource allocatio policy, Γ P i (t) is defied as the achieved dowlik throughput of user i at time t, that is the umber of bits received by user i at time t. For a resource allocatio policy, we defie the feasible log-term achieved dowlik throughput per-user, Γ P (), such that lim if Γ PD i (t) =Γ PD (). () Γ P () depeds o various factors icludig the maximum supported bit-rate, umber of users i the coverage area, ad the wireless chael temporal characteristics. he defiitio i () is similar to that preseted i [7], for ad-hoc etworks. III. MULI-USER DIVERSIY o exploit multi-user diversity, a resource allocatio policy, P D, is employed. his policy, i its simplest form, allocates the maximum access-poit trasmit bit-rate to a user i at each time t, where i (t) = arg max{g i (t)}. (2)

2 Note that, selectig i (t) based o the chael coditio may result i a ufair resource allocatio. o resolve the fairess issue, some corrective schedulig methods are ofte used (see e.g., [3], [8], [9]). Sice our focus is o the multi-user diversity gai, we simply cosider a log-term fairess requiremet i which lim sup i,j Γ P i (t) ΓP j (t) =0;that is a direct cosequece of the i.i.d. wireless chaels betwee differet users i the coverage area ad the base-statio. Note that, i order to exploit multi-user diversity, accordig to P D, a user s packets have to be delayed util the chael becomes the best relative to other users. herefore, the timescale of chael variatios that ca be exploited by P D is limited by the delay tolerace of the correspodig applicatio. It is show that for the above metioed resource allocatio policy, P D, the overall system throughput performace is sigificatly higher tha that of simultaeous trasmissio [], [3], [0]. he greater the umber of users i the coverage area, the higher is the probability of occurrece of a good chael, which results i a greater improvemet i the accesspoit throughput. However, the achieved dowlik throughput per-user is still limited by the maximum trasmissio bit-rate ad coverage area, thus limited by fudametal architectural costraits. Cosider a CDMA-based radio iterface; for trasmissio with a rate R i (t) bits/s to a user i, the basic bit-eergy to the iterferece-plus-oise spectral desity costrait should be satisfied. hus W P max g i (t) ρ i (t), (3) R i (t) I 0 where I 0 is the iterferece-plus-oise power, W is the chip rate ad ρ i (t) is the miimum required bit-eergy to the iterferece-plus-oise spectral desity for the data trasmissio with bit-rate R i (t). Forauseri selected for trasmissio, usig (3) we write, R i (t) ξ 0 g i (t) (4) where ξ 0 = ( ρ i (t)i 0 ) WPmax. Propositio : For large umber of users i the coverage area, the feasible log-term achieved dowlik throughput peruser, Γ PD (), is upper-bouded by ξ 0 lim if where i (t) is give by (2). Proof: We have lim if g i (t), Γ PD i (t) = lim if i a i (t)r i (t) (5) where a i (t) is the selectio idicator; i.e., a i (t) =,ifuseri is selected for trasmissio at time t, ad 0 otherwise. Summig (5) over all users, we have Γ PD () ξ 0 = ξ 0 lim if i lim if i i= a i (t)g i (t) (6) g i (t) (7) which complete the proof. Propositio shows that the dowlik throughput per-user is upper-bouded by g i (t). o icrease multi-user diversity gai, i [3], multiple trasmit ateas are used to iduce large ad fast chael fluctuatios, i.e., greater g i (t). Also i a multiple-cell sceario, the idepedet time variatios of the wireless chaels betwee a user ad the eighborig access-poits is itroduced i [], [2], as a ew dimesio i multi-user diversity. his form of diversity is exploited by joit access-poit assigmet ad packet schedulig, which results i greater g i (t) ad thus greater multi-user diversity gai per-user. o exploit the multi-user diversity i a multi-hop etwork, a relayig method is proposed i [6]. I this method, usig a sequetial optimizatio approach, multi-user diversity is exploited i each hop by selectig the ext relay based o the istataeous chael quality. However, selectig oly oe relay reduces the opportuity of capturig a good chael i the ext hop. I the followig sectio, we propose a basestatio coordiated cooperative relayig method, which exploit multi-user diversity i multi-hop cellular etworks. IV. COOPERAIVE INDUCED MULI-USER DIVERSIY RELAYING Cooperative Iduced Multi-user Diversity Relayig (CIMDR) (Fig.) exploits the broadcast ature of wireless chael to iduce multi-user diversity through a two-phase process. I the first phase, access-poit broadcasts data packets with its maximum bit-rate. Some users i the coverage area are likely to receive the trasmitted data packets. hese users, act as potetial relays i the secod phase; each potetial relay wait util the occurrece of a good chael to the destiatio user ad the trasmit the data packets. As soo as the trasmissio is carried out by oe of the potetial relays, the access-poit maages to release the packets saved buffered i other potetial relays. We cosider a 2-hop ifrastructure-based etwork. Accesspoit is located at the ceter of the coverage area ad its maximum trasmit power is P max. Packets ca be trasmitted directly from the access-poit to the users, or they ca go through aother mobile user servig as a relay. Access-poit trasmits sigallig o dedicated cotrol chael(s) that ca be received by all users i the coverage area. here are mobile users, idexed by i, distributed uiformly i the coverage area. Mobile users are able to receive, temporarily save ad relay packets i the same frequecy bad of access-poit trasmissio. hey also trasmit sigalig iformatio o a uplik sigalig chael. Mobile termials have a large eough buffer to store relay packets. Each packet

3 Fig.. CIMDR. Fig. 3. CIMDR sigalig: Normal trasmissio. Fig. 2. CIMDR two-phase protocol ad packet structure. has a large delay tolerace ad icludes the idetity (e.g., physical address) of the destiatio user. Each user i the coverage area broadcasts a pilot sigal to idicate its idetity. his pilot sigal is also utilized by the relays for chael estimatio. o decrease power cosumptio, broadcastig of users pilot chael ca be activated upo receivig a sigal (from the access-poit) idicatig the existece of a data packet destied to that mobile user. Sice by this sceario we iduce multi-user diversity through geeratig idepedet paths betwee the destiatio user ad m relays, we ame it Cooperative Iduced Multi-user Diversity Relayig (CIMDR). A. CIMDR Protocol he proposed sceario, P I, has two phases: the feedig phase ad the delivery phase. hese two phases occur sequetially i time (Fig.2). he time-spa of each phase (i.e., τ F ad τ D ) is assiged based o the etwork traffic ad the commuicatio eviromet characteristics. Fig.3 shows the sigallig procedure of CIMDR protocol. Feedig Phase: I the feedig phase, packets are broadcasted by the access-poit with its maximum bit-rate; the total umber of trasmitted bits i the feedig phase would be τ F., where τ F is the time duratio of the feedig phase. Durig the feedig phase multiple packets are trasmitted usig time domai schedulig. Ay user which receives a data packet i the feedig phase acts as potetial relays i the delivery phase. he trasmissio order of the queued packets i the accesspoit is maaged by a higher-layer fuctioality. If the destiatio user is amog those who receive packets i the feedig phase, it seds a received ackowledge sigal, R-ACK, to the access-poit. Cosequetly, the access-poit broadcasts a data release sigal, D-REL, ad all other relays release that packet. Here we assume that the umber of users i the coverage area is high eough that i each time istat there is, at least, oe user that ca receive the trasmitted data i the feedig phase. I cases where o mobile user i the coverage area ca receive the trasmitted packet i the feedig phase, the access-poit should reduce its bit rate. I the feedig phase, multi-user diversity gai comes from the fact that the access-poit radio resource is oly allocated for trasmissio with its maximum bit-rate. Note that for a large umber of users i the coverage area, it is likely that some users will have a chael state that supports the accesspoit s highest bit-rate. Delivery Phase: I the delivery phase, the access-poit is kept iactive ad oly trasmissios from relays to the fial destiatios are allowed. Each relay cotiuously tracks the quality of the wireless liks to the eighborig users as well as their idetity. If a relay is able to achieve a trasmissio bitrate greater tha or equal to a system parameter R 0 bits/s, the that relay trasmits to the destiatio user. he selectio of R 0 critically affects the system performace ad is elaborated i the Propositio give below. Medium access cotrol ca be either a cotetio-based method or a access-poit coordiated o-cotetio based method. Upo successful trasmissio, destiatio user seds a R-ACK sigal to the access-poit. Cosequetly, the access-poit broadcasts a D-REL sigal ad other relays release that packet. If the access-poit does ot receive a R- ACK correspodig to a packet i a predefied time iterval, τ max secods, that packet is cosidered lost ad a D-REL sigal is broadcasted by the access-poit. hat packet may be cosidered for retrasmissio i a later time. Multi-user diversity i the delivery phase is exploited by trasmissio o chaels with the achieved bit-rate greater tha or equal to R 0. Note that i practice the trasmit bit-rate may be adjusted based o the chael status which is fed back ito the access-poit by the users.

4 B. Icetive system for users cooperatio here should be a icetive system to provide reasoable motivatio to the users for participatio i CIMDR. he icetive problem is heavily studied i the cotext of mobile ad-hoc etworks (see e.g., [3]). I a ifrastructure-based multi-hop system it is possible to have a etwork-based icetive system which makes this problem a lot easier. Here, we propose a simple credit based icetive system. I this system a user i is grated a participatio credit of µ(t) upo participatig i relayig process at time t. hisis due to the fact that the mobile user allocates a part of its processig power for trackig the eighborig mobile statios ad for ivolvig i the correspodig sigalig processes. As soo as fidig the destiatio user ad detectig a chael with available bit-rate greater tha or equal to R 0,the relay trasmits the data packet thus allocates a portio of its trasmissio power to relayig. I this case, the etwork grats a relayig credit of ν(t) to that mobile user. he values of µ(t) ad ν(t) are related to the etwork chargig strategy ad ca be varied i differet times of the day based o the etwork traffic. I such a sceario with m mobile users participatig i CIMDR, the total grated credit per packet trasmissio is m.µ(t) +ν(t) which would be cosidered as part of the trasmissio cost for each data packet. V. PERFORMANCE EVALUAION For a give medium access cotrol techique, 0 <γ is defied as the medium access cotrol gai, which shows the average portio of the radio resource (e.g., trasmissio time) that ca be allocated to the competitors for a shared media. For o-cotetio based medium access cotrol mechaisms γ =.LetR be the average access-poit trasmissio bit-rate for sigle-hop trasmissio with multi-user schedulig. he followig propositio provides the coditio o the system parameters for CIMDR. Propositio 2: For a large umber of users i the coverage area, by usig CIMDR the access-poit throughput is icreased compared to the sigle-hop trasmissio if ( <γ R 0 R ). (8) Proof: Let Φ D (t) ad Φ F (t) be the idicator fuctios for delivery ad feedig phases, respectively. herefore, Φ D (t) = (Φ F (t) =) whe the system is i delivery (feedig) phases ad Φ D (t) =0(Φ F (t) =0), otherwise. he total data bits i the system at time t, Θ(t), ca be calculated as t ( ) Θ(t) = γr 0 Φ D (α) Φ F (α) dα. 0 For system stability, it is required that lim Θ( )=0, thus τ D γr 0 = τ F. (9) O the other had, compared to the sigle-hop trasmissio, the total access-poit throughput will be icreased if τ F > (τ F + τ D )R. (0) Hece, usig (9) ad (0), ( <γ R 0 R ). () his proves the propositio. O oe had, if is very large, the a smaller umber of users i the coverage area will receive the data packets i the feedig phase. O the other had, decreasig will icrease the umber of potetial relays but will decrease the overall rate. he trasmissio rate mayalsobeadjusted based o the umber of potetial relays; if data packets are ot received by a reasoable umber of mobile users, the may be decreased. Give the coditio i (8) holds, withi the iterval [0, ] for all packets trasmitted to the relays will be delivered to the users. herefore, for CIMDR, it is simple to show that (7) is modified as Γ PI () ξ ǧ, (2) where ξ is defied similar to ξ 0 i (7) ad ǧ is the miimum time-average value of the chael gai betwee the accesspoit ad the relay with the maximum trasmissio bit-rate. Note that lim if g i (t) < ǧ, (3) which is a direct cosequece of smaller path-losses because of multi-hop trasmissio. his directly results i Γ PI () > Γ PD (). I other words, usig CIMDR, the achieved average throughput per user is icreased. Note that τ max has a importat role i the performace of CIMDR. If τ max, the a packet ca be kept waitig i a potetial relay util the occurrece of a very high rate chael (i.e., very large R 0 ). For moderate values of τ max, the mobility is very importat. he higher the users mobility the higher is the probability of the occurrece of a high bitrate chael i the secod hop. For a give mobility profile, a larger value of τ max results i the exploitatio of the mobility i a more efficiet way. VI. SIMULAION RESULS We simulate a sigle-cell DS-CDMA system with active users based o UMS stadard [4]. Users are uiformly distributed i the coverage area. he simulatio parameters are preseted i able I. A simple mobility model has bee implemeted, i which at each time istat, a user radomly located withi a circle with its previous locatio i the ceter ad a diameter of 2.5 meters. o show the effect of multi-user diversity, we cosider three differet systems: i System I, for each user the accesspoit trasmits packets i first-come-first-serve fashio usig a time domai schedulig scheme. I System II, packets are scheduled based o P D. rasmissio i System III is based o P I, with a o-cotetio based medium access cotrol techique i the delivery phase.

5 ABLE I SIMULAION PARAMEERS. Parameter Value Cell radius 00 m Access-poits trasmit power 0 W Stadard dev. of log-ormal fadig 8dB Backgroud oise desity dbm/hz Propagatio loss expoet 4 ime-slot legth 0 ms 2 Mbps R 384 Kbps Medium access cotrol gai (γ) Miimum required E b /I 0 2dB PDR Sys. II Sys. III, τ max = 2s Sys. III, τ max = 0s System II System III Iduced Multi user diversity Fig. 5. Packet-drop-ratio (PDR) of CIMDR ad sigle-hop multi-user diversity schedulig for τ max = 2 ad 0 secods. Γ() Multi user diversity 2-hop forwardig scheme ad the exploited to improve per user achieved data throughput. Simulatio results cofirm that by usig the proposed method, the throughput per-user ad the packet-drop-ratio are sigificatly improved. Fig Normalized average achieved et throughput vs. the umber of users. System I is cosidered as the bechmark, ad the average achieved et throughput of Systems II ad III are ormalized by the average achieved et throughput of System I. Fig.4 illustrates the ormalized average achieved et throughput versus the umber of users i the coverage area for τ max = 2 s. he differece betwee the throughput gais of Systems II ad III idicates the achieved multi-user diversity gai resultig from exploitig the iduced multi-user diversity by CIMDR. As it is expected, this gai is icreased by the umber of users. Note that ormalized throughput curve will saturate because of the access-poit total throughput costrait. We also compare the packet-drop-ratio for System II ad System III. Packets are cosidered lost whe they caot be trasmitted withi a delay threshold of τ max =2s. As it ca be see i Fig.5, usig CIMDR improves the packet-drop-ratio performace. he greater improvemet i the packet drop ratio is archived by a larger umber of users i the coverage area ad a larger delay tolerace of 0 s. VII. CONCLUSION I this paper we study the multi-user diversity gai i the dowlik of sigle-hop ad multi-hop ifrastructure-based etworks. We proposed Cooperative Iduced Multi-user Diversity Relayig (CIMDR) scheme to overcome the fudametal limitatios o the average achieved et throughput per-user. I the proposed method, multi-user diversity is iduced i a ACKNOWLEDGMEN his work was supported by Special Research Opportuities (SRO) grat from Natural Scieces ad Egieerig Research Coucil (NSERC) of Caada for participatio i the Wireless World Iitiative New Radio (WINNER) project. REFERENCES [] R. Kopp ad P. A. Humblet, Iformatio capacity ad power cotrol i sigle-cell multiuser commuicatios, i Proc. IEEE ICC 95, pp , 995. [2] D. se, Optimal power allocatio over parallel Gaussia chaels, i Proc. IEEE ISI 97, p. 27, 997. [3] P. Viswaath et al., Opportuistic beamformig usig dumb ateas, IEEE ras. o Ifo. heory, pp , [4] H. Yaikomeroglu, Fixed ad mobile relayig techologies for cellular etworks, i Proc. ASWN 02, pp. 75 8, [5] R. Pabst et al., Relay-based deploymet cocepts for wireless ad mobile broadbad cellular radio, IEEE Commu. Mag., vol. 42, o. 9, pp , Sept [6] P. Larsso ad N. Johasso, Multi-user diversity forwardig i multihop packet radio etworks, i Proc. IEEE WCNC 05, [7] P. Gupta ad P. R. Kumar, he capacity of wireless etworks, IEEE ras. o Ifo. heory, pp , [8] S. Shakkottai ad A. Stolyar, Schedulig for multiple flows sharig a time-varyig chael: he expoetial rule, i AMS raslatios, ser. 2, Y. M. Suhov, Ed., [9] K. Navaie et al., Fairess of resource allocatio i cellular etworks: A survey, i Resource Allocatio i Next Geeratio Wireless Networks, W. Li ad Y. Pa, Eds. Nova Sciece Publishers, [0] P. Beder et al., CDMA/HDR: a badwidth-efficiet high-speed wireless data service for omadic users, IEEE Commu. Mag., pp , [] K. Navaie ad H. Yaikomeroglu, Optimal dowlik resource allocatio for o-realtime traffic i cellular CDMA/DMA etworks, to appear i IEEE Commu. Letters, [2], Dowlik joit base-statio assigmet ad packet schedulig algorithm for cellular CDMA/DMA etworks, IEEE ICC 06, [3] L. Buttya ad J. Hubaux, Stimulatig cooperatio i self-orgaizig mobile ad hoc etworks, ACM Mobile Networks ad Applicatios, vol. 8, o. 5, pp , [4] H. Holma ad A. oskala, WCDMA for UMS: Radio Access for hird Geeratio Mobile Commuicatios. Joh Wiley ad Sos, 2000.