ADAPTIVE ALGORITHMS FOR IMPROVING THE THROUGHPUT IN AN INDOOR MOBILE S-ALOHA DS-CDMA SYSTEM

Transcription

1 ADAPTIVE ALGORITHMS FOR IMPROVIG THE THROUGHPUT I A IDOOR MOBILE S-ALOHA DS-CDMA SYSTEM Oriol Sallet, Ramo Agustí Deartamet de Teoria del Seyal i Comuicacios Uiersitat Politècica de Cataluya C/ Gra Caita s/ Barceloa - Sai Abstract: This aer resets a oel Adatie DS- CDMA Slotted-ALOHA acket radom access scheme with trasmitter-based sreadig codes for mobiles. It is aimed at imroig the throughut ad message delay deliery whe traffic load alues below the saturatio oit of the coetioal DS-CDMA Slotted-ALOHA system are sesed i the chael. For this urose, oe Mobile ad two Base Statio assisted algorithms are eisaged to cotrol the chage of the trasmissio rate accordig to the traffic load. These algorithms reealed that the otimum behaior, obtaied usig a Marko Chai model, may be almost reached at a low comlexity cost. I. ITRODUCTIO Slotted ALOHA (S-ALOHA) has bee widely recogized for acket radio alicatios because of both its simlicity to maage bursty traffic ad its ability to delier a shorter delay tha that of fixed multile access techiques i the resece of low traffic loads. The S- ALOHA CDMA scheme allows seeral users to trasmit at the same time usig a differet sreadig code for each oe [1, [, so that usuccessful trasmissios are caused etirely by multile access iterferece, roided egligible thermal oise effects are assumed. This aer is iteded to imroe the throughut achieed with a multi-receier DS-CDMA S-ALOHA whe a light load is offered to the system, ad accordigly decreasig the delay. For this urose we will focus o the trasmissio rate used istead of the access rotocol itself. I articular, differet trasmissio rates are roosed to be used i order to utilize the most suitable rate accordig to the chael load at ay time slot. The, fast ad simle algorithms that commad the rocessig gai of the DS-CDMA scheme as a fuctio of the chael load are eisaged. The aer is orgaized as follows. I Sectio a aalytical model for the DS-CDMA S-ALOHA system is reseted, which is used to ealuate the otimum achieable throughut with a adatie chage of trasmissio rate algorithm. I Sectio 3 seeral S- ALOHA adatie algorithms are roosed ad assessed deedig o whether the algorithm is maaged by mobile statio (MS) or by base statio (BS). Fially, some coclusios close the aer i Sectio. II. MODEL FOR A S-ALOHA DS-CDMA ACCESS SYSTEM From ow owards registered users will be cosidered. These users ca be i two differet oeratio modes: origiatio mode ad backlogged mode. I the former mode there is o acket to be retrasmitted ad ew ackets are geerated with robability o. Termials eter the backlogged mode whe a attemt to trasmit a ew acket fails. I this mode, the retrasmissio of the backlogged acket occurs i ay gie slot with robability r. While i the backlogged mode the user does ot geerate ay ew acket. Let k deote the umber of backlogged users at the begiig of the k th slot. The rocess defied by k k=0,1,... is a Marko chai, whose erformace has bee studied amog others i [3,. Whe cosiderig DS- CDMA, the formulatio must be modified as it was doe i [5. The same otatio is followed i this aer, ad it is summarized i Table 1 ad Fig. 1. Throughut measuremets ca be obtaied from the markoia model cosiderig that ( S) ( T) T { } ( ) { = } S = s P = s = P (1) = 0 s= 0

2 where S is i ackets er slot ad, if all users emloy the same modulatio ad trasmissio rate, the followig exressio arises ( S) ( T) s { } ( ) ( ) P = s = = s 1 () beig P c () the robability of correctly detectig a acket whe users hae attemted trasmissio i a time slot. P{ (T) = } i (1) stads for the equilibrium distributio of the comosite acket arrials i a time slot, which could be reached i terms of the equilibrium distributio of the Marko chai. The comlete deriatio of the exressio for this robability ca be foud i [5. IDLE (I) o () (R) (T) r (S) DS-CDMA CHAEL BACKLOGGED s Fig. 1. Model for a S-ALOHA DS-CDMA system. k (I) k (T) k (R) k () k (S) k Table 1. List of arameters for the S-ALOHA DS-CDMA model. umber of mobiles i the system umber of mobiles i B-mode at start of slot k umber of mobiles i I-mode at start of slot k umber of mobiles trasmittig i slot k umber of mobiles i B-mode trasmit i slot k umber of mobiles i I-mode trasmit i slot k umber of ackets correctly receied i slot k A. S-ALOHA DS-CDMA ACCESS SYSTEM A BPSK DS-CDMA access system with a rocessig gai gie by G is cosidered. All users hae bee assiged radom P sigature sequeces. A erfect ower cotrol caable of mitigatig fadigs of the chael is itroduced, i.e., the termial trasmits the ower leel ecessary to kee always the same receied ower. With a iew to achieig this, a cotiuous lik betwee BS ad MS is eeded so that the mobile trasmits at rate whe there is iformatio to be set ad at rate (i geeral much lower) whe the termial is ot actie. A istataeous ower cotrol ermits a huge imroemet i the system erformace whe comared with a oe loo ower cotrol, ad it is, i fact, cosidered i the already oeratie [7 ad the roosed CDMA systems [8 resectiely. Although a istataeous ower cotrol could be eisaged for acket radio o a acket by acket basis [9, we hae retaied the cotiuous lik aroach because i this case o sychroizatio oerhead at the begiig of time slot would be required. B. S-ALOHA DS-CDMA PERFORMACE By assumig a ideal istataeous ower cotrol, the chael ca be see as a AWG if we use the gaussia hyothesis to model the iterferece origiated by other users [10, [. Uder these coditios the followig exressios hold for the ealuatio of the BER Pb ( )= Q E b ; E b 1 = o ( 1) ( ) ' o + 3G 3G where is the umber of simultaeous users. The robability of detectig correctly a acket cotaiig αb bits is [ 1 Pb P ( ) = ( ) c αb Usig the same badwidth, to trasmit at double rate ( bits/s, where is the basic rate) half the sread caacity should be gie u, thus allowig a rocessig gai of G /, or G / if we trasmit at bits/s. Howeer, i exchage for this reductio i rocessig gai, B or B bits er time slot ca be allocated istead of oly B. The achieable throughut with each oe of the trasmissio rates obtaied with the reious Marko model ca be see i Fig., where G stads for the offered load -aerage umber of attemted trasmissios er time slot-, ad o is set equal to r. It is clear from Fig. that the high rate () is iterestig whe the system is lightly loaded sice more bits er acket ca be set through the chael, due to the fact that the little iterferece obsered ermits a correct trasmissio. Whe the offered load icreases so does the iterferece leel, ad therefore the rate bits/s is o loger iterestig because of the higher BER if it is comared to the medium rate ( bits/s). Ee for higher offered loads the same trade-off aears betwee medium rate ( bits/s) ad low rate ( bits/s). Trasmissio rate could be icreased oer bits/s, but the margial gai is each time lower as Fig. shows. So, for the sake of breity oly three rates, ad bits/s will be cosidered i the sequel. (3) ()

3 mi(, s) mi(, si) S = [ Bi+ Bj + B si j ( ) = 0 i= max( 0, s ) j= max( 0, si ) i [ P [ P c i c i j j. 1 1 j s i j s i j s ++ i j T. 1 P{ } bits slot ( ) = / (6) where B is the umber of bits i a BPSK rate acket (B=100 bits all oer this aer) ad S is exressed i bits/slot. Fig. 3 shows this otimum throughut i comariso with the idiidual behaiors. Fig.. Throughut erformace for differet trasmissio rates. C. ADAPTIVE S-ALOHA DS-CDMA I the light of the aboe results, a algorithm able to chage the trasmissio rate used by the MS as a fuctio of the traffic load of the system could be foresee so that the maximum ossible throughut could always be obtaied. The otimum throughut attaied ca be aalytically obtaied from the aboe Marko model, ad this result ca be later used as a referece to assess the erformace of the trasmissio rate chage algorithm roosed below. I order to attai this otimum erformace, the best combiatio of trasmissio rates should be obtaied roided that simultaeous users are reset. The resultig otimal table would be reached after a exhaustie search, as it is further exlaied i [6. Sice the same chael is shared by users trasmittig at differet rates, the robability of gettig s successful ackets oer trasmitted ackets is gie ow by mi(, s) mi(, si) ( S) ( T) { = = } = i [ P i c i= max( 0, s ) j= max( 0, si ) [ P c i j j. 1 1 j s i j. s ++ 1 i j P s si j istead of (), which alies whe all users emloy the same trasmissio rate. I turs, it is assumed that otimal combiatio (,, ) is aailable for ay ad users are i fact usig it. That is, is readily associated to a (,, ) set, ad so P{ (S) = s (T) = } is ealuated cosiderig all ossible cases giig s correctly-receied ackets whe users trasmit at low rate, at medium ad at high, leadig to (5). Thus, the modified throughut exressio is (5) Fig. 3. The ery best achieable throughut for the S-ALOHA DS-CDMA system. III. CHAGE OF TRASMISSIO RATE ALGORITHMS I this sectio seeral roosals of adatie chage of the trasmissio rate algorithms are addressed. The commo idea cosist i sesig the traffic load through the chael i order to accommodate trasmissio rates accordigly. Three algorithms with icreasig order of comlexity are roosed i the followig subsectios: A. Mobile cotrolled algorithm The roosed simle algorithm carried out by the mobile statio (MS) works as follows: each termial traces its ow eolutio durig the trasmissio time, that is, termials cout their successful ad erroeous ackets. I the absece of errors the mobile will assume a low traffic load ad tries to use a higher trasmissio rate. The throughut should be icreased i this way. If errors occur, the mobile decides that the chael is too loaded ad tries a lower trasmissio rate. I this case fewer bits er acket are trasmitted, but a global imroemet of the throughut should also follow sice rocessig gai icreases accordigly. Let us ote that this decisio is take by the mobile without ay exchage of iformatio with the Base Statio (BS) excet for the ackets ackowledgmet.

4 Ee the MS does ot eed to idicate its choice of trasmissio rate before usig it because the ery same BS could be able to detect which oe is arriig. Secifically, the MS oly eeds to establish two arameters: the umber of cosecutie acket failures before chagig to a lower rate (max_tr) ad the umber of cosecutie acket successes before tryig a higher rate (mi_suc). Fig. shows the throughut attaied for =60 registered mobiles. I site of the simlicity of the algorithm, the eeloe of the three idiidual grahs is almost reached, ad this is ot far from the otimum behaior. Results hae bee obtaied with max_tr=1 ad mi_suc=6, which is the best choice, oce other ossibilities hae bee studied. Results obtaied followig the aboe rocedure ca be see i Fig. 5. It shows that a so simle iformatio as it is the rude estimatio of the mea offered load is ery aluable. Howeer, if we comare BS-I algorithm erformace with that of the mobile cotrolled algorithm, o imroemet is obtaied i site of the comlexity icrease. The behaior of a simlified ersio of the algorithm is also reseted i Fig. 6. It cosists i decidig the trasmissio rates to broadcast by the BS based o thresholds alues. That is, threshold L1 meas that for <L1 all mobiles will use bits/s i the ext time slot, ad L idicates that for L1<<L bits/s rate will be used durig the followig time slot ad >L will use bits/s. Otimum thresholds obtaied by simulatio for =60 would be L1=11 ad L=6. Fig.. Throughut erformace for MS algorithm. B. Base cotrolled algorithm Coordiatio betwee mobiles ca be achieed if BS maages the algorithm. By doig so a erformace imroemet is exected at the exese of losig certai degree of simlicity i its imlemetatio. 1) BS-I algorithm This algorithm is maaged by the BS, which decides the trasmissio rate to be used. The BS estimates the umber of users,, which hae attemted to trasmit i a gie time slot. The, the BS exects users to be the most likely alue for the ext time slot. Sice the BS does ot kow which mobiles are goig to trasmit, it has to broadcast the robability of usig each trasmissio rate that the BS is able to kow from (1) ad (13) i [6. For examle, whe G =17 ad there has bee 1 users trasmittig their ackets i the last time slot, the BS exects 1 users agai for the ext time slot, ad it kows that the best throughut uder this situatio is achieed with 9 users trasmittig at bits/s ad 5 users trasmittig at bits/s. Thus, a robability of 9/1 of trasmittig at bits/s ad a robability of 5/1 for bits/s is broadcasted for ext time slot. This rocess is reeated eery time slot. Fig. 5. BS-I algorithm with otimal combiatio of rates. Fig. 6. BS-I algorithm with thresholds. ) BS-II algorithm A imroemet i throughut erformace attaied with BS-1 algorithm could be exected if the BS kew i

5 adace the exact umber of users ready to trasmit i the reset slot. For this urose the time slot is slit i two arts: the first art is simly used to idicate that a acket is scheduled for trasmissio, ad i the secod art iformatio is eetually trasmitted. I the so-called re-slot (first art of time slot), mobiles oly trasmit a sigal i a S-ALOHA mode idicatig that they are ready to trasmit. After this, the BS couts how may users will attemt to trasmit ad selects the otimum combiatio of trasmissio rates to be emloyed. This iformatio is sulied to the MS ad the ayload bits are set at the most suitable rate i the secod art of the slot. The mai differece with BS-I is that BS-II makes the BS to kow the exact umber of users whe there are ot errors i the re-slot eriod. Pre-slot uses a S-ALOHA oeratio mode with the BS-I threshold ersio algorithm because, whe ossible, users emloy a higher rate ad the the oerhead caused by the re-slot is reduced (a fixed amout of bits is assumed to be ecessary to idicate the BS that there is iformatio ready to be set). Worthly of ote is the fact that i the re-slot it is comulsory to use the thresholds ersio i the BS-I because all users hae to use the same rate ad so they will eed the same time to trasmit the required re-slot bits. Fig. 7. BS-II algorithm reaches almost otimal erformace. Performace of BS-II is reseted i Fig. 7, where a 5 bits re-slot has bee cosidered. BS-II outerforms BS-I excet for the high loads, where it is clear that maily bits/s hae to be used because of the high iterferece leel. Moreoer, each mobile ca idetify itself whe idicatig its urose to trasmit a acket durig re-slot, so that it allows a riority olicy betwee mobiles accordig to their serice eeds. Cosequetly, a mobile with high traffic eeds could be allocated the highest trasmissio rate comosig the otimum combiatio. A ew Adatie DS-CDMA S-ALOHA techique for acket mobile commuicatios access based o the choice of the most suitable trasmissio rate at ay time slot has bee addressed i this aer. Two kids of lowcomlexity algorithms hae bee itroduced accordig to whether the Mobile Statio or the Base Statio are i charge of them. The throughut attaied outerforms by far those obtaied with a coetioal DS-CDMA S- ALOHA scheme whe the offered load is uder the saturatio oit. I articular, the mobile assisted algorithm allows to obtai almost the maximum attaiable erformace at a ery low comlexity cost. V. REFERECES [1 D. RAYCHAUDHURI, Performace Aalysis of Radom Access Packet-Switched Code Diisio Multile Access Systems, IEEE Trasactios o Commuicatios, Vol. Com. 9, o. 6, Jue 1981, [ R. K. MORROW, J.S. LEHERT Packet Throughut i Slotted ALOHA DS/SSMA Radio Systems with Radom Sigature Sequeces, IEEE Trasactios o Commuicatios, Vol. Com. 0, o., July 199. [3 R. ROM, M. SIDI, Multile Access Protocols. Performace ad Aalysis, Sriger-Verlag, [ L. KLEIROCK, S.S. LAM, Packet Switchig i a Multi-access Broadcast Chael: Performace Ealuatio, IEEE Trasactios o Commuicatios, Vol. Com.-3, o., Aril 1975,.10-. [5 Z. LIU, M. EL ZARKI, Performace Aalysis of DS-CDMA with Slotted ALOHA Radom Access for Packet PCs, Wireless etworks 1 (1995), [6 O. SALLET, R. AGUSTÍ, A Proosal of a Adatie S-ALOHA Access System for a Mobile CDMA Eiromet, submitted for ublicatio. [7 A. SALMASI, K. S. GILHOUSE, O the System Desig Asects of CDMA Alied to Digital Cellular ad Persoal Commuicatios etworks, Proc. 1 st IEEE Vehicular Techology Coferece, May [8 A. BAIER et al., Desig Study for a CDMA-Based Third-Geeratio Mobile Radio System, IEEE Joural o Selected Areas i Commuicatios, Vol. 1, o., May 199, [9 M. J. McTIFFI et al., Mobile Access to a ATM etwork Usig a CDMA Air Iterface, IEEE Joural o Selected Areas i Commuicatios, Vol. 1, o. 5, Jue 199, [10 M. B. PURSLEY, Performace Ealuatio for Phase-Coded Sread-Sectrum Multile-Access Commuicatio - Part I: System Aalysis, IEEE Trasactios o Commuicatios, Vol. Com.-5, o. 8, Aug IV. COCLUSIOS