Performance of Single User vs. Multiuser Modulation in Wireless Multicarrier (MC) Communications

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1 erformance of Singe User vs. Mutiuser Moduation in Wireess Muticarrier (MC) Communications Anwaru Azim, ecturer, East West University Bangadesh Abstract-- he main objective of this paper is to compare boc transmission system performance anayticay for Code Divisiona Mutipe Access (CDMA) moduations in Generaized Muticarrier environment against inear moduation techniues for both singe user and mutiuser. he effectivity of GMC-CDMA for mutiuser wi aso be judged for different Direct Seuence CDMA (DS-CDMA) such as MC-CDMA, MC-DS-CDMA, MC-SS-CDMA. he anaytica comparison wi be in terms of computing probabiity of bit error for freuency seective and sow fat fading channes for different moduation techniues. he Bit Error ate shoud be a good indication for of the performance. he toerance characteristics of DS-CDMA in freuency-seective channes and MC-CDMA in fat fading channes wi be shown anayticay and improved capacity and Bit Error ate performance wi be derived for Boc Spread Mutiuser Muticarrier by reying boc symbo spreading and ast ourier ransform () operations. GMC-CDMA shoud give guaranteed symbo recovery regardess of channe imitations. Index terms Code Division Mutipe Access (CDMA), Direct Seuence CDMA (DS-CDMA), Generaized Muticarrier CDMA (GMC- CDMA). I. INODUCION In digita wireess communication systems [1, Chapter 6], severa factors comes in to consideration to choose the suitabe digita moduation techniue. A desired moduation techniue produces ow Bit Error ates (BE) at ow received Signa-to-Noise ratios (SN), performs we in mutipath and fading conditions, occupies a minimum of bandwidth (BW) and is easy and cost effective to impement. he tradeoffs between BE performances vs. bandwidth efficiency is aways made in terms of signa fideity at ow power eves and data accommodation within a imited bandwidth.inear moduation techniues such as M-ary eying ensure bandwidth efficiency to perform for both singe user and mutiuser demands where as in mutiuser mutipe access environment, Spread Spectrum (SS) systems shows more robustness against interference and mutipath fading with many users can simutaneousy same BW without significant interference. erformance of Mutiuser Moduation techniues [1] hod great potentia in signa processing chaenges in various appications such as audio/video broadcasting, cabe networs, modem design, mutimedia services, mobie oca area networs and future generation wideband ceuar systems. II. BOCK SEAD MUICAIE MUIUSE COMMUNICAION Wireess muticarrier (MC) systems consist of mutipe compex exponentias as information carriers became effective after impementation. MC combining with direct seuence code division mutipe access (DS-CDMA) spread spectrum (SS) systems have produced various wideband ceuar communication standards. hese MC and DS-CDMA systems transmit information in bocs of symbos processed through inverse fast fourier transform (I) and they both faces inter boc interference (IBI) because of impuse response of mutipath channes. edundancy added to each transmission boc to eiminate the effects of IBI at

2 the receiver. MC and SS transmissions offer comparabe features to dea with the effects of freuency seective mutipath fading channes causing inter symbo interference (ISI), mutiuser interference (MUI) from high power near users masing user powers ocated far-away and ayeigh fading channes. As MC systems are has the orthogona features through inear time invariant (I) impementation, it mitigates the effect of MUI. But DS-CDMA systems reuire mutipath detection aiding MUI in wireess communication environment. But as an SS system, DS-CDMA spreads the information across the bandwidth (BW) which provides toerance to mutipath and freuency seective fading. Now to improve the capacity and bit error rate (BE) performance by reducing the effects of MUI and fading, MC and DS-CDMA can be combined into a generaized MC-CDMA (GMC-CDMA) communication system. he GMC-CDMA wi be MUI and ISI resiient and has high probabiity of symbo recovery at the receiver even in freuency seective mutipath fading environment with ensuring BW efficiency. ransmission in wireess environment, channe induced ISI effects the system performance severey. o aeviate ISI which aso resuts in freuency seective fading issues, information bearing chips wi be transmitted in bocs. A. Generaized Muticarrier Code Divisiona Mutipe Access (GMC-CDMA) o buid MUI and ISI toerant transceiver, et us suppose there are M users using the channes using impuse response for user, d N ( z) z h ( z N 0 where is the max. deay spread of the chip samped mutipath channes. GMC-CDMA empoys the freuency divisiona mutipe access (DMA) principes by transmitting on channes with non-overapping freuencies. It wi aso oad the same symbo to more than subcarriers that resuts in at east one subcarrier being received in adverse channe conditions. his symbo recovery without BW over-expansion is caed symbo bocing. he idea is to et each user send (1) K symbos using J K subcarriers instead of individua pacement in separate subcarrier. he users subcarriers woud have DMA standard separation in freuency domain. o eiminate MUI and ISI, each user boc of K symbos is precoded to J symbos paced on J subcarriers so that a K symbos in the boc are recoverabe from any J- of J subcarriers. or M users system, tota number of subcarriers is reuired is N=MJ and transmitting chips shoud have bocs of N. Now for the boc transmission design, the transmitted seuences are grouped in bocs of size = N+1 that incude ength of cycic prefixing or zero padding to cance out IBI. he th user s channe wi be represented as an N X N circuant matrix [ig.1]. So user s transmitting N X 1 vector woud be u ( Cs ( where C is defined as N X K spreading matrix for user. So received vector, x( C s ( ( 0 and user s boc-symbo estimate, s ( G x( where G is the K X N receiving matrix for user. his genera system mode can describe the principe operation of various DS-CDMA standards incuding combinations of MC and DS systems such as MC-CDMA, MC-DS-CDMA, M-CDMA, and MC-SS-MA. his GMC-CDMA is depicted in figure 1. Aso to present the MUI/ISI resiience of the mode, et us consider C and G for spreading and despreading matrices. ere is a J X K matrix that ineary maps the K information symbos of the nth boc ( s to J symbos s ( through N X J subcarrier matrix. he I matrix impements an ODM moduation at the fina stage. At the

receiver end, euaizes the channe precoder combination whie represents the operation on x( where decodes J symbos from subcarriers for user. igure 1.

3 receiver end, euaizes the channe precoder combination whie represents the operation on x( where decodes J symbos from subcarriers for user. igure 1. Boc Spreading System So the user s symbo estimate can be re-written as, s ( 0 s ( ( (2) Using the properties from [3], we can eiminate the effect of MUI and express the estimate as, s ( D s ( ( x ( (3) where x ( is the MUI-free vector for user. Considering the associated noise terms are white Gaussian we can concude that this GMC-CDMA design deivered the mutipath euivaent to independent parae singe user freuency seective channes with AWGN. his mode performs better with symbo detectabiity with robustness to freuency fading. or other design considerations such as boc ength K, the number of redundant symbos and the number of users M, it has to be done in terms of BW utiization. If each user sends K information symbos for =N+=MJ+ transmitted symbos. So the MK MK M ( K 1) bandwidth (BW) efficiency shoud be, 1. So arger K wi give higher BW efficiency which wi be affecting system performance. GMC-CDMA aso has the capabiity to assign variabe transmission rates to different users. As a resut, users have different number of subcarriers aocated for their symbo transmission. his mutirate feature wi enabe the transmission scheme to rate switching, better ate resoution and in genera better BE performance [5].his mutirate service is incorporated whie preserving MUI and ISI resiience. A. Mutirate Capabiities of GMC-CDMA Mutirate systems are attractive for their variabe rates for different services such as text, images, data and ow rate video which can meet different Quaity of Service (QoS) reuirements. DS-CDMA systems have the design fexibiity and improved capacity through muticarrier to provide mutirate services by choosing appropriate chip rate, variabe spreading ength, number of mutipe

4 codes and moduation methods. he receiver design for DS-CDMA systems may vary from maximum ieihood (M) decoders, conventiona matched fiters (M) or de-correating mutichanne euaizers, minimum mean suare error (MMSE) receivers, successive interference canceers, decision feedbac receivers. But in mutipath environment, these designs fai to mitigate MUI and ISI effects without high compexity and may aso cause noise enhancement. he proposed GMC-CDMA gives a receiver design which can be MUI/ISI free and it can guarantee symbo recovery in presence freuency seective mutipath fading and ayeigh fading. Such a system is shown in figure 2 [5]. igure 2. Mutirate Boc recoded CDMA System Now for user, a boc of K symbos is transmitted using chips; information rate for the user woud be K c symbos/second where C the chip/samping period is. So the tota rate MK M ( J ) ( MJ ) 0 c c woud be, (4) or mutirate design, has to come cose to the avaiabe BW 1 with a decoding deay. After reuired rate adjustment and c code design, GMC-CDMA converts a freuency seective channe to a set of fat fading channes and ineary precoded symbos on the subcarriers. As a resut, each symbo is spread on mutipe subcarriers achieving freuency diversity mitigating fading effect. he main advantage of the GMC-CDMA is that it uses mutipath diversity which comes from the boc-spreading to enabe MUI/ISI eimination by design. Now if the symbos of S are binary and iid, the BE performance of the th symbo in terms of generic vector and moment generating functions of the symbo, ISI and noise woud be [5], 1 1 symbo ISI noise, ( ), ( ), ( d (5) 2j e,, ) his BE performance is to ensure MUI/ISI resiient transmission with guaranteed channe identification abiity and symbo recovery at the receiver in freuency seective mutipath environment.

5 III. CONCUSION he proposed GMC-CDMA mode reies on symbo bocing and bocs precoding but it mae decoding computationay simpe. Its mutirate receiver design performs better than existing framewors in fading conditions through MUI eimination, bind channe estimation and I channe irrespective symbo recovery with finer rate resoution and easier rate switching capabiities. It expoits the ength of boc spreading codes at the expense of onger deays in decoding at the receiver side. But reduced compexity and increased fexibiity in the design maes GMC-CDMA idea for robust MUI/ISI free mutipath muticarrier communication. EEENCES [1].S. appaport, Wireess Commnications : rincipes and ractice, rentice-a, New Jersey, 2 nd ed., [2] Z. Wang, X. Ma and Georgios B. Giannais, ODM or Singe-Carrier Boc ransmissions?, IEEE ransactions on Communications, vo.52, No. 3, March [3] Z. Wang and Georgios B. Giannais, Wireess Muticarrier Communications: Where ourier Meets Shannon, IEEE Signa rocessing Magazine, pp , May [4] J.G. roais, Digita Communications, Mcgraw-i, New Yor, 4 th ed., [5] Z.Wang and G.B. Giannais, Boc proceding for MUI/ISI esiient generaized Muticarrier CDMA with Mutirate Capabiities, IEEE ransactions on Communications, vo.49, No. 11, Nov

Channel Division Multiple Access Based on High UWB Channel Temporal Resolution

Channel Division Multiple Access Based on High UWB Channel Temporal Resolution Channe Division Mutipe Access Based on High UWB Channe Tempora Resoution Rau L. de Lacerda Neto, Aawatif Menouni Hayar and Mérouane Debbah Institut Eurecom B.P. 93 694 Sophia-Antipois Cedex - France Emai: