Armour, SMD

Bian, YQ

Cooper, MA

McGeehan, JP

English

This paper focuses on the bit error rate (BER) performance of equalisation strategies for a downlink MC-CDMA based system. Simulation results utilising maximal ratio combining (MRC), orthogonal restoring combining (ORC), equal gain combining (EGC) and minimum mean square error combining (MMSEC) for a multiuser scenario are presented. In addition, we analyse the relative complexities of each scheme and discuss their suitability for implementation in future consumer product

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                          Cooper, M. A., Armour, S. M. D., Bian, Y. Q., & McGeehan, J. P. (2003).Comparison of equalisation strategies for multi-carrier CDMA. In IEEEInternational Conference on Consumer Electronics, 2003 (ICCE 2003). (Vol.1, pp. 298 - 299). Institute of Electrical and Electronics Engineers (IEEE).10.1109/ICCE.2003.1218936Link to published version (if available):10.1109/ICCE.2003.1218936Link to publication record in Explore Bristol ResearchPDF-documentUniversity of Bristol - Explore Bristol ResearchGeneral rightsThis document is made available in accordance with publisher policies. Please cite only the publishedversion using the reference above. Full terms of use are available:http://www.bristol.ac.uk/pure/about/ebr-terms.htmlTake down policyExplore Bristol Research is a digital archive and the intention is that deposited content should not beremoved. However, if you believe that this version of the work breaches copyright law please contactopen-access@bristol.ac.uk and include the following information in your message:• Your contact details• Bibliographic details for the item, including a URL• An outline of the nature of the complaintOn receipt of your message the Open Access Team will immediately investigate your claim, make aninitial judgement of the validity of the claim and, where appropriate, withdraw the item in questionfrom public view.THAM 15.7 COMPARISON OF EQUALISATION STRATEGIES FOR MULTI-CARRIER CDMA M.A. Cooper', S.M.D. Armour', Y.Q. Bian'and J.P. McGeehanl,z 'Centre for Communications Research, University of Bristol, Woodland Road, Bristol. BS8 IUB. U.K. 'Toshiba Research Europe Ltd., 32 Queen Square, Bristol. BSI 4ND. U.K. ABSTRACT This paper focuses on rhe bil error rare (BER) performance of equalisation srraregies for a downlink MC-CDMA based system. Simulurion results urilising MRC, ORC, EGC and MMSEC for a mulri-user scenario will be presenred. In addition, an analysis of rhe relnrive complexiries of each scheme is discussed as well as rheir suitability for implemenrurion in furure consumer producu. INTRODUCTION Multi-Carrier Code Division Multiple Access (MC-CDMA) technology is a possible candidate for bridging the gap between existing cellular mobile networks and fully integrated self organising ad-hoc networks through the realisation of the folllth generation (4G) standard. The support of macro-cellular as well as shorter range Wireless Local Area Network air interface standards poses a unique set of design requirements in terms of mobility, traffic density, radio propagation environments. coverage and spectrum usage, which must be accommodated by the 4G standard [I]. One suggestion for 4G technology aims to combine the Physical Layer capabilities of COFDM and CDMA through the hybrid technique of MC-CDMA. Multi-Carrier CDMA, as described in [Z], operates using two principles. Exploitation of the frequency selective nature of a widehand channel through COFDM implementation requires transmission of coded data on narrowband carriers separated in the frequency domain by a bandwidth greater than the coherence bandwidth. Carriers may he overlapped to achieve good spectral efficiency with Inter-Carrier Interference prevented by the insertion of a guard interval between each symbol in the time domain. The utilisation of a spreading code in the frequency domain results in data being transmitted over a number of sub-carriers. Tlus provides increased immunity to frequency selective fading though the copying of multiple data symbols placed in the frequency domain. Spreading through the use of orthogonal Hadamard codes provides a multi-user capability. However, frequency selective fading can destroy orthogonality between these codes, reducing performance as the number of users and delay spread increases. Different equalisation strategies with varying performance characteristics may be employed to compensate for the effects of frequency selective fadmg. EQUALISATION STRATEGIES MC-CDMA schemes employing coherent detection techniques are able to employ the equalisation and despreading techniques of Maximal Ratio Combining (MRC). Orthogonal Restoring Combining (ORC), Equal Gain Combining (EGC) and Minimum Mean Square Error Combining (MMSEC), often in tandem with an iterative technique such as the LMS or RLS algorithms. MRC provides each sub-carrier branch with a combining weight in proportion to its signal-to-noise (SNR) power ratio with a strong signal being given a greater weight than a weak signal. These weights are subsequently utilised in the summation. Co-phasing is also required in order to align the phases of each sub-carrier and avoid signal cancellation when summation occurs. EGC is identical to MRC except that a gain of unity is applied to each sub-carrier irrespective of the received SNR. Consequently, signals received with high SNRs are not fully utilised in bit decision calculations. ORC achieves channel compensation by deriving the complex conjugate of the channel gain which is then applied to provide phase cancellation. Consequently, sub-carriers with a low signal power level are compensated for by use of a large gain. MMSEC provides equalisation coefficients designed to minimise the mean square value of the error. SOFTWARE SIMULATIONS A software simulation using BPSK modulation (as shown in Figure 1) was designed and built to investigate the BER performance of the equalisation schemes detailed above. Forward Error Correction was implemented though the application of a % rate convolutional code with constraint length 7 [133,171]0a,, along with the subsequent utilisation of soft decision Viterbi decoding. Simulations are conducted for a quasi-stationary Rayleigh fading wideband channel where it can be assumed that a number of OFDM symbols are transmitted within the coherence time of the channel. The channel is modelled by a 16 tap Rayleigh fading profile with a maximum delay spread of 366211s and an rm delay spread of 65511s. representing an indoor to outdoor scenario based on channel 4 of [41. A bandwidth of 4.096MHe is assumed which leads to a sampling time and tap spacing of 444ns .  0-7803-7721403 $17.00 0 2003 IEEE 298 A processing gain is acheved by spreamng in the frequency domain by a factor > 1. As such, under the parameter of SC sub-caniers wluch is defined by the size of the FFT used in the OFDM modulation, it is possible to have a spreading code of length M for the transmission of P data bits in one OFDM symbol where SC = M x P . A spreading factor of M makes it possible to support M users through the use of orthogonal Hadamard codes. The spread data bits corresponding to identical coded data bits aTe block interleaved over the sub-caniers at a spacing of M sub-carriers, in order to reduce the effects of correlated fading on adjacent sub-carriers. SIMULATION RESULTS Downlink transmission simulations canied out to investigate the effects of multi-user interference on MRC, EGC and ORC are shown in Figure 2 with parameters SC = 64; M = 8; P = 8 where perfect CSI is assumed. The slightly superior perfoirmance of MRC over EGC for the single user case is achieved due to the pre-detection weighting of each sub-canier with respect to its SNR. ORC suffers from severe noise amplification in the sub-carriers with a low SNR leading to an increase in the overall BER. In cases where the maximum number of users are supported, ORC is seen to produce superior performance over MRC by its ability to maintain orlhogonality between the k users in a frequency selective environment, and hence will never suffer an error floor. Comprehensive simulations canied out have revealed that ORC is able to eliminate multi-user interference with a penalty paid for the noise enhancement effects whilst avoiding an error floor. This concurs with 131. MRC and EGC both succumb to an error floor in the multi-user case due to the loss of orthogonality between users. Here, EGC performs better than MRC due to the greater interference amplification present in MRC which compounds the effect of the loss of orthogonality. Due to the ability of ORC to maintain orthogonality at high values of EbINo, ORC will out-perform EGC in a maximum multi-user scenario. 10 16 20 21 Jo EWND ldS, Figure 2: BERperformoncr compodson of eq~olmzlion techniques using SC = M, M = 8; P = 8fora  "ingle ondmarimum u s ~ r s c c ~ n d  CONCLUSIONS Simulated results show the effects of frequency diversity on equalisation schemes for a MC-CDMA based system. Comprehensive results for these schemes as well as MMSEC along with a time domain least squares method will be presented showing multi-user performance, along with a complexity evaluation review. REFERENCES [l] J.M. Perelm, "Founh Generation: Now i t  is Personal!.. PIMRC ZWO. Va1.2,pp. 1009-1016 [ZI N. Ye, J:P. Linnm,  "Multi-Carrier CDMA in lndoor Wireless Radio NelworW Confcrencc Praeedmgs, IMRC 1993. pp. 109-113 131 S. Haru, R. Prarad. "Overview of Multicarrier C D M A  EEE CommunicarionsMagazine, Decembcr 1997. pp. 126-133 [41 ETSUSMGISMGZ, ' l h e  ETSI UMTS Tcmluial Radio ACCCSS (UTRA) Im-R R T I  Candidate Submission" ETSI Proposal for IMT-ZOW 299 

Founh Generation: Now it is Personal!.. PIMRC ZWO.

Comparison of equalisation strategies for multi-carrier CDMA

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Comparison of equalisation strategies for multi-carrier CDMA

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