Minimum probability of error detection for asynchronous uplink MC-CDMA

Abstract

Multi-Carrier Code Division Multiple Access (MC- CDMA) multiplexes several users using a spreading code in the frequency domain. Different chips of the same user travel on or- thogonal subcarriers and each subcarrier undergoes independent flat fading. As a result a single tap equalizer is required at the receiving end before forming the decision variable. Minimum Mean Square Error (MMSE) detector is a popular choice for this. In this paper we propose a Minimum Probability of Error (MPOE) detector for MC-CDMA uplink signals and compare the performance of this detector with the MMSE detector. Simulation results have been provided, which show the superiority of the MPOE detector over the MMSE detector in terms of Bit Error Rate (BER). This paper also studies system performance when complex Zhadoff-Chu sequences are used, which offers a low Crest Factor. Uplink MC-CDMA system based on the MPOE criterion. Perfomance of this detector has been compared with the MMSE detector. Theoretical and simulation results for the same have been presented. It can be seen that the MPOE de- tector outperforms the MMSE detector in terms of BER albeit at a slightly higher computational complexity when in weight adaptation mode. Once in the decision directed mode, both MMSE and MPOE detectors have linear complexity. Ways to reduce the computational complexity of the MPOE detector in the weight adaptation mode have been proposed in (7) for the downlink case and the same approach can be extended to the uplink case also. Another advantage of the MPOE detector over the MMSE detector is its improved resistance to the near far problem widely prevalent in mobile communication. We also discuss the performance of these detectors when using complex Zhadoff-Chu sequences to spread the users' data. These sequences have been shown to exhibit a lower crest factor (3), resulting in a reduced Peak to Average Power Ration (PAPR) problem, widely prevelant in FFT based schemes. Organization of the paper is as follows. Section II describes the signal model assumed. Section III derives the expression for the probability of error for MC-CDMA and using this expression, arrives at the MPOE detector. Simulation results are presented in Section IV. The conclusions are presented in Section V.