CDMA communications with error correcting codes in rayleigh fading channels

Abstract

In this thesis, several attempts are made to improve the performance of the coded CDMA system in Rayleigh fading channels. First, we find that the asymptotic bit error probability of the convolutionally coded code division multiple access (CDMA) system in a Rayleigh fading channel depends on the length of the shortest error event path and the product of symbol distances along that path. Based on this observation, we propose a new spreading scheme for the convolutionally coded CDMA communication which maximizes the length of the shortest error event path by expanding the size of the spreading sequence set. It is shown from Monte Carlo simulations that the proposed scheme yields an improvement of 1.0~1.3dB at the bit error probability of $10^{-5}$ over the conventional convolutionally coded CDMA system, and even a higher improvement can be achieved as the required bit error probability is decreased. Second, we propose a multilevel coded CDMA system for a Rayleigh fading channel, which introduces the mutilevel coded modulation technique with multistage decoding into the CDMA system. The advantages of the proposed system over the conventional convolutionally coded CDMA system for a given bandwidth are making the multiple access interference smaller by using a longer spreading sequence, while achieving the same length of the shortest error event path. We show from Monte Carlo simulations that the proposed multilevel coded CDMA system attains a performance improvement of -1.0~3.5dB at the bit error probability of $10^{-5}$ over the conventional convolutionally coded CDMA system, and even a higher improvement can be achieved as the required bit error probability is decreased. Finally, we try to find out how we should combine the channel coding, spreading and modulation in order to maximize the capacity (the number of users satisfying a certain bit error probability) for a given bandwidth and a given transmitted signal power. We consider five coded CDMA schemes combi...