Impact of the spreading sequences on the performance of forward link MC-CDMA systems

We investigate the impact of the spreading sequences on the performance of forward link MC-CDMA systems. We show that between the extreme cases of uncorrelated and highly correlated channels, the spreading sequences have a great influence on the mutual interference power. This is true for Walsh-Hadamard and Fourier orthogonal sequences where the aperiodic correlation function varies drastically with respect to the spreading sequences. Thus, in this context, selecting the subset of less mutually interfering sequences for a given system load can significantly improve the system performance. Based on this observation, we propose a recursive algorithm that at a given recursion selects the sequence which has less mutual interference powers with the already selected sequences. This algorithm is shown to provide significant gains of up to 3.5 dB in terms of minimum achieved SINR when compared to bad sequence selection. This algorithm requires knowledge of the equalized channel correlations for all active users, which may be difficult to provide in practice. To overcome this difficulty, we show that by making some assumptions on the channel correlation model, one can greatly reduce the algorithm knowledge requirements without significant loss in performance.