Adaptive assignment of binary user spreading codes in DS-CDMA systems

In direct-sequence code-division-multiple-access (DS-CDMA) systems, the pre-detection signal-to-interference-plus-noise ratio (SINR) at the output of the single-user minimum-mean-square-error (MMSE) filter is a function of the specific user spreading code (signature). In this paper, we consider the adaptive optimization of the user signature assignment such that the output SINR of the MMSE filter is maximized under a transmitter power constraint. In the context of binary signatures, the complexity of the signature optimization procedure is exponential in the processing gain. A low-cost suboptimum adaptive binary signature assignment algorithm is derived based on conditional optimization principles. We use this algorithm to design an efficient system-wide multiuser adaptive signature set assignment scheme. The performance of the proposed scheme is evaluated under asynchronous multipath fading DS-CDMA channel models and is compared to the performance of systems with arbitrarily chosen signature sets.