Combined coding and spreading in downlink CDMA systems

Abstract

Changing from orthogonal spreading codes in a direct-sequence CDMA (DS-CDMA) system to more powerful forward error correcting convolutional codes is proven superior in an asynchronous uplink system. This approach is referred to as code-spread CDMA (CS-CDMA). In this paper these two systems are compared in a synchronous downlink scenario. Both systems are assumed to operate over channels with nonzero delay spread, making it difficult to maintain orthogonality between users at the receiver. Using synchronous transmission, the DS-CDMA system is found to outperform the CS-CDMA system at high loads on all simulated channels. However, it is found that if the CS-CDMA system is allowed to transmit different users asynchronously through independent fading channels, like in an uplink scenario, it outperforms the synchronous downlink DS-CDMA system already on channels with modest delay spread.