Multi-layer coded direct sequence CDMA

We consider the problem of multi-user detection for randomly spread direct-sequence (DS) coded-division multiple access (CDMA) over flat fading channels. The analysis focuses on the case of many users, and large spreading sequences such that their ratio, defined as the system load, is kept fixed. Single layer and multi layer coding are analyzed in this setup. The spectral efficiency, for linear multiuser detectors, is derived for different decoding strategies. Iterative decoding of multilayered transmission with successive interference cancellation (SIC) is optimized, and the optimal layering power distribution is obtained. For small system loads, the achievable spectral efficiency with the broadcast approach and a matched filter detector exhibits significant gains over single layer coding.