Precoding optimization for the sparse MC-CDMA downlink communication

We introduce a novel Multi-Carrier Code Division Multiple Access (MC-CDMA) system, where random sparse signatures are deployed in the frequency domain. Data symbols transmitted from base station (BS) to mobile stations (MSs) are drawn from discrete finite alphabets, such as M-QAM constellations. Transmitter-based precoding is introduced so as to allow simple despreading followed by single-user detection at MSs. A power-efficient non-linear precoding optimization problem is formulated by imposing minimum Symbol Error Probability (SEP) targets at MSs. We first elaborate on how to translate the SEP targets into exact constraint regions on noiseless received components at MSs. With relaxation on the exact regions, a tractable convex problem is obtained. A dual-decomposition-based algorithm is then developed to accommodate parallel processors to perform precoding calculation. The signature sparsity turns out to be vital to reduce interprocessor communication overhead and computational complexity for pre-coding. The scheme proposed offers considerable transmit power reduction compared with the conventional zero-forcing precoder.