Superposition Coding for Cooperative BICM-OFDM Systems

In this paper, we study a cooperative diversity scheme for wireless systems based on symbol level superposition coding and the combination of bit-interleaved coded modulation (BICM) and orthogonal frequency division multiplexing (OFDM). The considered system comprises multiple sources, a single relay, and one common destination. The relay decodes the signal received from all sources over orthogonal channels, linearly combines the decoded symbols, and forwards the combined symbols to the destination. A simple cooperative maximum-ratio combining scheme is used at the destination which can successfully exploit the full spatial and frequency diversity offered by the channel for arbitrary numbers of sources and arbitrary linear modulation schemes. To gain insight for system design, we derive a closed-form upper bound for the asymptotic worst-case pairwise error probability and the diversity gain of the considered superposition-coded cooperative diversity (SCCD) scheme for BICM-OFDM systems. The diversity gain is a function of the free distance of the code and the frequency diversity of the involved links, but is independent of number of sources. Simulation results corroborate the derived analytical results and confirm that SCCD schemes enjoy better coding and diversity gains compared to Galois field network coding for the considered system and may be more attractive for application in wireless cooperative networks.