Cooperative multiple access channels in OFDM systems: Diversity order analysis and code design

In this paper, we consider the half-duplex cooperative multiple-access channel (CMAC) in block-fading channels. Each link employs orthogonal frequency division multiplexing (OFDM) which is suitable for high rate wideband transmission and the modulated symbols are drawn from a finite constellation set. We first obtain the diversity order of the CMAC, as a function of the time sharing variables of the users and the rates of the codes. We also design multiple turbo codes that minimize the information outage and give a code structure for the multiple turbo codes to achieve the full diversity when the conditions for full diversity are satisfied. The codes are optimized using the extrinsic information transfer (EXIT) chart analysis, based on the convergence thresholds of the iterative decoding tailored for CMAC. Numerical examples show that with our design technique, the achieved frame error rate is within 0.8dB from the information outage. A suboptimum approach which optimizes the code of each user separately, however, is 2.7dB away from the information outage.