OFDM Modulated Cooperative Multiple-Access Channel with Network-Channel Coding

In this paper, we consider the half-duplex cooperative multiple-access channel (CMAC) with frequency-selective block-fading. Each link employs an orthogonal frequency division multiplexing (OFDM) system, where 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. To achieve this rate-diversity tradeoff, we use the principle of network coding where messages of the two sources are jointly encoded. Both separate and joint network-channel coding approaches are considered. Specifically, we design multiple turbo codes that minimize the outage probabilities of these approaches. We also give a code structure for the multiple turbo codes to achieve full diversity of the system. The codes are optimized using the extrinsic information transfer (EXIT) chart analysis with iterative decoding tailored for OFDM modulated CMAC. Numerical examples show that with our proposed design technique, the achieved frame error rate is within 0.5dB from the information outage. Without network-channel coding, the outage probability of distributed coding cannot achieve the diversity order given in the rate-diversity tradeoff.