Optimality of Separate Network-Channel Coding

The encoding and decoding of network coding at network layer and channel coding at the physical layer can be performed either jointly or separately. The largest achievable rate region is obtained when both are done jointly. Our interest in this paper lies in investigating the optimality of separate encoding and joint decoding of network-channel (NC) code (SJNC) assuming an orthogonal access scheme. Our key result is that SJNC is optimal as it achieves the capacity region of JJNC. However, SJNC is suboptimal if time sharing of codes is not allowed, or if separate decoding is used. We then apply these results to the cooperative multiple-access channel (CMAC) where time-sharing of codes is practically infeasible. Optimality of SJNC is compared to JJNC in terms of outage region. The conditions for full diversity order of orthogonal frequency division multiplexing (OFDM) modulated CMAC with JJNC and JSNC are then derived. Using numerical examples, multiple turbo codes are designed to show that the achieved frame error rate (FER) is within 0.6dB from the information outage.