Joint Noisy Network Coding and Decode-Forward Relaying for Non-Orthogonal Multiple Access

We consider a non-orthogonal multiple access channel in which multiple users communicate with a base-station through a dedicated relay. For this setup, we propose a novel joint-noisy network-coded (J-NNC)-decode-and-forward (DF) cooperation scheme that employs NNC relaying with opportunistic DF. The proposed scheme utilizes the DF cooperation when messages from all users are successfully decoded at the relay and the NNC when the relay is unable to decode any one of the users. In the scenario when the relay decodes only a subset of users, the proposed scheme utilizes joint DF and NNC encoding. We derive the information theoretic conditions required for successful message recovery and show that the J-NNC-DF has a strictly better outage performance than the conventional DF-only, NNC-only, or DF-or-NNC schemes. For the two-user case, we derive closed-form expressions for the outage performance under Rayleigh fading while assuming access only to the local channel state information at the relay. For these expressions, let us choose the NNC quantization parameter so as to attain optimum outage performance. The simulation results validate the theoretical findings and show that the J-NNC-DF achieves a performance gain of 0.75, 1.7, and 2.1 dB compared with the DF-or-NNC, DF-only and NNC-only strategies, respectively.