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This paper considers non-orthogonal multiple access (NOMA)-based coordinated direct and relay transmission (CDRT) system, where the base station (BS) directly communicates with the nearby users while it requires the help of a dedicated relay to communicate with the cell-edge users. We derive exact closed form expression for the outage probabilities experienced by the downlink users and the system outage probability of the considered CDRT network with full-duplex relaying (FDR) technique. Further, we derive approximate closed form expressions for the ergodic rates achieved by the users. The channel of all the links experience Nakagami fading distribution and the analysis takes into account the residual interference generated due to the imperfect successive interference cancelation (I-SIC) technique. We provide numerical and simulation results to identify the impact of key system parameters on the outage and ergodic rate performance of the users and the system outage performance. The outage and ergodic rate performance of users in the considered FDR-based NOMA-CDRT system has been observed to be significantly improved compared to a FDR-based OMA (orthogonal multiple access)-CDRT system. It is observed that random selection of NOMA power allocation coefficients at the BS leads to higher outage for the near users compared to the far users. We determine numerical results for the NOMA power allocation coefficient that leads to equal outage performance for both the users. Finally, we derive analytical expression for the optimal power allocation (OPA) coefficient at the BS that minimizes the system outage probability. Through extensive numerical and simulation studies, we establish that OPA can lead to significant reduction of system outage probability compared to random selection of power allocation coefficients at the BS.

Non-orthogonal multiple access in full-duplex-based coordinated direct and relay transmission (CDRT) system: performance analysis and optimization