Full/Half Duplex Cooperative NOMA Under Imperfect Successive Interference Cancellation and Channel State Estimation Errors

This paper investigates the application of non-orthogonal multiple access (NOMA) in a coordinated direct and relay transmission (CDRT) system, where the base station (BS) directly communicates with the near (i.e., cell-centric) user, while it requires the help of a dedicated full duplex (1-D) relay to communicate with the far (i.e., cell-edge) user. For the considered NOMA-FD-CDRT system, we derive closed-form expressions for the outage probabilities and ergodic rates experienced by the downlink users, under the realistic assumptions of imperfect channel state information (I-CSI) and imperfect successive interference cancellation (I-SIC). Expressions for system outage probability and ergodic sum rate are also presented. The links are assumed to experience independent, non-identically distributed Nakagami-m fading. The analysis of outage probabilities are carried out for both integer and non-integer values of the fading severity index. Extensive simulation results are described to validate the accuracy of analytical results, and to illustrate the performance gain of the considered system, as compared to NOMA-HD-CDRT system and the orthogonal multiple access (OMA) based CDRT system. Our results show that, both channel estimation error variance and I-SIC factor have significant impact on the performance of the considered NOMA-FD/HD-CDRT system. Further, to ensure fairness in terms of outage, we numerically determine the power allocation coefficient at the BS that provides equal outage performance for both the near and the far users in the presence of I-CSI and I-SIC. Furthermore, we derive closed-form expression for the optimal power allocation (OPA) coefficient at the BS that minimizes the system outage probability of the NOMA-FD/HD-CDRT network under I-CSI. With the help of numerical and simulation investigations, we establish that the proposed OPA significantly reduces the system outage probability, as compared to random (i.e., non-optimal) power allocation at the BS.