Abstract
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 (FD) 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.
Highlights
Non-orthogonal multiple access (NOMA) technique has been proposed for improving the efficiency of channel access in the fifth generation (5G) wireless networks
Both half duplex (HD) and full duplex (FD) systems have to achieve the same target rates; the threshold signalto-interference plus noise ratio (SINR) requirement is higher for HD systems, which makes Pout of user 1 (U1) and user 2 (U2) to be higher in non-orthogonal multiple access (NOMA)-HD-coordinated direct and relay transmission (CDRT) system
NOMA-HD-CDRT network suffers from outage floor under imperfect channel state information (I-CSI)/imperfect successive interference cancellation (I-successive interference cancellation (SIC)) alone, due to the interference generated by I-SIC, while outage floor is not observed under perfect channel state information (P-CSI)/P-SIC
Summary
Non-orthogonal multiple access (NOMA) technique has been proposed for improving the efficiency of channel access in the fifth generation (5G) wireless networks. To the best of our knowledge, this is the first paper that considers the impact of I-CSI and I-SIC together, on the outage and ergodic rate performance of NOMA-FD/HD-CDRT system, in the presence of RSI. In [24], the authors have analyzed the outage probabilities and ergodic rates of the users in an EH based FD C-NOMA system, assuming P-CSI/P-SIC. The outage and ergodic sum rate of NOMA-FD-CDRT system has been analyzed in [26], in Nakagami fading channels under P-CSI/P-SIC. The authors of [34] have investigated the joint effects of residual hardware impairments, channel estimation errors and I-SIC on the outage and ergodic rate performance of EH based C-NOMA system, assuming Nakagami fading channels.
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