Performance of NOMA-based coordinated direct and relay transmission using dynamic scheme

In this study, non-orthogonal multiple access (NOMA)-based coordinated direct and relay transmission (CDRT) is considered, where a base station ( BS ) directly communicates with two cell-centre users (CCUs) while communicating with a cell-edge user (CEU) via a relay. A novel dynamic scheme is proposed, in which the CCU close to the BS will act as a relay to assist the CEU in the second phase if it can obtain the prior information of the CEU in advance, otherwise it will switch to the receiving mode. To evaluate the system performance of the proposed scheme, the closed-form expressions of the outage probability, diversity order and ergodic sum rate (SR) are, respectively, derived. Simulation results demonstrate that the proposed scheme attains greater diversity order for the CEU as well as superior outage performance without performance loss of the ergodic SR, compared with conventional one in NOMA-based CDRT. In addition, a suboptimal power allocation (SPA) for this dynamic scheme is put forward, for which the closed-form expression is also obtained. It is also indicated that the presented SPA for dynamic scheme achieves near-optimal ergodic SR performance at high SNR region.