Cooperative NOMA for Simultaneous Uplink and Downlink Transmission.

In this paper, a cooperative non-orthogonal multiple access (NOMA) network is considered, where a strong user (SU) serves as a relay to provide the connection between the base station (BS) and a weak user (WU). A simultaneous uplink and downlink transmission strategy is designed to support the bidirectional communications between BS and users. The analytical expression of the ergodic sum rate (ESR) and its scaling law are derived to evaluate the performance of the proposed strategy. It proves that the proposed strategy can achieve an ESR scaling of $\log_{2}\rho$ , achieving $\frac{1}{2}\log_{2}\rho$ scaling gain compared with the conventional cooperative NOMA, and $\frac{1}{3}\log_{2}\rho$ scaling gain than the cooperative orthogonal multiple access and the three-phase cooperative NOMA, where $\rho$ is the transmit signal-to-noise ratio. Finally, simulation results are presented to show the superiority of our design.