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.

I. Introduction

As a promising enabling technique for 5G and its beyond, non-orthogonal multiple access (NOMA) has drawn significant attention in the recent years [1]–[3]. Different from the conventional orthogonal multiple access (OMA) that exploits the time/frequency/code domain for multiple access, NOMA achieves users multiplexing in the power domain. In particular, the superposition mechanism of the NOMA signal enables multiple users to simultaneously share the same frequency band with different power levels. Meanwhile, the receivers are supposed to extract the desired components from the superposed signals by utilizing the successive interference cancelation (SIC). As a result, NOMA can offer a higher spectrum efficiency and better link density than OMA at the expense of reliability degradation incurred by inter-user interference.