Capacity Enhancement for NOMA-based Coordinated Direct and Relay Transmission

In this paper, we propose a highly efficient protocol for non-orthogonal multiple access (NOMA) based coordinated direct and relay transmission (CDRT) to improve the spectral efficiency (SE) over Nakagami-m fading channel. In this scheme, the base station (BS) connects with a cell-center user (CCU) directly while communicating with a cell-edge user (CEU) via a relay and the CCU. Then, the relay and the CCU use network coding to process and retransmit the signals sent by the BS first and the CEU later. Finally, the BS and the relay simultaneously broadcast downlink signals. We derive the closed- form expressions for the ergodic sum capacity (ESC) using both perfect and imperfect successive interference cancellation (SIC). Simulations verify the correctness of our theoretical analysis and the superiority of the proposed scheme in ESC.