NOMA Based Coordinated Direct and Relay Transmission: Secure Design and Performance Analysis

In this paper, we propose a coordinated direct and relay transmission scheme based on non-orthogonal multiple access (NOMA), where the relay and cell- center user operate in full-duplex mode to help enhance the quality of service of the cell-edge user as well as ensure the secure transmission. In the proposed system, the base station directly serves the cell-center user and relay while communicating with the cell-edge user through the relay and cell-center user. Taking advantage of NOMA, the cell-center user and relay are able to cancel the mutual interference between themselves. Moreover, they can also cooperatively apply the artificial noise scheme to interfere any potential eavesdropper without impairing the legitimate cell-edge user. Exact and closed-form expressions for the outage probability, achievable rate, and achievable secure rate are derived. Numerical results prove that the proposed system outperforms the existing counterpart in the low signal-to-noise ratio region, and ensures secure communications with appropriate power allocation.