Secrecy sum rate maximization of RIS assisted downlink NOMA system

In this paper, a method is proposed to improve the strongest user's channel gain and thus enhance the system secrecy sum rate (SSR) of the Non-orthogonal multiple access (NOMA) system using a reconfigurable intelligent surface (RIS). Based on the observation that the decoding process of NOMA system utilizes the power allocation differences among multiple users and the SSR largely depends on the strongest user's channel gain, an optimal power allocation optimization problem is constructed under quality of service (QoS) and total power constraints. Then a low-complexity relaxation based algorithm is proposed to improve the channel gain of the strongest user by optimizing the RIS phase, thereby enhancing the SSR of the system. The final simulation results show that the proposed method not only maintains good performance but also reduces computational complexity by about 2 dB.