Sum Secrecy Rate Maximization for Active RIS-Assisted Uplink SIMO-NOMA Networks

In this letter, we investigate the physical-layer security of an active reconfigurable intelligent surface (ARIS)-assisted uplink single-input multi-output non-orthogonal multiple access (SIMO-NOMA) system, where multiple users transmit their information to a base station (BS) with the help of an ARIS in the presence of multiple eavesdroppers (Eves). We formulate an optimization problem targeted at maximizing the sum secrecy rate (SSR) by jointly designing receive beamforming vector at BS, ARIS reflecting coefficients, and transmit power of each user subject to the amplification power budget and individual maximum transmit power. To address this problem, we propose a block coordinate descent (BCD) method to decouple the formulated problem into multiple sub-problems and iteratively optimize one set of variables with the other set being fixed. Specifically, an optimal closed-form solution for the optimal receive beamforming vector is derived, and an efficient solution for the ARIS reflecting coefficients is obtained by a successive convex approximation (SCA) algorithm, while the optimal transmit power of all users is obtained by a customized iterative algorithm. Simulation results reveal that the proposed ARIS-assisted uplink NOMA (ARIS-NOMA) scheme outperforms the passive reconfigurable intelligent surface (PRIS)-based schemes and orthogonal multiple access (OMA)-based schemes in terms of the SSR.