Robust Beamforming for RIS-Assisted NOMA Systems With CSI Imperfection and Low-Resolution Phase Shifters

In this paper, we investigate the robust beamforming design for reconfigurable intelligent surface (RIS) assisted non-orthogonal multiple access (NOMA) communication systems with imperfect channel state information (CSI). More specifically, we propose to perform the joint design of base station (BS) active beamforming and RIS passive beamforming, by minimizing the total transmit power while guaranteeing the worst-case signal-to-interference-plus-noise ratio (SINR) of each user under the assumption of bounded CSI error. This result in a quadratic programming problem with unit-modulus and SINR constraints is non-convex and challenging to solve. Towards that end, the original problem is decoupled into two sub-problems, such that we can tackle the problem under the alternating optimization (AO) framework. Further, the design with low resolution of RIS phase shifters is also addressed. Numerical results are provided to demonstrate the effectiveness of the proposed algorithm.