Two-user SINR Region for Reconfigurable Intelligent Surface Aided Downlink Channel

A two-user downlink network assisted by a reconfigurable intelligent surface (RIS) is considered in this paper. We aim to maximize the weighted signal to interference plus noise ratio (SINR) of the two users by jointly optimizing the precoding vectors at the base station and the reflection coefficient matrix at the RIS. For the nonconvex sum ratio optimization problem, we first formulate the upper bound of the SINR using the zero-forcing technique and then maximize it instead. The reformulated problem provides a lower bound for the optimal value of the original problem. Then we apply the alternating direction method of multipliers to deal with the constraints individually. In simulations the two-user SINR region is depicted, which shows that the new model as well as the algorithm improve the system’s quality of service compared to the state of the arts.