RIS-Enabled Robust Signal Focusing and Nulling Against Angular Mismatch

In this paper, we propose a reconfigurable intelligent surface (RIS)-enabled signal focusing and nulling that is robust against angular mismatch. To our best knowledge, this is the first effort to control the shape of both mainlobe and null in RIS-aided systems. The proposed algorithm has two main objectives, where the first objective is to deliver the signal t o t he receiver of interest (RoI), whose direction is inaccurately given. The first objective is achieved by maximizing the sum of reflect beam gains in the angular region where RoI may exist. The other objective is to null the signal towards the receiver of uninterest (RoU), and this is achieved by suppressing every beam gain in the angular region where RoU may exist. However, the resulting optimization equation is unsolvable since it aims to maximize the sum of beam gains, which is convex. To solve this issue, the semi-definite programming (SDP) relaxation converts the optimization equation to a solvable SDP. Simulation results demonstrate that the proposed algorithm has an excellent signal focusing and nulling capability under angular mismatch.