Interference Exploitation Precoding for Reconfigurable Intelligent Surface Aided Multi-User Communications with Direct Links

In this letter, we study the multi-user multiple-input single-output (MU-MISO) transmission assisted by a reconfigurable intelligent surface (RIS). Specifically, for an arbitrary precoder employed at the base station (BS), by exploiting the concept of constructive interference (CI), we aim to design the phase shifts at the RIS such that the direct-link signals and the reflected signals can add constructively at each user side. An optimization problem is formulated to minimize the symbol error rate, where we also allow the on-off switch for the reflecting elements at the RIS. By relaxing the constant-envelope (CE) constraints, we arrive at a convex problem that can readily be solved, and a direct normalization is subsequently applied to satisfy the CE constraints. Moreover, we propose a greedy algorithm to further improve the system performance. Numerical results validate the effectiveness of the proposed scheme and demonstrate its superior performance compared to conventional schemes.