Joint Beamforming and User Association Design in Active RIS-Aided Cell-Free URLLC System

Reconfigurable intelligent surface (RIS) is considered as a promising technology thanks to its strong ability to reflect incident signals and modify propagation properties of wireless environment. In this paper, we investigate the sum-rate maximization problem for active RIS-aided cell-free multipleinput multiple-output (CF-MIMO) system in ultra-reliable and low-latency communication (URLLC) scenario. Specifically, a two-timescale user-RIS match optimization scheme is proposed to reduce the required channel training and feedback overhead, where beamforming matrices at the remote antenna unit (RAU) and active RIS are jointly optimized under the maximum transmit power constraints. Since the problem formulated is non-convex, an alternating optimization algorithm is proposed, wherein the original optimization problem is decomposed into transmission and association subproblems that can be efficiently solved. For the former subproblem, auxiliary variables are introduced in accordance with the successive convex approximation (SCA) method to overcome the non-convexity, while for the latter subproblem, a penalty-based association scheme is proposed to tackle the non-convex discrete user-RIS match constraints. Simulation results demonstrate that the proposed algorithm outperforms the baseline schemes in terms of spectral efficiency.