Beamforming Design for Active-RIS-Aided Cell-Free Massive MIMO Networks Under Imperfect CSI

In the pursuit of an efficient 6G network that achieves an enhanced capacity with minimal power consumption, reconfigurable intelligent surfaces (RISs) and cell-free (CF) massive multiple-input-multiple-output (MIMO) networks emerge as two key technologies. This paper investigates an active-RIS-aided CF massive MIMO system under imperfect channel state information (CSI) and proposes a two-step optimization algorithm to address the max-min achievable rate problem. Given that the original problem is non-convex, we decompose it into two subproblems, which allows us to optimize the AP transmit beamforming and the RIS reflecting precoding, respectively, in an alternating manner. Simulation results demonstrate the superiority of the proposed scheme over existing benchmarks, achieving significant performance gains in active-RIS-aided CF massive MIMO systems.