Capacity Enhancement of Optimized Deployment Active RISs-Assisted CF MIMO Networks

Cell-free (CF) networks, with their distributed architecture of access points, offer considerable potential for improving spectral efficiency and expanding coverage. However, the need for dense access point deployment leads to high infrastructure cost and energy consumption. This paper incorporates active reconfigurable intelligent surfaces (RISs)—a low-cost and energy-efficient technology—into cell-free multiple-input multiple-output (MIMO) systems to tackle these challenges and enhance network capacity. Unlike existing active RIS schemes, the proposed method optimizes the spatial configuration of the active elements under a fixed panel layout, harnessing element-level spatial freedom to suppress interference and improve system capacity. We establish a joint optimization framework for active element selection and precoding aimed at maximizing the weighted sum-rate (WSR). An adaptive tabu search (ATS) algorithm is applied to optimize the element topology, and a Lagrangian dual reformulation (LDR) method is introduced to handle the precoding optimization. Simulation results indicate that at a transmit power of 0dBm, the passive RIS yields only a 62.49% gain over the no-RIS baseline due to multiplicative fading, whereas the conventional active RIS achieves a 217.46% improvement and the proposed optimized deployment-active RIS further increases the gain to 269.43%; thus, our scheme delivers the most significant performance enhancement.