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

## TL;DR

This paper proposes a beamforming design for active-RIS-aided cell-free MIMO networks with imperfect CSI to improve performance in future 6G systems.

## Contribution

A two-step optimization algorithm for AP beamforming and RIS precoding in cell-free MIMO systems under imperfect CSI is proposed.

## Key findings

- The proposed algorithm outperforms existing benchmarks in active-RIS-aided cell-free MIMO systems.
- Decomposing the non-convex problem into subproblems enables efficient optimization of beamforming and precoding.

## Abstract

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.