Hybrid Extension of Generalized Reduced-WMMSE for Cell-Free Massive MIMO Systems

This paper proposes a hybrid analog-digital beamforming scheme for cell-free massive multiple-input multiple-output (CF-mMIMO) systems. Unlike fully-digital schemes, each AP uses a limited number of RF chains and applies analog beamforming under constant modulus constraints. The digital beamforming vectors are optimized via the weighted minimum mean square error (WMMSE) algorithm, while the analog phase shifters are updated using gradient descent. Simulation results show that the proposed scheme achieves sum-rate performance close to the fully-digital case as the number of RF chains increases, demonstrating its efficiency and hardware scalability.