Hybrid Beamforming Design for Multi-Functional RIS-Aided mmWave MIMO Systems

This paper delves into the hybrid beamforming problem in a millimeter-wave (mmWave) multiple-input multiple-output network aided by a multi-functional reconfigurable intelligent surface (MF-RIS). The goal is to overcome the path loss and blockage issues in mmWave networks. Unlike conventional RISs, our MF-RIS can reflect, refract, and amplify signals at the same time. By jointly optimizing the transmit beamforming, MF-RIS coefficients, and hybrid precoding matrices, we propose an iterative algorithm to maximize downlink spectral efficiency (SE), leveraging sequential rank-one convex relaxation and least square methods. Simulation results show the superior performance of MF-RIS over conventional RIS under various parameter settings, highlighting its potential to significantly enhance mmWave network effectiveness.