Spatially Correlated Rayleigh Fading Characteristics of RIS-Aided mmWave MIMO Communications

Reconfigurable intelligent surface (RIS) is a promising solution that can reconfigure a mmWave MIMO channel at low cost. This letter first investigates the improvement caused by multiple RISs on the rank and the condition number of the mmWave MIMO channel matrix. Then it demonstrates that as the number of RISs increases, the mmWave channel can be characterized by a spatially correlated Rayleigh channel, which has a rich scattering environment as a traditional microwave channel. Finally, the simulation results verify the derived closed-form expressions for the variance of the equivalent Rayleigh fading channel matrix with random or optimized RIS phase shifts.