Reconfigurable Intelligent Surface-Assisted Massive MIMO System to Reduce Pilot Contamination

In massive multiple-input multiple-output (MIMO) systems, cellular users are affected by inter-cell coherent interference, especially pilot contamination. To solve this problem and improve spectral efficiency (SE) of each cell, we propose a reconfigurable intelligent surface (RIS)-assisted cellular massive MIMO system. Based on the uplink pilot information and minimum mean square error (MMSE) method, base stations (BSs) can estimate all the uplink channels for local decoding. Then, we give an uplink signal-to-noise-plus-interference ratio (SINR) expression and an each user's SE expression based on maximum ratio combining (MRC) processing method. The effects of pilot contamination, beamforming gain, interference, and noise on the system can be seen from the expressions. Power control and RIS phase shift optimization methods are also proposed to reduce pilot contamination. The power control method is formulated to maximize each user's rate. The phase shift optimization is also formulated to maximize the target area's average power. Numerical results show that power control method and phase shift matrix optimization can improve the system performance.