Abstract
In this letter, we investigate the signal-to-interference-plus-noise-ratio
(SINR) maximization problem in a multi-user massive
multiple-input-multiple-output (massive MIMO) system enabled with multiple
reconfigurable intelligent surfaces (RISs). We examine two zero-forcing (ZF)
beamforming approaches for interference management namely BS-UE-ZF and
BS-RIS-ZF that enforce the interference to zero at the users (UEs) and the
RISs, respectively.Then, for each case, we resolve the SINR maximization
problem to find the optimal phase shifts of the elements of the RISs. Also, we
evaluate the asymptotic expressions for the optimal phase shifts and the
maximum SINRs when the number of the base station (BS) antennas tends to
infinity. We show that if the channels of the RIS elements are independent and
the number of the BS antennas tends to infinity, random phase shifts achieve
the maximum SINR using the BS-UE-ZF beamforming approach. The simulation
results illustrate that by employing the BS-RIS-ZF beamforming approach, the
asymptotic expressions of the phase shifts and maximum SINRs achieve the rate
obtained by the optimal phase shifts even for a small number of the BS
antennas.