Multiple Intelligent Reflecting Surfaces for Capacity Maximization in LOS MIMO Systems

The use of multiple intelligent reflecting surfaces (IRSs) is a promising solution to overcome the rank-deficiency of channel matrix by exploiting spatial multiplexing gains in high frequency communication. In this letter, we aim to maximize the capacity through optimizing the phase shifter matrices of multiple IRSs in pure line-of-sight multiple-input and multiple-output (MIMO) channel. However, this optimization problem is not mathematically straightforward. To tackle this challenge, we propose a new IRS-assisted beamforming scheme. First, the problem of maximizing the determinant of channel matrix is formulated. The problem is reformulated by the inequality of arithmetic and geometric means, which generates a system of equations for the phase shift values. We find the solution in a closed-form expression using the least squares method. Simulation results show that the proposed scheme can provide substantial capacity gains by increasing the rank of the channel matrix compared to the baseline schemes.