Intelligent Reflecting Surfaces for MIMO Communications in LoS Environments

In line-of-sight (LoS) environments, point-to-point (P2P) multiple-input multiple-output (MIMO) channel matrix turns out to be rank deficient such that spatial multiplexing becomes unattainable. In this paper, we propose the deployment of distributed intelligent reflecting surfaces (IRSs) to act as artificial scatterers and synthesize a sort of multi-path propagation such that additional degrees of freedom are created. We show that given the far-field deployment of the IRS, it simply resembles a full-duplex relay with a single effective reflection coefficient. However, to maximize the channel capacity both the effective reflection coefficients of all IRSs and the transmit covariance matrix should be jointly optimized, which is a nonconvex optimization problem. Thus, we develop an alternating optimization algorithm to iteratively find a sub-optimal solution. Moreover, we propose different schemes to enhance the composite channel power which would result in an improvement to the achievable rate. Our simulation results show that the deployment of distributed IRSs with P2P MIMO systems in LoS environments increases the rank of the channel matrix, and improves the achievable rate by making spatial multiplexing possible.