Intelligent Reflecting Surface Aided MIMO With Cascaded LoS Links: Joint Beamforming and Array Orientation Optimization

In this paper, we study the performance limit of the cascaded line-of-sight (LoS) multiple-input-multiple-output (MIMO) system consisting of a transmitter (Tx) and a receiver (Rx) both equipped with uniform linear arrays (ULAs), and an intelligent reflecting surface (IRS) that enables communications between the Tx and the Rx through the cascaded LoS Tx-IRS-Rx link. We investigate the potential gain of the cascaded LoS MIMO system achieved by the optimization of the array orientations, especially when the Tx and the Rx are in the near-field of the IRS. Under a recently established near-field channel model, we formulate the problem of maximizing the input-output mutual information (MI) of the cascaded LoS MIMO system over active and passive beamforming as well as Tx and Rx array orientations. We give analytical solutions to the problem under asymptotic conditions, such as in the high/low signal-to-noise ratio (SNR) regime or with sufficiently large Tx-IRS and IRS-Rx distances. For non-asymptotic cases, we propose an alternating optimization method to solve the problem. We show that compared with only optimizing active and passive beamforming, the cascaded LoS MIMO system can harvest a significant MI gain from the additional optimization over the array orientations.