A Multi-Route Cascaded IRSs Beamforming Scheme for mmWave Communication Systems

In this article, to improve the achievable sum-rate of the multi-user millimeter wave (mmWave) communication in densely blocked areas, we design a multi-route multi-hop (MRMH) cascaded intelligent reflecting surface (IRS)-aided system. In our designed system, we aim to maximize the achievable sum-rate for the multiple users by jointly optimizing the phase shifts (PSs) at the multiple IRSs and the active beamforming vector (ABV) at the base station (BS). To achieve this target, we formulate an optimization problem. However, it is non-convex and difficult to be solved. By considering the sparse scattering and near-orthogonality characteristics of the mmWave channels, we divide the optimization problem into multiple tractable sub-problems and obtain a near-optimal solution. The simulation results prove that the proposed scheme has better performance than the existing schemes. Furthermore, we prove the effectiveness of the multi-hop IRS-aided system by comparing the single-hop IRS-aided system.