Dual-Hop Hybrid IRS-Aided Outdoor-to-Indoor mmWave Communications

In this letter, we design a dual-hop hybrid intelligent reflecting surface (IRS)-aided millimeter-wave (mmWave) system with one passive and one active IRS. In this system, we aim to maximize the user received signal-to-noise ratio (SNR) by jointly optimizing the reflecting coefficients of the reflecting elements (REs) at the cascaded IRSs and the active beamforming vector at the base station. To achieve this target, we formulate a nonlinear fractional programming problem. However, it is difficult to be solved. Since it is non-convex and all the three design variables are deeply coupled. To address this problem, we decouple all variables using an alternating optimization method and divide the original problem into three sub-problems. By solving the three sub-problems iteratively until convergence, we obtain a sub-optimal solution. Simulation results show that the proposed scheme outperforms the traditional schemes.