A Cascaded Multi-IRSs Beamforming Scheme in mmWave Communication Systems

In this paper, we design a cascaded multiple intelligent reflecting surfaces (IRSs) assisted system to improve the user equipment (UE) received signal-to-noise-ratio (SNR) for mmWave wireless systems in dense blocked areas where a single IRS cannot establish visual path between the base station (BS) and UE. In our designed system, we aim to maximize the UE received SNR by optimizing the phase shifts (PSs) at cascaded IRSs and precoding vector at the BS and formulate an optimization problem. However, it is non-convex and difficult to solve. By employing the characteristics of the mmWave wireless channel, we divide the formulated problem into multiple sub-problems. After solving all the sub-problems, we obtain a near-optimal analytical solution with low computational complexity. However, in practical system, due to the hardware limitations, the PSs cannot to be infinite precision values. Therefore, we provide a n-bit discrete PS (DPS) scheme for practical deployment. The simulation results show that comparing with existing scheme, our proposed scheme has a better performance.