A Cascaded Multi-IRSs Beamforming Scheme in mmWave Communication Systems

Abstract

In this paper, we design a cascaded multi-IRSs (two or more IRSs) assisted system to improve the user equipment (UE) received 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 signal-to-noise-ratio (SNR) by optimizing the phase shifts (PSs) at cascaded IRSs and precoding vector at the BS and formulate an optimization problem. However, it is nonconvex 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 nearoptimal 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 system performance.INDEX TERMS intelligent reflecting surface (IRS), signal-to-noise-ratio (SNR), millimeter wave (mmWave), cascaded multi-IRSs.

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