Distributed Intelligent Reflecting Surfaces-Aided Communication System: Analysis and Design

Intelligent reflecting surface (IRS) is a promising solution to build a programmable wireless environment with reconfigurable passive elements, which can achieve high spectral and energy efficiency. In this paper, we investigate a distributed IRS-assisted multi-user communication system. Different from most previous works relying on instantaneous channel state information (CSI) for beamforming design, this paper proposes a low-complexity two-timescale beamforming scheme using both statistical CSI (S-CSI) and instantaneous CSI (I-CSI), where closed-form expressions of IRS phase shifts and the base station (BS) beamforming vectors are obtained. Moreover, a closed-form expression of the achievable rate is derived, which enables the evaluation of the impact of key parameters on system performance. The analytical results show that the achievable rate can be significantly improved by increasing the number of IRS elements. In addition, the performance of the distributed IRS-assisted multi-user communication system is better than that of the centralized IRS-assisted system. Numerical results show that the beamforming algorithm proposed in this paper is superior to the beamforming algorithm using I-CSI under low and moderate user rate requirements. In addition, the phase shift design scheme has almost the same performance as the genetic algorithm based on S-CSI, but with much lower complexity. Finally, a power allocation scheme utilized S-CSI has been proposed to improve the system performance.