Intelligent reflecting surface (IRS) has emerged as an enabling technology to achieve smart and reconfigurable wireless communication environment cost-effectively. Prior works on IRS mainly consider its passive beamforming design and performance optimization without the inter-IRS signal reflection, which thus do not unveil the full potential of multi-IRS assisted wireless networks. In this paper, we study a double-IRS assisted multi-user communication system with the cooperative passive beamforming design that captures the multiplicative beamforming gain from the inter-IRS channel. Under the general channel setup with the co-existence of both double- and single-reflection links, we jointly optimize the (active) receive beamforming at the base station (BS) and the cooperative (passive) reflect beamforming at the two distributed IRSs (deployed near the BS and users, respectively) to maximize the minimum signal-to-interference-plus-noise ratio (SINR) of all users. Moreover, for the single-user and multi-user setups, we analytically show the superior performance of the double-IRS cooperative system over the conventional single-IRS system in terms of the maximum signal-to-noise ratio (SNR) and multi-user effective channel rank, respectively. Simulation results validate our analytical results and show the practical advantages of the proposed double-IRS system with cooperative passive beamforming designs.

中文翻译：

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双 IRS 辅助多用户 MIMO：协同无源波束成形设计

智能反射面 (IRS) 已成为一种能够经济高效地实现智能和可重构无线通信环境的使能技术。先前关于 IRS 的工作主要考虑其无源波束成形设计和性能优化，没有 IRS 间信号反射，因此没有充分发挥多 IRS 辅助无线网络的潜力。在本文中，我们研究了一个双 IRS 辅助多用户通信系统协作无源波束成形设计，可从 IRS 间通道捕获倍增波束成形增益。在双反射和单反射链路共存的一般信道设置下，我们联合优化基站 (BS) 的（主动）接收波束成形和两个分布式 IRS 的协作（无源）反射波束成形（分别部署在 BS 和用户附近）以最大化所有用户的最小信号干扰加噪声比（SINR）。此外，对于单用户和多用户设置，我们在最大信噪比 (SNR) 和多用户设置方面分析表明双 IRS 协作系统优于传统的单 IRS 系统。分别为用户有效信道等级。