Intelligent reflecting surface (IRS) has emerged as a promising technique for wireless communication networks. By dynamically tuning the reflection amplitudes/phase shifts of a large number of passive elements, IRS enables flexible wireless channel control and configuration and thereby enhances the wireless signal transmission rate and reliability significantly. Despite the vast literature on designing and optimizing assorted IRS-aided wireless systems, prior works have mainly focused on enhancing wireless links with single signal reflection only by one or multiple IRSs, which may be insufficient to boost the wireless link capacity under some harsh propagation conditions (e.g., indoor environment with dense blockages/obstructions). This issue can be tackled by employing two or more IRSs to assist each wireless link and jointly exploiting their single as well as multiple signal reflections over them. However, the resultant double-/multi-IRS-aided wireless systems face more complex design issues as well as new practical challenges for implementation compared to the conventional single-IRS counterpart, in terms of IRS reflection optimization, channel acquisition, as well as IRS deployment and association/selection. As such, a new paradigm for designing multi-IRS cooperative passive beamforming and joint active/passive beam routing arises, which calls for innovative design approaches and optimization methods. In this article, we give a tutorial overview of multi-IRS-aided wireless networks, with an emphasis on addressing the new challenges due to multi-IRS signal reflection and routing. Moreover, we point out important directions worthy of research and investigation in the future.

中文翻译：

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智能反射表面辅助无线网络：从单反射到多反射设计和优化

智能反射面（IRS）已成为一种很有前途的无线通信网络技术。通过动态调整大量无源元件的反射幅度/相移，IRS 实现了灵活的无线信道控制和配置，从而显着提高了无线信号的传输速率和可靠性。尽管有大量关于设计和优化各种 IRS 辅助无线系统的文献，但之前的工作主要集中在仅通过一个或多个 IRS 增强单信号反射的无线链路，这可能不足以在一些恶劣的传播条件下提高无线链路容量（例如，有密集阻塞/障碍物的室内环境）。这个问题可以通过使用两个或多个 IRS 来协助每个无线链路并共同利用它们的单个和多个信号反射来解决。然而，与传统的单 IRS 对应物相比，由此产生的双/多 IRS 辅助无线系统在 IRS 反射优化、信道采集以及 IRS 方面面临更复杂的设计问题以及新的实施挑战。部署和关联/选择。因此，出现了一种用于设计多 IRS 协作无源波束成形和联合主动/被动波束路由的新范式，这需要创新的设计方法和优化方法。在本文中，我们给出了多 IRS 辅助无线网络的教程概述，重点是解决多 IRS 信号反射和路由带来的新挑战。此外，我们指出了未来值得研究和调查的重要方向。