Intelligent reflecting surface (IRS) is an emerging technology to enhance the spectral and energy efficiency of wireless communications cost-effectively. This letter considers a new multi-IRS aided wireless network where a cascaded line-of-sight (LoS) link is established between the base station (BS) and a remote user by leveraging the multi-hop signal reflection of selected IRSs. As compared to the conventional single-/double-hop IRS system, multi-hop IRS system provides more pronounced path diversity and cooperative passive beamforming gains, especially in the environment with dense obstacles. However, a more challenging joint active/passive beamforming and multi-hop beam routing problem also arises for maximizing the end-to-end channel gain. Furthermore, the number of IRS-associated channel coefficients increases drastically with the number of IRS hops. To tackle the above issues, in this letter we propose a new and efficient beam training based solution by considering the use of practical codebook-based BS/IRS active/passive beamforming without the need of explicit channel estimation. Instead of exhaustively or sequentially searching over all combinations of active and passive beam patterns for each beam route, a distributed beam training scheme is proposed to reduce the complexity, by exploiting the (nearly) time-invariant BS-IRS and inter-IRS channels and the cooperative training among the BS and IRSs’ controllers. Simulation results show that our proposed design achieves the end-to-end channel gain close to that of the sequential beam search, but at a much lower training overhead and complexity.

中文翻译：

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用于智能反射表面启用多跳路由的分布式波束训练


智能反射面（IRS）是一项新兴技术，可经济有效地提高无线通信的频谱和能源效率。这封信考虑了一种新的多 IRS 辅助无线网络，其中通过利用所选 IRS 的多跳信号反射，在基站 (BS) 和远程用户之间建立级联视距 (LoS) 链路。与传统的单跳/双跳IRS系统相比，多跳IRS系统提供更显着的路径分集和协作无源波束形成增益，尤其是在障碍物密集的环境中。然而，为了最大化端到端信道增益，还出现了更具挑战性的联合主动/被动波束成形和多跳波束路由问题。此外，IRS相关信道系数的数量随着IRS跳数的增加而急剧增加。为了解决上述问题，在这封信中，我们提出了一种新的、高效的基于波束训练的解决方案，考虑使用实用的基于码本的 BS/IRS 有源/无源波束成形，而不需要显式的信道估计。提出了一种分布式波束训练方案，通过利用（几乎）时不变的 BS-IRS 和 IRS 间信道来降低复杂性，而不是详尽地或顺序地搜索每个波束路径的有源和无源波束方向图的所有组合。 BS 和 IRS 管制员之间的合作培训。仿真结果表明，我们提出的设计实现了接近顺序波束搜索的端到端信道增益，但训练开销和复杂性要低得多。