Reconfigurable intelligent surfaces (RISs) are 2-D metasurfaces, which can intelligently manipulate electromagnetic waves by low-cost near passive reflecting elements. RIS is viewed as a potential key technology for the sixth-generation (6G) wireless communication systems mainly due to its advantages in tuning wireless signals, thus smartly controlling propagation environments. In this article, we aim at addressing channel characterization and modeling issues of RIS-assisted wireless communication systems. First, the concept, principle, and potential applications of RIS are given. An overview of RIS-based channel measurements and experiments is presented by classifying frequency bands, scenarios, system configurations, RIS constructions, experiment purposes, and channel observations. Then, RIS-based channel characteristics are studied, including reflection and transmission, the Doppler effect and multipath fading mitigation, channel reciprocity, channel hardening, rank improvement, far field, near field, and so on. RIS-based channel modeling works are investigated, including large-scale path loss models and small-scale multipath fading models. Finally, future research directions related to RIS-assisted channels are also discussed.

中文翻译：

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可重构智能表面：通道表征和建模


可重构智能表面（RIS）是二维超表面，它可以通过低成本的近无源反射元件智能地操纵电磁波。 RIS被视为第六代（6G）无线通信系统的潜在关键技术，主要是因为它在调谐无线信号方面具有优势，从而智能地控制传播环境。在本文中，我们旨在解决 RIS 辅助无线通信系统的信道表征和建模问题。首先，给出了RIS的概念、原理和潜在应用。通过对频段、场景、系统配置、RIS 结构、实验目的和信道观察进行分类，概述了基于 RIS 的信道测量和实验。然后，研究了基于RIS的信道特性，包括反射和传输、多普勒效应和多径衰落抑制、信道互易性、信道硬化、秩改进、远场、近场等。研究了基于RIS的信道建模工作，包括大规模路径损耗模型和小规模多径衰落模型。最后，还讨论了与 RIS 辅助通道相关的未来研究方向。