A reconfigurable intelligent surface (RIS) is a planar structure that is engineered to dynamically control the electromagnetic waves. In wireless communications, RISs have recently emerged as a promising technology for realizing programmable and reconfigurable wireless propagation environments through nearly passive signal transformations. With the aid of RISs, a wireless environment becomes part of the network design parameters that are subject to optimization. In this tutorial article, we focus our attention on communication models for RISs. First, we review the communication models that are most often employed in wireless communications and networks for analyzing and optimizing RISs and elaborate on their advantages and limitations. Then, we concentrate on models for RISs that are based on inhomogeneous sheets of surface impedance and offer a step-by-step tutorial on formulating electromagnetically consistent analytical models for optimizing the surface impedance. The differences between local and global designs are discussed and analytically formulated in terms of surface power efficiency and reradiated power flux through the Poynting vector. Finally, with the aid of numerical results, we discuss how approximate global designs can be realized by using locally passive RISs with zero electrical resistance (i.e., inhomogeneous reactance boundaries with no local power amplification) even for large angles of reflection and at high power efficiency.

中文翻译：

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可重构智能表面的通信模型：从表面电磁学到无线网络优化

可重构智能表面 (RIS) 是一种平面结构，旨在动态控制电磁波。在无线通信中，RIS 最近已成为一种很有前途的技术，可通过几乎无源的信号转换来实现可编程和可重新配置的无线传播环境。在 RIS 的帮助下，无线环境成为需要优化的网络设计参数的一部分。在本教程文章中，我们将注意力集中在 RIS 的通信模型上。首先，我们回顾了无线通信和网络中最常用于分析和优化 RIS 的通信模型，并详细阐述了它们的优点和局限性。然后，我们专注于基于不均匀表面阻抗片的 RIS 模型，并提供有关制定电磁一致分析模型以优化表面​​阻抗的分步教程。在表面功率效率和通过坡印廷矢量的再辐射功率通量方面讨论和分析地制定了局部和全局设计之间的差异。最后，借助数值结果，我们讨论了如何通过使用具有零电阻的局部无源 RIS（即不均匀的电抗边界，没有局部功率放大）来实现近似全局设计，即使对于大反射角和高功率效率也是如此. 在表面功率效率和通过坡印廷矢量的再辐射功率通量方面讨论和分析地制定了局部和全局设计之间的差异。最后，借助数值结果，我们讨论了如何通过使用具有零电阻的局部无源 RIS（即不均匀的电抗边界，没有局部功率放大）来实现近似全局设计，即使对于大反射角和高功率效率也是如此. 在表面功率效率和通过坡印廷矢量的再辐射功率通量方面讨论和分析地制定了局部和全局设计之间的差异。最后，借助数值结果，我们讨论了如何通过使用具有零电阻的局部无源 RIS（即不均匀的电抗边界，没有局部功率放大）来实现近似全局设计，即使对于大反射角和高功率效率也是如此.