Recently there has been a flurry of research on the use of reconfigurable intelligent surfaces (RIS) in wireless networks to create smart radio environments. In a smart radio environment, surfaces are capable of manipulating the propagation of incident electromagnetic waves in a programmable manner to actively alter the channel realization, which turns the wireless channel into a controllable system block that can be optimized to improve overall system performance. In this article, we provide a tutorial overview of reconfigurable intelligent surfaces (RIS) for wireless communications. We describe the working principles of reconfigurable intelligent surfaces (RIS) and elaborate on different candidate implementations using metasurfaces and reflectarrays. We discuss the channel models suitable for both implementations and examine the feasibility of obtaining accurate channel estimates. Furthermore, we discuss the aspects that differentiate RIS optimization from precoding for traditional MIMO arrays highlighting both the arising challenges and the potential opportunities associated with this emerging technology. Finally, we present numerical results to illustrate the power of an RIS in shaping the key properties of a MIMO channel.

中文翻译：

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用于无线通信的可重构智能表面：原则、挑战和机遇

最近，有大量关于在无线网络中使用可重构智能表面 (RIS) 来创建智能无线电环境的研究。在智能无线电环境中，表面能够以可编程方式操纵入射电磁波的传播，以主动改变信道实现，从而将无线信道变成可控系统块，可以对其进行优化以提高整体系统性能。在本文中，我们提供了用于无线通信的可重构智能表面 (RIS) 的教程概述。我们描述了可重构智能表面 (RIS) 的工作原理，并详细说明了使用超表面和反射阵列的不同候选实现。我们讨论了适用于两种实现方式的信道模型，并检验了获得准确信道估计的可行性。此外，我们讨论了区分 RIS 优化与传统 MIMO 阵列预编码的方面，重点介绍了与这种新兴技术相关的挑战和潜在机遇。最后，我们提供数值结果来说明 RIS 在塑造 MIMO 信道的关键特性方面的能力。