[论文解读] Wireless Communications Through Reconfigurable Intelligent Surfaces

Ertuğrul Başar, Marco Di Renzo+4 more|arXiv (Cornell University)|Jun 22, 2019

Advanced Wireless Communication Technologies参考文献 93被引用 145

一句话总结

本文综述了支持可重配置智能表面的无线网络，介绍基本概念、建模、性能极限，以及与中继和背散之间的区别，辅以两射模型示例和统一的 SISO RIS 框架。

ABSTRACT

The future of mobile communications looks exciting with the potential new use cases and challenging requirements of future 6th generation (6G) and beyond wireless networks. Since the beginning of the modern era of wireless communications, the propagation medium has been perceived as a randomly behaving entity between the transmitter and the receiver, which degrades the quality of the received signal due to the uncontrollable interactions of the transmitted radio waves with the surrounding objects. The recent advent of reconfigurable intelligent surfaces in wireless communications enables, on the other hand, network operators to control the scattering, reflection, and refraction characteristics of the radio waves, by overcoming the negative effects of natural wireless propagation. Recent results have revealed that reconfigurable intelligent surfaces can effectively control the wavefront, e.g., the phase, amplitude, frequency, and even polarization, of the impinging signals without the need of complex decoding, encoding, and radio frequency processing operations. Motivated by the potential of this emerging technology, the present article is aimed to provide the readers with a detailed overview and historical perspective on state-of-the-art solutions, and to elaborate on the fundamental differences with other technologies, the most important open research issues to tackle, and the reasons why the use of reconfigurable intelligent surfaces necessitates to rethink the communication-theoretic models currently employed in wireless networks. This article also explores theoretical performance limits of reconfigurable intelligent surface-assisted communication systems using mathematical techniques and elaborates on the potential use cases of intelligent surfaces in 6G and beyond wireless networks.

研究动机与目标

• 激励使用可重配置智能表面将无线传播环境从失控转变为可控。
• 提出一个数学框架来分析 RIS 辅助链路并推导关键性能趋势与增长定律。
• 区分 RIS、中继和背散，并讨论实际设计考虑与未解决的研究问题。
• 探讨潜在的 6G/及更高场景中的用例以及 RIS 在智能射频环境中的作用。

提出的方法

• 介绍 RIS 概念并与传统传播及相关技术进行对比。
• 使用简单解析模型（两射模型和 RIS 辅助的两射模型）来说明有无 RIS 的功率尺度。
• 推导 LOS、地面反射和 RIS 辅助路径的接收功率表达式，包括多单元 RIS 增益。
• 给出统一的 SISO RIS 系统模型 r = g^T Phi h x + n 并讨论相位对齐以实现相干叠加。
• 讨论硬件实现和可重构超表面，包括反射阵列、基于 Varactor 的可调谐贴片以及软件定义表面。

实验结果

研究问题

• RQ1 在理想相位对齐下，RIS 如何将接收功率的距离衰减从 d^-4（无 RIS）转变为 d^-2（有 RIS）？
• RQ2 在尺度定律和能量效率方面，RIS、中继和背散的基本差异是什么？
• RQ3 哪些简单模型能够捕捉 RIS 对无线信道与性能的关键影响？
• RQ4 实现 RIS 及其实时相控的实际硬件方法有哪些？
• RQ5 将 RIS 集成到未来无线网络（6G 及更高）时出现哪些未解决的研究问题？

主要发现

• 单个 RIS 在理想相位对齐下可将接收功率的比例尺度从逆四次幂转为逆二次幂，在大 N 时带来显著增益。
• 总 RIS 增益在完美相位对齐下按 (N+1)^2 增长，表明可控超表面元件数量的二次幂增益。
• RIS 提供几乎被动、可软件编程、全带宽的解决方案，不依赖主动中继或背散扩散，具有与传统中继不同的路径损耗行为。
• 简单的 RIS 辅助 SISO 模型 r = g^T Phi h x + n 捕捉了多个反射路径在相位移 Phi 下的相干叠加。
• 物理实现包括超表面瓷砖、基于 Varactor 的可调谐谐振器、液晶方法，以及实现软件定义电磁响应的 HyperSurfaces。
• 两射图解展示了 RIS 如何将 LOS 与反射分量相干叠加，从而减轻对地反射的破坏作用并改善链路预算。