Reconfigurable Intelligent Surfaces for 6G: Nine Fundamental Issues and One Critical Problem

Thanks to the recent advances in metamaterials, Reconfigurable Intelligent Surface (RIS) has emerged as a promising technology for future 6G wireless communications. Benefiting from its high array gain, low cost, and low power consumption, RISs are expected to greatly enlarge signal coverage, improve system capacity, and increase energy efficiency. In this article, we systematically overview the emerging RIS technology with the focus on its key basics, nine fundamental issues, and one critical problem. Specifically, we first explain the RIS basics, including its working principles, hardware structures, and potential benefits for communications. Based on these basics, nine fundamental issues of RISs, such as What's the differences between RISs and massive MIMO? and Is RIS really intelligent?, are explicitly addressed to elaborate its technical features, distinguish it from existing technologies, and clarify some misunderstandings in the literature. Then, one critical problem of RISs is revealed that, due to the multiplicative fading effect, existing passive RISs can hardly achieve visible performance gains in many communication scenarios with strong direct links. To address this critical problem, a potential solution called active RISs is introduced, and its effectiveness is demonstrated by numerical simulations.

Automated summary

This article systematically outlines the basics, nine fundamental issues, and one critical problem of Reconfigurable Intelligent Surfaces (RIS) technology. RISs have emerged as a promising technology for future 6G wireless communications due to their high array gain, low cost, and low power consumption. However, existing passive RISs face a critical problem of limited performance gains in communication scenarios with strong direct links, which can be addressed by introducing active RISs as a potential solution.