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.

翻译：得益于超材料的最新进展，可重构智能表面（RIS）已成为未来6G无线通信的一项有前景技术。凭借其高阵列增益、低成本与低功耗优势，RIS有望显著扩大信号覆盖范围、提升系统容量并提高能量效率。本文系统综述了新兴RIS技术，重点阐述其核心基础、九大基本问题及一个关键挑战。具体而言，我们首先阐释RIS基础，包括其工作原理、硬件结构及对通信的潜在益处。基于这些基础，本文明确探讨了九大RIS基本问题，例如"RIS与大规模MIMO有何区别？"以及"RIS是否真正智能？"，以阐述其技术特征、区分现有技术并澄清文献中的某些误解。进而揭示了RIS的一个关键挑战：由于"乘性衰落"效应，现有无源RIS在多数具有强直射链路的通信场景中难以实现显著性能增益。针对该关键问题，提出有源RIS这一潜在解决方案，并通过数值仿真验证其有效性。