Current discussions on the sixth Generation (6G) of wireless communications are envisioning future networks as a unified communication, sensing, and computing platform that intelligently enables diverse services, ranging from immersive to mission critical applications. The recently conceived concept of the smart radio environment, enabled by Reconfigurable Intelligent Surfaces (RISs), contributes towards this intelligent networking trend, offering programmable propagation of information-bearing signals, which can be jointly optimized with transceiver operations. Typical RIS implementations include metasurfaces with nearly passive meta-atoms, allowing to solely reflect the incident wave in an externally controllable way. However, this purely reflective nature induces significant challenges in the RIS orchestration from the wireless network. For example, channel estimation, which is essential for coherent communications in RIS-empowered wireless networks, is quite challenging with the available RIS designs. This article introduces the concept of Hybrid reflecting and sensing RISs (HRISs), which enables metasurfaces to reflect the impinging signal in a controllable manner, while simultaneously sense a portion of it. The sensing capability of HRISs facilitates various network management functionalities, including channel estimation and localization. We discuss a hardware design for HRISs and detail a full-wave proof-of-concept. We highlight their distinctive properties in comparison to reflective RISs and active relays, and present a simulation study evaluating the HRIS capability for performing channel estimation. Future research challenges and opportunities arising from the concept of HRISs are presented.

翻译：当前关于第六代（6G）无线通信的讨论正将未来网络设想为统一通信、感知与计算平台，能够智能地支持从沉浸式到关键任务型等各种应用。由可重构智能表面（RISs）赋能提出的智能无线电环境概念，通过提供可编程的信息承载信号传播路径，并与收发器操作进行联合优化，推动了这一智能化网络发展。典型的RIS实现包含近乎无源的超原子构成的超表面，仅能以外部可控方式反射入射波。然而，这种纯反射特性给无线网络中RIS的协调带来了重大挑战。例如，在RIS赋能无线网络中，对相干通信至关重要的信道估计，在现有RIS设计下极具挑战性。本文提出混合反射与感知RIS（HRIS）概念，使超表面既能以可控方式反射入射信号，同时又能感知其中部分能量。HRIS的感知能力可支持多种网络管理功能，包括信道估计与定位。我们讨论了HRIS的硬件设计，并详细介绍了全波概念验证。通过与反射式RIS和有源中继器的对比，突出了其独特特性，并开展了评估HRIS信道估计能力的仿真研究。最后提出了HRIS概念所带来的未来研究挑战与机遇。