The latest discussions on the upcoming sixth Generation (6G) of wireless communications are envisioning future networks as a unified communications, sensing, and computing platform. The recently conceived concept of the smart radio environment, enabled by Reconfigurable Intelligent Surfaces (RISs), contributes towards this vision offering programmable propagation of information-bearing signals. Typical RIS implementations include metasurfaces with almost passive unit elements capable of reflecting their incident waves in controllable ways. However, this solely reflective operation induces significant challenges for the RIS optimization from the wireless network orchestrator. For example, RISs lack information to locally tune their reflection pattern, which can only be acquired by other network entities, and then shared with the RIS controller. Furthermore, channel estimation, which is essential for coherent RIS-empowered communications, is challenging with the available RIS designs. This article reviews the emerging concept of Hybrid reflecting and sensing RISs (HRISs), which enables metasurfaces to reflect the impinging signal in a controllable manner, while simultaneously sensing a portion of it. The sensing capability of HRISs facilitates various network management functionalities, including channel parameter estimation and localization, while giving rise to potentially computationally autonomous and self-configuring metasurfaces. We discuss a hardware design for HRISs and detail a full-wave electromagnetic proof of concept. The distinctive properties of HRISs, in comparison to their solely reflective counterparts, are highlighted and a simulation study evaluating their capability for performing full and parametric channel estimation is presented. Future research challenges and opportunities arising from the HRIS concept are also included.

翻译：关于即将到来的第六代（6G）无线通信的最新讨论，将未来网络设想为通信、感知与计算一体化的平台。最近提出的由可重构智能超表面（RIS）赋能的新型智能无线电环境概念，通过实现信息携带信号的可编程传播，为这一愿景做出了贡献。典型的RIS实现采用近乎无源单元的超表面，能够以可控方式反射入射波。然而，这种纯反射操作给无线网络协调器的RIS优化带来了重大挑战。例如，RIS缺乏信息来本地调整其反射模式，而这些信息只能由其他网络实体获取，然后与RIS控制器共享。此外，对于RIS赋能通信至关重要的信道估计，在现有RIS设计中也难以实现。本文综述了混合反射与感知RIS（HRIS）这一新兴概念，它使超表面能够以可控方式反射入射信号，同时感知其中的一部分。HRIS的感知能力有助于实现各种网络管理功能，包括信道参数估计与定位，同时催生了可能具有计算自主性和自配置能力的超表面。我们讨论了HRIS的硬件设计，并详细介绍了全波电磁概念验证。与纯反射型RIS相比，突出了HRIS的独特特性，并展示了评估其执行全参数信道估计和参数化信道估计能力的仿真研究。最后还包含了HRIS概念带来的未来研究挑战与机遇。