Reconfigurable Intelligent Surfaces (RISs) are envisioned to play a key role in future wireless communications, enabling programmable radio propagation environments. They are usually considered as almost passive planar structures that operate as adjustable reflectors, giving rise to a multitude of implementation challenges, including the inherent difficulty in estimating the underlying wireless channels. In this paper, we focus on the recently conceived concept of Hybrid Reconfigurable Intelligent Surfaces (HRISs), which do not solely reflect the impinging waveform in a controllable fashion, but are also capable of sensing and processing an adjustable portion of it. We first present implementation details for this metasurface architecture and propose a convenient mathematical model for characterizing its dual operation. As an indicative application of HRISs in wireless communications, we formulate the individual channel estimation problem for the uplink of a multi-user HRIS-empowered communication system. Considering first a noise-free setting, we theoretically quantify the advantage of HRISs in notably reducing the amount of pilots needed for channel estimation, as compared to the case of purely reflective RISs. We then present closed-form expressions for the MSE performance in estimating the individual channels at the HRISs and the base station for the noisy model. Based on these derivations, we propose an automatic differentiation-based first-order optimization approach to efficiently determine the HRIS phase and power splitting configurations for minimizing the weighted sum-MSE performance. Our numerical evaluations demonstrate that HRISs do not only enable the estimation of the individual channels in HRIS-empowered communication systems, but also improve the ability to recover the cascaded channel, as compared to existing methods using passive and reflective RISs.

翻译：根据设想,未来无线通信中,可重新配置的智能表面(RIS)将在未来无线通信中发挥关键作用,使可编程的无线电传播环境成为能够调节的无线电传播环境。通常,它们被视为几乎被动的平板结构,作为可调整的反射器运作,产生许多执行挑战,包括在估计基本无线频道方面固有的困难。在本文件中,我们侧重于最近构想的混合再配置的智能表面(HRIS)概念,它不仅以可控制的方式反映波形的断裂,而且能够感测和处理其中可调整的部分。我们首先为这一元表层结构提供实施细节,并提出一个方便的数学模型,用以描述其双重操作的特性。作为无线通信中应用HRIS的指示性应用,我们为多用户的HRIS动力通信系统(HRIS)的连接设计了单个频道估算问题。首先考虑到无噪音环境,我们从理论上量化了HRIS在显著减少频道估算所需的试点数量方面的优势,但与纯粹的反射效果相比,我们首先展示了该系统内部系统的能力,然后用闭路式的表达方式来评估这些内部系统,然后在内部系统对内部系统进行内部系统进行升级分析。我们用智能分析,然后用内部系统对内部系统进行内部系统进行内部结构分析,然后在评估,我们为内部系统进行内部系统进行内部结构分析,然后在估算,然后在估算,我们用内部系统进行内部系统进行内部结构分析,在估算,我们为内部结构分析,然后为内部结构分析,然后在评估,在估算,我们为内部结构分析后算,在估算,我们为内部系统进行内部分析后算算算算。