Evolving from massive multiple-input multiple-output (MIMO) in current 5G communications, ultra-massive MIMO emerges as a seminal technology for fulfilling more stringent requirements of future 6G communications. However, widely-utilized phased arrays relying on active components make the implementation of ultra-massive MIMO in practice increasingly prohibitive from both cost and power consumption perspectives. In contrast, the development of reconfigurable holographic surface (RHS) provides a new paradigm to solve the above issue without the need of costly hardware components. By leveraging the holographic principle, the RHS serves as an ultra-thin and lightweight surface antenna integrated with the transceiver, which is a promising alternative to phased arrays for realizing ultra-massive MIMO. In this paper, we provide a comprehensive overview of the RHS, especially the RHS-aided communication and sensing. We first describe the basic concepts of RHS, and introduce its working principle and unique practical constraints. Moreover, we show how to utilize the RHS to achieve cost-efficient and high-performance wireless communication and sensing, and introduce the key technologies. In particular, we present the implementation of RHS with a wireless communication prototype, and report the experimental measurement results based on it. Finally, we outline some open challenges and potential future directions in this area.

翻译：作为当前5G通信中大规模多输入多输出（MIMO）技术的演进，超大规模MIMO正成为满足未来6G通信更严苛要求的关键技术。然而，广泛采用的依赖有源器件的相控阵，从成本和功耗角度使得超大规模MIMO的实际部署日益困难。相比之下，可重构全息表面（RHS）的发展为解决上述问题提供了新范式，无需昂贵的硬件组件。通过利用全息原理，RHS作为一种与收发信机集成的超薄轻量化表面天线，是实现超大规模MIMO的相控阵的有前景的替代方案。本文对RHS，特别是RHS辅助的通信与感知技术进行了全面综述。我们首先描述了RHS的基本概念，并介绍了其工作原理和独特的实际约束。此外，我们展示了如何利用RHS实现高性价比、高性能的无线通信与感知，并介绍了关键技术。特别地，我们展示了基于无线通信原型系统的RHS实现，并报告了基于该原型的实验测量结果。最后，我们概述了该领域的一些开放性挑战和潜在未来方向。