Recent advances in the fabrication and experimentation of Reconfigurable
Intelligent Surfaces (RISs) have motivated the concept of the smart radio
environment, according to which the propagation of information-bearing
waveforms in the wireless medium is amenable to programmability. Although the
vast majority of recent experimental research on RIS-empowered wireless
communications gravitates around narrowband beamforming in quasi-free space,
RISs are foreseen to revolutionize wideband wireless connectivity in dense
urban as well as indoor scenarios, which are usually characterized as strongly
reverberant environments exhibiting severe multipath conditions. In this
article, capitalizing on recent physics-driven experimental explorations of
RIS-empowered wave propagation control in complex scattering cavities, we
identify the potential of the spatiotemporal control offered by RISs to boost
wireless communications in rich scattering channels via two case studies.
First, an RIS is deployed to shape the multipath channel impulse response,
which is shown to enable higher achievable communication rates. Second, the
RIS-tunable propagation environment is leveraged as an analog multiplexer to
localize non-cooperative objects using wave fingerprints, even when they are
outside the line of sight. Future research challenges and opportunities in the
algorithmic design and experimentation of smart rich scattering wireless
environments enabled by RISs for sixth Generation (6G) wireless communications
are discussed.

本文介绍了可重构智能表面（RIS）在改善复杂多路径环境下的宽带无线连接中的应用，提出了两种案例研究，即通过控制多径信道脉冲响应来提高通信速率和利用 RIS 调谐的传播环境作为模拟复用器进行物体定位，同时讨论了 6G 无线通信中由 RIS 实现的智能场景中的算法设计和实验的未来挑战和机遇。

智能可重构表面用于丰富散射无线通信：近期实验、挑战和机遇

Reconfigurable Intelligent Surfaces for Rich Scattering Wireless  Communications: Recent Experiments, Challenges, and Opportunities

Wireless communication in the TeraHertz band (0.1--10 THz) is envisioned as
one of the key enabling technologies for the future sixth generation (6G)
wireless communication systems scaled up beyond massive multiple input multiple
output (Massive-MIMO) technology. However, very high propagation attenuations
and molecular absorptions of THz frequencies often limit the signal
transmission distance and coverage range. Benefited from the recent
breakthrough on the reconfigurable intelligent surfaces (RIS) for realizing
smart radio propagation environment, we propose a novel hybrid beamforming
scheme for the multi-hop RIS-assisted communication networks to improve the
coverage range at THz-band frequencies. Particularly, multiple passive and
controllable RISs are deployed to assist the transmissions between the base
station (BS) and multiple single-antenna users. We investigate the joint design
of digital beamforming matrix at the BS and analog beamforming matrices at the
RISs, by leveraging the recent advances in deep reinforcement learning (DRL) to
combat the propagation loss. To improve the convergence of the proposed
DRL-based algorithm, two algorithms are then designed to initialize the digital
beamforming and the analog beamforming matrices utilizing the alternating
optimization technique. Simulation results show that our proposed scheme is
able to improve 50\% more coverage range of THz communications compared with
the benchmarks. Furthermore, it is also shown that our proposed DRL-based
method is a state-of-the-art method to solve the NP-hard beamforming problem,
especially when the signals at RIS-assisted THz communication networks
experience multiple hops.

本文提出了一种基于可重构智能表面的多跳信道下的 THz 通信网络混合波束赋形方案，使用近期的强化学习技术，通过协同设计基站和可重构智能表面的数字波束和模拟波束矩阵，提高了 THz 通信的覆盖范围，并证明了该方案优于传统的基于多跳的波束赋形方案。

多跳 RIS 强化太赫兹通信：基于 DRL 的混合波束成形设计

Multi-hop RIS-Empowered Terahertz Communications: A DRL-based Hybrid  Beamforming Design

While fifth-generation (5G) communications are being rolled out worldwide,
sixth-generation (6G) communications have attracted much attention from both
the industry and the academia. Compared with 5G, 6G will have a wider frequency
band, higher transmission rate, spectrum efficiency, greater connection
capacity, shorter delay, broader coverage, and more robust anti-interference
capability to satisfy various network requirements. This survey presents an
insightful understanding of 6G wireless communications by introducing
requirements, features, critical technologies, challenges, and applications.
First, we give an overview of 6G from perspectives of technologies, security
and privacy, and applications. Subsequently, we introduce various 6G
technologies and their existing challenges in detail, e.g., artificial
intelligence (AI), intelligent surfaces, THz, space-air-ground-sea integrated
network, cell-free massive MIMO, etc. Because of these technologies, 6G is
expected to outperform existing wireless communication systems regarding the
transmission rate, latency, global coverage, etc. Next, we discuss security and
privacy techniques that can be applied to protect data in 6G. Since edge
devices are expected to gain popularity soon, the vast amount of generated data
and frequent data exchange make the leakage of data easily. Finally, we predict
real-world applications built on the technologies and features of 6G; for
example, smart healthcare, smart city, and smart manufacturing will be
implemented by taking advantage of AI.

本篇综述了 6G 无线通信的要求、特征、重要技术、挑战和应用，介绍新技术发展及其在智能表面、THz、空地海一体化网络、无蜂窝大规模 MIMO 等方面的应用。同时，细述了其在传输速率、时延、全球覆盖面等方面的卓越表现，最后预测 AI、智慧医疗、智慧城市和智能制造等实际应用的实现。