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Digital metasurfaces based on space‐time coding have established themselves as a powerful and versatile platform for joint spatial/spectral control of electromagnetic waves. However, their advanced design remains a largely open problem with significant computational challenges. This study introduces a novel approach, based on deep neural networks, to address this challenge. The proposed technique enables the simultaneous and independent multi‐frequency synthesis of scattering patterns, allowing precise tailoring of the harmonic equivalent currents (both in magnitude and phase), and enhancing spectral efficiency. These results, experimentally validated at X‐band microwave frequencies, substantially broaden the capabilities of space‐time coding digital metasurfaces, paving the way for advanced applications in wireless communications, sensing, and imaging. [ABSTRACT FROM AUTHOR]

Summary The performance of millimeter‐wave (mmWave) multiple‐input multiple‐output (MIMO) systems has been significantly enhanced by the incorporation of dynamic reconfigurable intelligent surfaces (RIS). This paper proposes a novel dynamic channel estimation technique that combines dynamic atomic norm minimization with dynamic RIS to optimize RIS‐aided mmWave MIMO systems. Leveraging the dynamic nature of both atomic norm minimization and RIS, the proposed approach efficiently adapts to changing environmental conditions, providing robust and accurate channel estimation. By dynamically optimizing the RIS configuration, the system achieves improved spectral and energy efficiency, enabling high‐speed and reliable communication in challenging mmWave environments. Theoretical analysis and simulation results demonstrate the effectiveness of the proposed dynamic channel estimation technique, highlighting its potential for enhancing the performance of future wireless communication systems. [ABSTRACT FROM AUTHOR]

The sixth generation (6G) wireless network is envisioned to provide faster and more reliable communication by utilizing electromagnetic waves in the terahertz spectrum. However, terahertz waves suffer from high propagation loss and, hence, wavefront manipulation solutions to focus and redirect terahertz waves are of paramount importance. Reconfigurable intelligent surface (RIS) is one such promising dynamic wavefront engineering solution proposed for 6G networks, but the technological realization of 6G (>100 GHz) RIS is still largely lacking. Here, spatially‐selective vanadium dioxide (VO2) patches incorporating coding metasurfaces for dynamic beam steering of terahertz waves are presented. The reversible and abrupt insulator‐to‐metal phase transition property of VO2 is exploited to achieve a reconfigurable transmission angle for the normal incident terahertz wave. By designing metasurfaces with specific linear phase gradients and designer phase sequencing, electrically switchable beam steering is achieved from co‐polarized 0° (normal transmission) in the OFF state to cross‐polarized 0° (control), 11°, 23°, 36°, and 52° in the ON state at 0.6 THz. Scaling up the proposed concept to achieve individual control of each active element (VO2) within the metasurface will enable a complete 6G RIS solution for full 3D terahertz beam manipulation to support the future THz wireless communication networks. [ABSTRACT FROM AUTHOR]

Recently, reconfigurable intelligent surfaces (RISs) have attracted extensive attentions from the communication community due to their powerful wavefront manipulation and signal modulation abilities. Optically transparent RISs are needed in some scenarios requiring both optical transparency and high‐quality communication services. In RIS‐based wireless communications, the channel reciprocity is an important factor to be considered since the electromagnetic responses of RISs usually depend on the incident and receiving angles. To address this problem, an optically transparent 2‐bit RIS with low angular sensitivity is proposed. By applying a transparent dielectric substrate and metal‐mesh‐based patterns, the designed RIS achieves a light transmittance of 49.3%. Both the simulation and measurement results demonstrate the low angular sensitivity at transverse magnetic incidence from 0° to 60°. Moreover, beam steering experiments under various coding sequences and various incident angles are simulated and measured, and the results are consistent with the theoretically predicted results. A further study on the broadcast mode of the RIS shows that its phase response is weakly dependent to the incident and receiving directions. [ABSTRACT FROM AUTHOR]

Summary: In practice, hardware impairments (HIs) often exist in wireless devices, especially in low‐cost device systems. However, previous reports usually ignored HIs due to the computational complexity in mathematical derivations, particularly in millimeter‐wave (mmWave) communications. This article proposes the usage of transmit antenna selection (TAS) and multiple reconfigurable intelligent surfaces (RISs) to address HIs in mmWave communications. In particular, TAS is utilized at the source to increase the received signal power at the destination and reduce the effects of HIs. Multiple channel gains from RISs are combined with the classical source‐destination channel gain at the destination. The symbol error probability (SEP) expression of the mmWave communications with TAS and RISs under the effects of HIs (shortened as the TAS‐RISs‐HIs communications) is formulated over Nakagami‐ m channels and verified through Monte Carlo simulations. The effects of HIs on the SEP of TAS‐RISs‐HIs communications are demonstrated through numerical illustrations. Specifically, the SEP of the TAS‐RISs‐HIs communications is significantly higher than the SEP of the ideal hardware communications with TAS and RISs, especially in the high transmit power regime. As a result, ignoring HIs when evaluating the SEP of the mmWave communications can result in some inappropriate conclusions. Moreover, the joint and separate advantages of TAS and RISs are thoroughly analyzed when comparing the SEPs in the cases with and without TAS, with and without RISs. [ABSTRACT FROM AUTHOR]

Summary: Wireless communications often suffer from legitimate transmissions regarding malicious jamming attacks launched through the smart jammer. The drone or unmanned aerial vehicle (UAV) communication networks derived with reconfigurable intelligent surfaces (RIS) increase the issues of beam selection and proactive handoff in terahertz (THz). Thus, a new heuristic strategy is designed for efficient and incorporated optimization of the beamforming vector and anti‐jamming transmit power allocation in undefined environments. Here, the transmit power allocation and beamforming matrix of UAV are optimized with the developed hybrid heuristic algorithm of the Hybrid Crow Black Widow Search Optimization (HCBWSO) algorithm for maximizing the system achievable rate. Here, the HCBWSO algorithm is implemented to integrate with the Crow search algorithm (CSO) and Black Widow Optimization (BWO). The second contribution is to adopt RIS into THz–UAV communications, a new Enhanced Deep Temporal Convolutional Network (EDTCN) for predicting the future beam and proactive handoff of UAVs based on their prior analysis of the UAV locations, where the HCBWSO algorithm is utilized for recommending EDTCN. Here, the training of the EDTCN needs to be done with the collection of UAV information from the DEEPMIMO dataset for predicting the future beams and, also, tracking the location of the UAV. EDTCN helps in increasing the possibility of expanding the UAV coverage and also increases the consistency of the THz communication system. Thus, the prediction of the future beam increases the coverage area of the UAV and also maximizes the system rate in the THz communication system. [ABSTRACT FROM AUTHOR]

Intelligent metasurfaces have gained significant importance in recent years due to their ability to dynamically manipulate electromagnetic (EM) waves. Their multifunctional characteristics, realized by incorporating active elements into the metasurface designs, have huge potential in numerous novel devices and exciting applications. In this article, recent progress in the field of intelligent metasurfaces are reviewed, focusing particularly on tuning mechanisms, hardware designs, and applications. Reconfigurable and programmable metasurfaces, classified as space gradient, time modulated, and space–time modulated metasurfaces, are discussed. Then, reconfigurable intelligent surfaces (RISs) that can alter their wireless environments, and are considered as a promising technology for sixth‐generation communication networks, are explored. Next, the recent progress made in simultaneously transmitting and reflecting reconfigurable intelligent surfaces (STAR‐RISs) that can achieve full‐space EM wave control are summarized. Finally, the perspective on the challenges and future directions of intelligent metasurfaces are presented. [ABSTRACT FROM AUTHOR]

Summary: The continuous growth of wireless connectivity and the emergence of the concept of the Internet of Everything in the future sixth‐generation (6G) network require a new communication paradigm. Different from the prior works that focus on performance optimization for communication networks, in this paper, we attempt to analyze the energy efficiency of unmanned aerial vehicles (UAVs) communication aided by reconfigurable intelligent surfaces (RIS) which is a new disruptive technology, for the reason that UAV has the line‐of‐sight (LoS) link transmission and flexibility and RIS has the low power for improving the communication reliability and network coverage. At firstly, when central limit theorem (CLT) assumption is used and the number of RIS reflecting elements is large, we derive the closed‐form expression of the average signal‐to‐interference‐plus‐noise ratio (SINR) of each user based on the new BS‐RIS‐user communication link and RIS‐user/BS association. Following this, we further gain the energy efficiency (EE) in closed‐form based on random access between BS and user. Finally, we further show the performance of EE under certain assumptions. In particular, the offered results demonstrate that the use of the RIS can significantly improve EE due to RIS assist in improving the SINR and has very low transmission power; meanwhile, the UAV equipped with RIS can also improve EE for the reason that RIS can be used as a mobile relay to take advantage of the LoS link transmission. [ABSTRACT FROM AUTHOR]