Terahertz Reconfigurable Intelligent Surfaces (RISs) for 6G Communication Links

Abstract

The forthcoming sixth generation (6G) communication network is envisioned to provide ultra-fast data transmission and ubiquitous wireless connectivity. The terahertz (THz) spectrum, with higher frequency and wider bandwidth, offers great potential for 6G wireless technologies. However, the THz links suffers from high loss and line-of-sight connectivity. To overcome these challenges, a cost-effective method to dynamically optimize the transmission path using reconfigurable intelligent surfaces (RISs) is widely proposed. RIS is constructed by embedding active elements into passive metasurfaces, which is an artificially designed periodic structure. However, the active elements (e.g., PIN diodes) used for 5G RIS are impractical for 6G RIS due to the cutoff frequency limitation and higher loss at THz frequencies. As such, various tuning elements have been explored to fill this THz gap between radio waves and infrared light. The focus of this review is on THz RISs with the potential to assist 6G communication functionalities including pixel-level amplitude modulation and dynamic beam manipulation. By reviewing a wide range of tuning mechanisms, including electronic approaches (complementary metal-oxide-semiconductor (CMOS) transistors, Schottky diodes, high electron mobility transistors (HEMTs), and graphene), optical approaches (photoactive semiconductor materials), phase-change materials (vanadium dioxide, chalcogenides, and liquid crystals), as well as microelectromechanical systems (MEMS), this review summarizes recent developments in THz RISs in support of 6G communication links and discusses future research directions in this field.

Ag/n–Si/p–MgSe/(Ag, C, Au, Pt) devices designed as current rectifiers, photodetectors and as ac signal filters suitable for VLC, IR and 6G technologies

“…For our device structure, it is clear that when the applied signal frequency exceeds 1.55 GHz the cutoff frequency reaches 8.0 GHz and then sharply increases arriving at a value of 1.0 THz. Such a feature indicates the suitability of the Ag/SM/Ag device for 6G (100-600 GHz) and THz communication technologies [38,39]. 6G communication technology based switches operative at frequencies close to those reported here are mentioned for MoS 2 atomic layers [38].…”

Inverse design of electromagnetic metamaterials: from iterative to deep learning-based methods

“…A promising direction for inverse design is to address the growing complexity of design objectives. Multi-objective optimization is often a necessity for multi-functional [148] or reconfigurable [149][150][151] metasurface devices. Solving multi-objective problems imposes additional demands on already limited computing resources, requiring modifications to optimization algorithms for enhanced efficiency [92].…”

Active anisotropic polarization conversion meta-surface for 6G communication bandgap in the reflection mode

“…Significantly, the THz band from [0.1-0.4] THz has been a viable research interest in the scientific community, as it is the main transmission band for 6 G telecommunication and a radar band for autonomous vehicles [14]. These technologies based on THz communication and sensing would play a key role in establishing future social infrastructure providing intelligent services such as 4 K highdefinition services, virtual reality, augmented reality, and unmanned vehicle maneuvering [15]. Furthermore, 6 G communication near 0.3 THz have been recently reported.…”