Seven Defining Features of Terahertz (THz) Wireless Systems: A Fellowship of Communication and Sensing

Abstract

Wireless communication at the terahertz (THz) frequency bands (0.1 − 10 THz) is viewed as one of the cornerstones of tomorrow's 6G wireless systems. Owing to the large amount of available bandwidth, if properly deployed, THz frequencies can potentially provide significant wireless capacity performance gains and enable high-resolution environment sensing. However, operating a wireless system at high-frequency bands such as THz is limited by a highly uncertain and dynamic channel. Effectively, these channel limitations lead to unreliable intermittent links as a result of an inherently short communication range, and a high susceptibility to blockage and molecular absorption. Consequently, such impediments could disrupt the THz band's promise of highrate communications and high-resolution sensing capabilities. In this context, this paper panoramically examines the steps needed to efficiently and reliably deploy and operate next-generation THz wireless systems that will synergistically support a fellowship of communication and sensing services. For this purpose, we first set the stage by describing the fundamentals of the THz frequency band. Based on these fundamentals, we characterize and comprehensively investigate seven unique defining features of THz wireless systems: 1) Quasi-opticality of the band, 2) THztailored wireless architectures, 3) Synergy with lower frequency bands, 4) Joint sensing and communication systems, 5) PHYlayer procedures, 6) Spectrum access techniques, and 7) Real-time network optimization. These seven defining features allow us to shed light on how to re-engineer wireless systems as we know them today so as to make them ready to support THz bands and their unique environments. On the one hand, THz systems benefit from their quasi-opticality and can turn every communication challenge into a sensing opportunity, thus contributing to a new generation of versatile wireless systems that can perform multiple functions beyond basic communications. On the other hand, THz systems can capitalize on the role of intelligent surfaces, lower frequency bands, and machine learning (ML) tools to guarantee a robust system performance. We conclude our exposition by presenting the key THz 6G use cases along with their associated major challenges and open problems. Ultimately, the goal of this article is to chart a forward-looking roadmap that exposes the necessary solutions and milestones for enabling THz frequencies to realize

THz-Empowered UAVs in 6G: Opportunities, Challenges, and Trade-Offs

“…To guarantee a certain level of reliability under delay constraint, some handovers may be skipped at the expense of rate reduction Further, we recall that sensing and localization capability in THz can assist UAVs to operate reliably by precisely positioning the intended transmitters/receivers. In addition, intelligent reflective surface (IRS) enabled THz architecture can ease effective beam alignment by offering enhanced sensing and localization [11].…”

“…However, a co-design of such multi-function system is not an easy task. In addition to the general challenges such as waveform design for integrated sensing and communication [11], the development of novel models and powerful algorithms with low-complexity that can be executed on UAVs with limited power and computation capability is a major challenge. Furthermore, the algorithms should take into account the sensitivity of several UAVs applications to delay caused by the integrated design.…”

Reliable Beam Tracking with Dynamic Beamwidth Adaptation in Terahertz (THz) Communications

“…However, unleashing the potential of THz frequency bands necessitates overcoming key THz challenges, stemming from the channel's uncertainty. Particularly, two major factors that restrain the communication at THz frequency bands are the high path loss and the molecular absorption effect [4], [5]. To compensate the effect of these phenomena, a very narrow beam (so-called pencil beam) is needed to focus the power towards the receiver [6].…”

Channel Estimation for Large Intelligent Surface-Based Transceiver Using a Parametric Channel Model

“…The millimeter-wave (mmWave) and terahertz (THz) frequency bands will play a pivotal role in next-generation (e.g., beyond 5G) wireless networks since they would be able to provide abundant spectrum resources, higher data rates, and lower latency [1], [2]. However, sever path-loss, atmospheric absorption, human blockage, and other environmental obstruction are key challenges for enabling wireless communication in the mmWave and THz bands [3], [4].…”

“…In order to realize massive MIMO BSs with a large antenna array, different architectures such as fully-digital, hybrid analog/digital, and lens array antennas architectures have been proposed [4]. Since the beamforming gain of massive MIMO increases with the number of antennas, it would be highly desirable to have infinitely many antennas 1 , which is not feasible with large antenna arrays [7], [8]. In this direction, large intelligent surfaces (LISs) [9], also known as holographic massive MIMO [8], holographic MIMO surfaces [10], and dynamic metasurface antennas [11], were introduced to go beyond massive MIMO systems [8]- [16].…”