Scintillation due to atmospheric turbulence is one of the effects challenging the reuse of extremely wideband and high-rate optical satellite-to-satellite communication systems for satellite-to-Earth and Earth-to-satellite transmissions. In this article, we study the possibility of utilizing links in the terahertz (THz) frequency bands for these uplink and downlink transmissions instead. Built upon the physics-based model, originally developed for optical wave propagation, we present a mathematical framework for the THz signal scintillation to analyze atmospheric turbulence's impact on ground-satellite and airplane-satellite connections. Our results indicate that, while the scintillation still significantly impacts the power of the received THz signal (especially at lower elevation angles and under specific weather conditions), the effect is drastically less profound than the extreme losses the optical link will experience in the same weather conditions. We further explore a notable asymmetry of up to 10 dB between uplink and downlink losses. Finally, we illustrate that, even at relatively low airplane altitudes, the airplane-to-satellite link is much less affected than the Earth-to-satellite link, making THz communications a promising candidate technology for future high-rate airplane connectivity systems as a part of 6G and beyond.

The terahertz (THz) spectrum holds immense potential for advanced wireless communication systems due to abundant available bandwidth and high data rates. However, a major challenge in deploying THz systems is susceptibility to blockage, which can greatly impair signal propagation and negatively affect the reliability of the communication link. This paper focuses on applying self-accelerated beams for blockage mitigation at THz frequencies. These beams can follow curved trajectories as they propagate in free space. Trajectories can be engineered to overcome physical obstacles present in the beam path. As we show through extensive simulations, self-accelerated beams perform better than conventional Gaussian beams in terms of received power when obstacles are present. This approach offers new possibilities for reliable THz communications in practical deployment scenarios.

Terahertz (THz) band communications is envisioned as a key technology for future wireless standards. Advances in hardware design, channel models, and signal processing have all contributed significantly to advancing the field. Practical THz wireless has been demonstrated in high data-rate backhaul links. However, the next great leap for adopting THz-band frequencies in widespread communication systems must cover a massive canyon. Such communication systems must operate in the massive near field of the high-gain devices that are required to overcome the very high spreading losses of THz frequencies while providing all the promises of very high data rates and sensing resolution. Recent years have seen progress toward near-field THz, with investigations centered around the physical layer, combining both wave and communication theory to provide meaningful solutions to the challenges of THz signal propagation in the near field. In this article, an in-depth look is presented on the aspect of near-field THz. The aspect of signal propagation is first explained from a symbiosis of array and wave theory, following which it is conclusively shown how canonical beamforming is decimated in the near field. It is further explained why THz wireless must necessarily be near field, at least in some cases. Then, a vision of beamshaping is presented in which wavefront engineering is presented to address the design of new beams, specifically beamfocusing, Bessel beams, and Airy beams, which each offer distinct attractive advantages in creating THz links. Issues related to their generation and reception and issues involving narrowband limitations are presented. Finally, the article ends by discussing some of the more promising and upcoming applications of these beams as well as the exciting challenges and opportunities in this new and intriguing research area.

 Movable antennas represent an emerging field in telecommunication research and a potential approach to achiev- ing higher data rates in multiple-input multiple-output (MIMO) communications when the total number of antennas is limited. Most solutions and analyses to date have been limited to narrowband setups. This work complements the prior studies by quantifying the benefit of using movable antennas in wideband MIMO communication systems. First, we derive a novel uplink wideband system model that also accounts for distortion from transceiver hardware impairments. We then formulate and solve an optimization task to maximize the average sum rate by ad- justing the antenna positions using particle swarm optimization. Finally, the performance with movable antennas is compared with fixed uniform arrays and the derived theoretical upper bound. The numerical study concludes that the data rate improvement from movable antennas over other arrays heavily depends on the level of hardware impairments, the richness of the multi- path environments, and the number of subcarriers. The present study provides vital insights into the most suitable use cases for movable antennas in future wideband systems.

Movable antennas represent an emerging field in telecommunication research and a potential approach to achieving higher data rates in multiple-input multiple-output (MIMO) communications when the total number of antennas is limited. Most solutions and analyses to date have been limited to narrowband setups. This work complements the prior studies by quantifying the benefit of using movable antennas in wideband MIMO communication systems. First, we derive a novel uplink wideband system model that also accounts for distortion from transceiver hardware impairments. We then formulate and solve an optimization task to maximize the average sum rate by adjusting the antenna positions using particle swarm optimization. Finally, the performance with movable antennas is compared with fixed uniform arrays and the derived theoretical upper bound. The numerical study concludes that the data rate improvement from movable antennas over other arrays heavily depends on the level of hardware impairments, the richness of the multipath environments, and the number of subcarriers. The present study provides vital insights into the most suitable use cases for movable antennas in future wideband systems.

Terahertz (THz) communications are envisioned as a key technology to enable massive Internet-of-things (IoT) applications in the 6G era. In particular, THz communications intend to alleviate the capacity limitations of future wireless networks by exploiting the THz electromagnetic spectrum. However, despite the fact that the transmitted signals are highly directional in the THz frequencies, an adversary can intercept signals in line-of-sight transmissions. This chapter will provide a detailed review of physical layer attacks against THz communications and examine existing physical layer security (PLS) schemes to identify potential PLS countermeasures for protecting THz communications and the massive IoT applications relying on them in the 6G era. First, the fundamental trade-offs are presented associated with data eavesdropping from directional THz links. Later, potential attacks are detailed and analyzed, specifically targeting THz communications. The discussion proceeds with possible countermeasures to further increase the level of security in IoT services using directional THz radios. Finally, the chapter overviews other security-related challenges, such as the jamming of THz links.

Massive multiple-input multiple-output (MIMO) systems exploit the spatial diversity achieved with an array of many antennas to perform spatial multiplexing of many users. Similar performance can be achieved using fewer antennas if movable antenna (MA) elements are used instead. MA-enabled arrays can dynamically change the antenna locations, mechanically or electrically, to achieve maximum spatial diversity for the current propagation conditions. However, optimizing the antenna locations for each channel realization is computationally excessive, requires channel knowledge for all conceivable locations, and requires rapid antenna movements, thus making real-time implementation cumbersome. To overcome these challenges, we propose a pre-optimized irregular array (PIA) concept, where the antenna locations at the base station are optimized a priori for a given coverage area. The objective is to maximize the average sum rate and we take a particle swarm optimization approach to solve it. Simulation results show that PIA achieves performance comparable to MA-enabled arrays while outperforming traditional uniform arrays. Hence, PIA offers a fixed yet efficient array deployment approach without the complexities associated with MA-enabled arrays.

In this paper, the physical layer security of terahertz (THz) band communications in the near field is discussed and evaluated. First, a novel class of design approaches to enhance the security of near-field THz links is proposed. These approaches are referred to as wavefront hopping schemes, as they exploit the properties of different THz wavefronts when propagating beams in the near field (particularly, THz Bessel beams and THz Airy beams). Then, a compound evaluation framework is delivered to assess both the performance and the security level of a given single-beam or multi-beam solution for a near-field THz system. Finally, a performance evaluation is conducted illustrating the performance/secrecy trade-offs associated with the modeled single-beam and multi-beam security schemes for both a single Attacker and a team of cooperating Attackers. Our results indicate that the proposed multi-beam solutions combining THz Airy beam(s) with THz Bessel beam through wavefront hopping lead to a notably decreased probability of message eavesdropping while maintaining nearly identical performance with the single-beam schemes. The contributed security schemes, evaluation methodology, and numerical results facilitate further development in the emerging area of secure near-field THz communications for 6G and beyond wireless networks.

In this work, we present an on-chip Hafnium Zirconium Oxide-based millimeter wave (mmWave) programmable antenna. The device exhibits a resonance frequency of 36.623 GHz, which can be programmed by varying the polarization state of a thin Hafnium Zirconium Oxide (HZO) layer enclosed in the structure of the antenna. By polarizing such HZO film, we demonstrate, for the first time, a variation in the resonance frequency for ferroelectrically programmable antennas without continuously applying a bias. In this regard, the non-volatile behavior of HZO allows to retain the polarization state of the film even after removing the DC voltage, therefore enabling the antenna's resonance frequency retention. The full ferroelectric switching of the HZO layer occurs approximately at ±3 V, resulting in a maximum resonance frequency shift of 3.019 GHz (8.3% fractional frequency change) between different programming voltages. The device's response to the applied programming voltage generates a ferroelectrically-induced hysteresis on its resonance frequency. In contrast to prior devices of this class, the proposed approach showcases a more compact size, full post-CMOS compatibility, low programming voltage, and non-volatile programmability.

Future 6G and beyond-6G cellular systems are expected to operate in the sub-terahertz (0.1-0.3 THz) and terahertz (THz, 0.3-3 THz) frequency bands. However, the small wavelength coupled with large antenna apertures force a part of the (sub-)THz access points coverage to be in the near field. There, conventional far-field propagation models (e.g., free space path loss, FSPL) are not applicable, as their use leads to substantial errors in the analysis. At the same time, existing exact near-field models following the electromagnetic principles are relatively complex and require additional efforts when using them for simple link budget predictions. In this paper, we fill this gap by developing an accurate yet easy-to-use propagation model for near-field THz communications. The proposed model has a simple algebraic structure, is applicable to both near field and far field, and requires no fine-tuning for different sets of input parameters. We analyze the key dependencies in the THz near field channel with the developed model and also contrast the results with those by FSPL, noticing up to 20 dB difference.