Scintillation and Attenuation Modelling of Atmospheric Turbulence for Terahertz UAV Channels

Terahertz (THz) wireless communications have the potential to realize ultra-high-speed and secure data transfer with miniaturized devices for unmanned aerial vehicle (UAV) communications. Existing THz channel models for aerial scenarios assume a homogeneous medium along the line-of-sight propagation path. However, the atmospheric turbulence due to random airflow leads to temporal and spatial inhomogeneity of the communication medium, motivating analysis and modelling of the THz UAV communication channel. In this paper, we statistically modelled the scintillation and attenuation effect of turbulence on THz UAV channels. Specifically, the frequency- and altitude-dependency of the refractive index structure constant, as a critical statistical parameter characterizing the intensity of turbulence, is first investigated. Then, the scintillation characteristic and attenuation of the THz communications caused by atmospheric turbulence are modelled, where the scintillation effect is modelled by a Gamma-Gamma distribution, and the turbulence attenuation as a function of altitude and frequency is derived. Numerical simulations on the refractive index structure constant, scintillation, and attenuation in the THz band are presented to quantitatively analyze the influence of turbulence for the THz UAV channels. It is discovered that THz turbulence can lead to at most 10dB attenuation with frequency less than 1THz and distance less than 10km.

Molecular Absorption Effect: A Double-edged Sword of Terahertz Communications

Communications in the terahertz band (THz) (0.1--10~THz) have been regarded as a promising technology for future 6G and beyond wireless systems, to overcome the challenges of evergrowing wireless data traffic and crowded spectrum. As the frequency increases from the microwave band to the THz band, new spectrum features pose unprecedented challenges to wireless communication system design. The molecular absorption effect is one of the new THz spectrum properties, which enlarges the path loss and noise at specific frequencies. This brings in a double-edged sword for THz wireless communication systems. On one hand, from the data rate viewpoint, molecular absorption is detrimental, since it mitigates the received signal power and degrades the channel capacity. On the other hand, it is worth noticing that for wireless security and covertness, the molecular absorption effect can be utilized to safeguard THz communications among users. In this paper, the features of the molecular absorption effect and their impact on the THz system design are analyzed under various scenarios, with the ultimate goal of providing guidelines to how better exploit this unique THz phenomenon. Specifically, since the molecular absorption greatly depends on the propagation medium, different communication scenarios consisting of various media are discussed, including terrestrial, air and space, sea surface and nano-scale communications. Furthermore, two novel molecular absorption enlightened secure and covert communication schemes are presented, where the molecular absorption effect is utilized as the key and unique feature to boost security and covertness.

WASABI: Widely-Spaced Array and Beamforming Design for Terahertz Range-Angle Secure Communications

Terahertz (THz) communications have naturally promising physical layer security (PLS) performance in the angular domain due to the high directivity feature. However, if eavesdroppers reside in the beam sector, the directivity fails to work effectively to handle this range-domain security problem. More critically, with an eavesdropper inside the beam sector and nearer to the transmitter than the legitimate receiver, i.e., in close proximity, secure communication is jeopardized. This open challenge motivates this work to study PLS techniques to enhance THz range-angle security. In this paper, a novel widely-spaced array and beamforming (WASABI) design for THz range-angle secure communication is proposed, based on the uniform planar array and hybrid beamforming. Specifically, the WASABI design is theoretically proved to achieve the optimal secrecy rate powered by the non-constrained optimum approaching (NCOA) algorithm with more than one RF chain, i.e., with the hybrid beamforming scheme. Moreover, with a low-complexity and sub-optimal analog beamforming, the WASABI scheme can achieve sub-optimal performance with less than 5% secrecy rate degradation. Simulation results illustrate that our proposed widely-spaced antenna communication scheme can ensure a 6bps/Hz secrecy rate when the transmit power is 10dBm. Finally, a frequency diverse array, as an advocated range security candidate in the literature, is proven to be ineffective to enhance range security.