Terahertz (THz) band unmanned aerial vehicle (UAV) links exploit ultra-wide spectra and high directivity to deliver multi-Gbps secure data for remote sensing and wireless backhaul, but their open three-dimensional flight paths increase vulnerability to covert detection and jamming. Altitude-dependent atmospheric loss, negligible in microwave or terrestrial THz studies, becomes critical in this band owing to triple selectivity, where propagation varies sharply with frequency, distance, and environment. In this paper, a spatially-aware transmission framework is proposed that jointly allocates spectrum and power according to node altitude and beam orientation to maximize jam-resilience covert throughput. Specifically, a three-dimensional propagation model incorporating altitude-dependent molecular absorption, weather loss, and turbulence is established, closed-form expressions for covert outage probability and throughput are derived, and the resulting nonconvex band-wise optimization is solved. Simulation results verify significant gains in covert throughput and jamming robustness and reveal that downward transmissions are more secure than upward counterparts, as their propagation path traverses denser and more absorptive air, whereas the upward path quickly rises into thinner layers that expose the signal to remote eavesdroppers. These analytical insights furnish a quantitative basis for altitude-aware spectrum planning and multilayer topology design in future space-air-ground integrated networks.

The growing demand for ultra-high data rates in space-air–ground integrated networks (SAGINs) has rendered terahertz (THz) communications a promising technology due to its exceptionally broad and continuous spectrum resources. Nevertheless, in air–ground (AG) scenarios, the high mobility of aircraft induces intense and rapidly fluctuating turbulence, leading to additional propagation attenuation that is often overlooked in existing studies. To bridge this gap, an AI-empowered THz AG communication framework is proposed in this paper that models turbulence-induced attenuation and incorporates it into a joint power-attitude optimization. Specifically, a fluid-dynamics-informed attenuation model is established to characterize the turbulent impact on THz signal propagation. Building upon this model, a joint power-attitude optimization problem is formulated to adaptively allocate transmit power and adjust aircraft attitude for maximizing link capacity.The optimization is efficiently solved using a diffusion-based algorithm to adaptively allocate transmit power and adjust aircraft attitude for maximizing link capacity. Comprehensive numerical evaluations demonstrate that the turbulence-induced attenuation ranges from 18 to 28 dB under attacking angles between −10° and 10° at 0.7 Mach. Moreover, the proposed framework achieves an average capacity of 11.241 bps/Hz, outperforming existing strategies by 12.4% to 22.8%, and reaching approximately 98% of the theoretical capacity limit.

Terahertz (THz) band ($0.1-10 \text{THz}$) possesses multi-gigahertz continuous bandwidth resources, making it a promising frequency band for high-speed wireless communications and environment sensing. The interaction between the THz wave and the external environment has been studied for various scenarios. However, it has recently been revealed that the friction forces in dust storms as well as the irradiation of sunlight and solar wind lead to the electrification of dust particles on Earth and the Moon. The THz wave propagation in these charged dust has not been fully investigated, which is essential for THz aerial communications in dust storms and lunar communications. In this paper, a channel model for THz wave propagation in charged dust is developed for wireless communications. Specifically, an extended Mie scattering model for charged dust is first introduced, which captures the electrodynamic feature of the interaction between THz wave and charged particles. Then, the diameter and density distributions of dust particles are modeled, based on which the propagation loss of THz wave in charged dust is modeled and elaborated. Finally, numerical results on the additional loss caused by these charged dust with different sizes in the THz band are evaluated and compared. Extensive results demonstrate that as the number of dust charges increases, the extinction cross section of smallersized particles significantly increases, and the overall attenuation led by charged dust increases by at most 50% at 0.3 THz.

Terahertz (THz) wireless communications have the potential to realize ultra-high-speed data transfer with miniaturized devices for unmanned aerial vehicle (UAV) communications. Existing THz UAV channel models assume a homogeneous medium along the propagation path. However, the atmospheric turbulence due to random airflow leads to temporal and spatial inhomogeneity of the communication medium, motivating the analysis and modeling of the influence of atmospheric turbulence on $\mathbf{T H z}$ wave propagation. In this paper, we statistically modeled the attenuation effect of turbulence on THz UAV channels. Specifically, the refractive index structure constant, as a critical statistical parameter characterizing the influence of turbulence on channel medium, is first investigated. Then, the scintillation characteristic and attenuation of the THz communications caused by atmospheric turbulence are modeled, where the scintillation effect is characterized 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 THz UAV channels. It is discovered that THz turbulence can lead to at most 10 dB attenuation with frequency less than 1 THz and distance less than $10 \mathbf{k m}$.

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. The atmospheric turbulence due to random airflow leads to spatial inhomogeneity of the communication medium, which is yet missing in most existing studies, leading to additional propagation loss and even loss of spatial coherence (LoSC) in MIMO systems. In this paper, the attenuation and loss of spatial coherence for atmospheric turbulence are modeled in THz UAV MIMO channels. Specifically, the frequency- and altitude-dependency of the refractive index structure constant (RISC), as a critical statistical parameter characterizing the intensity of turbulence, is first investigated. Then, the LoSC, fading, and attenuation caused by atmospheric turbulence are modeled, where the turbulence-induced fading is modeled by a Gamma-Gamma distribution, and the turbulence attenuation as a function of altitude and frequency is derived. Numerical results show that the turbulence leads to at most 10 dB attenuation with frequency less than 1 THz and distance less than 10 km. Furthermore, when the distance is 10 km and the RISC is 10-9m-2/3, the loss of spatial coherence effect leads to 10 dB additional loss for a 1024 × 1024 ultra-massive MIMO system.

Despite the narrowbeam nature of Terahertz (THz) communications, the physical layer security in the THz band is challenging when eavesdroppers are inside the beam radiation sector. In this paper, a deep neural network (DNN)-based self-interference cancellation (SIC)-free receiver artificial noise (AN) assisted mechanism is proposed to address the in-beam security challenge, by considering randomly distributed eavesdroppers in THz secure communications. By exploiting the different temporal broadening effects of the AN signals at distinct distances, the SIC can be saved with a proper signal detection design rather than using conventional high-complexity cancellation techniques. To combat the non-linearity and non-convexity of the optimization problem, the system parameters including carrier frequency, power of transmission signal and AN signal power, and frame time are designed by an efficient deep neural network (DNN) algorithm to minimize the secrecy outage probability. Numerical results demonstrate that the maximum secrecy rate of our proposed DNN-powered SIC-free receiver AN scheme is up to 3.3 bps/Hz over 10 m transmission when the eavesdropper is in close proximity. Moreover, the secrecy outage probability is less than 0.5% when the eavesdropper density is 0.001 per square meter, which is approximately 33% lower than that of conventional transmitter AN schemes.

Terahertz (0.1-10 THz) band is envisioned to enable ultra-broadband communications for beyond fifth generation (B5G). Although recent efforts focus on high-speed wireless communications, the transmission of THz electromagnetic (EM) waves in free space experiences high path loss and severe molecular absorption effect and is vulnerable to eavesdropping. It is therefore motivated to explore polymer fiber-based THz wireline transmission, which is promising for short-range, ultrafast, and secure communications in intra-vehicle scenarios. In this work, an end-to-end propagation of a THz wireline transmission system is modeled and characterized, by accounting for solid-core circular polymer fiber as the medium. A case study of a 2-mm-diameter solid-core circular polytetrafluoroethylene (PTFE) fiber is provided, which demonstrates the feasibility of the attenuation constant smaller than 6.5 dB/m and dispersion parameter smaller than 1.8 ps/GHz/m between 100 GHz and 160 GHz. Simulation results indicate that when a polymer fiber features a 5-dB/m attenuation constant and a 2-ps/GHz/m dispersion parameter, the system with this polymer fiber can reach a signal-to-interferenceplus-noise ratio (SINR) of 14 dB, corresponding to BER of ten to the power of -11 with the input power of -10 dBm.

The Terahertz (THz) band is envisioned as a promising technique to support bandwidth-hungry and secure applications. Although the significant path loss and strong directivity make THz communications secure naturally, the information security is still imperfect at near regions along the beam propagation path. In this paper, a novel distance-adaptive absorption peak modulation (DA-APM) is developed for THz covert communications, by exploring the unique spectrum features of frequency-dependent molecular absorption. Although high-attenuation molecular absorption is unfavored for communications, the main principle to enhance covertness or equivalently, minimize the eavesdroppable distance, is dynamically modulating signals under the molecular absorption peaks in the THz spectrum, where the eavesdroppable distance is defined as the threshold distance within which an eavesdropper can wiretap the transmission. Furthermore, an optimization framework is proposed to minimize the eavesdroppable distance, to which the sub-optimal solutions are derived for the multi-wideband waveform by controlling carrier frequencies, power allocation, and rate distribution on each sub-band. Extensive numerical results show that the THz-spectrum-based DA-APM approach can reduce the eavesdroppable distance by 60% compared with random spectrum selection methods, which significantly reduce the insecure area and enhance the covertness of THz wireless transmission.

Although with narrow-beam transmissions, physical layer security of Terahertz (THz) communications faces great challenges with the presence of eavesdroppers in close proximity inside the Terahertz beam sector. This paper proposes a novel self-interference cancellation (SIC)-free receiver artificial noise (AN) assisted mechanism to address this challenge for THz secure communications. By exploiting the different temporal broadening effects of the artificial noise signals at the receiver side versus the eavesdropper side, the high-complexity SIC part can be mitigated with a proper receiver design. Moreover, the system parameters including power allocation, carrier frequency, transmit power, symbol time, pulse waveform type, and receiver parameter that maximize the secrecy rate of the THz communication system are solved, based on an efficient deep neural network (DNN) algorithm. Numerical results demonstrate that our proposed SIC-free receiver artificial noise scheme achieves 4 bps/Hz secrecy rate, with substantially lower hardware complexity than SIC-based receiver AN systems and reduced computational complexity than exhaustive search.

Covert communication, aiming at concealing the existence of data transmission from an eavesdropper, is attracting increasing concerns for communication security. With the trend of moving to higher carrier frequencies, Terahertz (THz) band communication, i.e., wavelength at 0.03-3mm, is envisioned as a promising technique to support bandwidth-hungry applications, as well as improve physical layer security due to its naturally strong directivity and high path loss. Narrow-beam transmission widely adopted in THz communications can effectively prevent from eavesdropping outside the beam sector. However, the challenge still remains when an eavesdropper resides inside the beam sector. To ensure the covertness under such condition, in this paper, a distance-adaptive absorption peak hopping (DA-APH) modulation scheme is proposed for THz covert communications, which takes advantages of distance- and frequency-selective spectral windows, and the frequency-hopping mechanism over the THz spectrum. In particular, the pulse waveform model with polarization is developed to guarantee the reliability of transmission and covertness from eavesdropping. Furthermore, a distance-adaptive carrier frequency selection scheme is designed to choose optimal hopping frequencies at molecular absorption peaks in the THz band.