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arXiv:2009.00894

Physical Layer Security of Terahertz and Infrared Wireless Links in Atmospheric Turbulence

Yu Mei, Xiao Zhang, Rong Wang, Jianping An, Jianjun Ma

Published Sep 2, 2020• 3 citations
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Abstract

The future applications of terahertz (THz) wireless communication require investigations on link secrecy performance in all kinds of atmospheric conditions, including fog, snow, rain and atmospheric turbulence. Here, we present theoretical studies on physical layer security of point-to-point THz and infrared (IR) wireless links in atmospheric turbulence with a potential eavesdropper outside of the link path. Attenuations due to turbulence, gaseous absorption and beam divergence are included in the model to predict the propagation of both links. Secrecy capacity and outage probability of the THz links are calculated and compared with that of an IR (1550 nm) link. Dependences of link security on eavesdropper's position, atmospheric visibility, turbulence strength and intended data transmission rate are also presented and analyzed. We find that the THz link owns higher security at physical layer than the IR link.

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•Feb 23, 2023•0 citations
The future applications of terahertz (THz) wireless communication require investigations on link secrecy performance in all kinds of atmospheric conditions, including fog, snow, rain and atmospheric turbulence. Here, we present theoretical studies on physical layer security of point-to-point THz and infrared (IR) wireless links in atmospheric turbulence with a potential eavesdropper outside of the link path. Attenuations due to turbulence, gaseous absorption and beam divergence are included in the model to predict the propagation of both links. Secrecy capacity and outage probability of the THz links are calculated and compared with that of an IR (1550 nm) link. Dependences of link security on eavesdropper's position, atmospheric visibility, turbulence strength and intended data transmission rate are also presented and analyzed. We find that the THz link owns higher security at physical layer than the IR link.
The terahertz (THz) band is expected to become a key technology to meet the ever-increasing traffic demand for future 6G wireless communications, and a lot of efforts have been paid to develop its capacity. However, few studies have been concerned with the transmission security of such ultra-high-speed THz wireless links. In this paper, we comprehensively investigate the physical layer security (PLS) of a THz communication system in the presence of multiple eavesdroppers and beam scattering. The method of moments (MoM) was adopted so that the eavesdroppers' channel influenced by the PEC can be characterized. To establish a secure link, the traditional beamforming and artificial noise (AN) beamforming were considered as transmission schemes for comparison. For both schemes, we analyzed their secrecy transmission probability (STP) and ergodic secrecy capacity (ESC) in non-colluding and colluding cases, respectively. Numerical results show that eavesdroppers can indeed degrade the secrecy performance by changing the size or the location of the PEC, while the AN beamforming technique can be an effective candidate to counterbalance this adverse effect.
Physical layer security of a free-space optical (FSO) communication link is investigated for certain eavesdropping scenarios in which the eavesdropper exploits the divergence of the transmitted laser beam, which is subject to atmospheric turbulence, to compromise the security of the transmission link. A solution for physical layer security in which the legitimate receiver is positioned to receive only the coherent component of the transmitted laser beam is proposed. The irradiance fluctuations are modeled with separate distributions for the coherent and non-coherent components of the transmitted beam, corresponding to the main channel and the eavesdropper’s channel, respectively. Complete secrecy analysis of this setup is presented, and closed form expressions of the probability of strictly positive secrecy capacity, average secrecy capacity, and secrecy outage probability are derived. The analytical results are verified by Monte-Carlo simulations. Secrecy performance for worst-case scenarios corresponding to an adversary with powerful resources to capture all of the leaked information is explored. Results are analyzed for different turbulence regimes, and it was found that secure communication is possible even in the region of strong turbulence.
Hybrid Free-Space Optical (FSO) and millimeter Wave (mmWave) systems have emerged as a promising candidate for high data rate wireless transmissions due to the unique complementary properties against the different channel and environment conditions. Consequently, in this study, we investigate the hybrid FSO-mmWave systems from a physical-layer security point of view in the presence of different types of eavesdroppers, where the communication between two legitimate peers takes place over both FSO and RF links simultaneously. In particular, exponential atmospheric turbulence and Weibull fading channels are considered for FSO and mmWave links, respectively. We examine practical scenarios to eavesdrop the legitimate communication, and discuss the effects of random radio power of mmWave link and optical irradiance of FSO link on the probability of achieving a secure transmission. The impact of the fundamental physical layer parameters on the secrecy performance of the hybrid system is analyzed by obtaining analytical derivations of the probability of strictly positive secrecy capacity (SPSC) for different types of eavesdroppers, namely RF-, FSO- and Hybrid-Eve. In the light of results, we show that the analytical expressions are in exact agreement with the Monte-Carlo simulations.
The use of the Terahertz (THz) frequency band and free-space optical (FSO) communication represents a promising solution to address the spectrum scarcity challenges encountered in emerging applications related to 6G. In this direction, a mixed THz-FSO relaying system is proposed in this paper. The study takes into account the presence of fading effects and pointing errors affecting both THz and FSO links. The THz link is modeled using α−μ\alpha - \muα−μ distribution, while the FSO link is characterized using Malaga (M) distribution. The physical layer security (PLS) analysis is conducted, and secure outage probability is derived into closed-form for two scenarios: 1) THz eavesdropping attack and 2) FSO eavesdropping attack. The study further investigates the effect of THz and FSO links parameters on the secrecy performance in detail. The results demonstrate that security attacks on FSO links are more severe than those on THz links. Interestingly, improved THz channel conditions negatively affect the security of the proposed mixed THz-FSO system, whereas better FSO channel conditions provide enhanced secrecy.
In this work, the secrecy performance of a multiple-input–multiple-output (MIMO) terahertz (THz) communication system is investigated. To enhance physical layer security (PLS), a novel transmission protocol is proposed, which dynamically tunes the carrier frequency to align with the centers of molecular absorption loss (MAL) lines. This frequency-selection strategy increases atmospheric attenuation, thereby limiting the signal’s effective range and making it significantly more difficult for a distant eavesdropper (Eve) to intercept the transmission. To assess the secrecy performance, the exact statistical characteristics of the signal-to-noise ratios (SNRs) at both the legitimate user and Eve are derived. Using these statistical characteristics, a unified analytical framework is developed, which yields closed-form expressions for key secrecy metrics, such as the average secrecy capacity (ASC), the lower bound on the secrecy outage probability (SOP), and the probability of strictly positive secrecy capacity (SPSC). Asymptotic analyses in the high-SNR regime further reveal the secrecy diversity order and secrecy coding gain, providing deeper insight into the system’s robustness. Numerical results show that the proposed MAL-aligned frequency tuning can improve ASC by up to 25% and achieves nearly a twofold reduction in insecure area compared to conventional fixed-frequency transmission. These gains are achieved while imposing only a marginal reduction in the legitimate user’s capacity, thereby maintaining an efficient balance between security and communication performance.
Terahertz (THz) frequency band and free-space optical (FSO) communication can be proven potential solutions to the spectrum scarcity challenges in the emerging applications in the 6G. In this direction, a mixed THz-FSO relaying system is proposed in this paper. It is assumed that both THz and FSO links encounter fading effects and pointing errors, where the THz link is modeled using α-μ distribution while Málaga ( M ) distribution is used to characterize the FSO link. Particularly, we conduct the physical layer security (PLS) analysis and derive the secure outage probability into closed-form for two scenarios: 1) THz eavesdropping attack and 2) FSO eavesdropping attack. The effect of THz and FSO links parameters on the secrecy performance are then further investigated in detail. The results show that the security attacks on FSO links are more severe than those on THz links. Interestingly, the improved THz channel conditions adversely affect the security of the proposed mixed THz-FSO system, whereas better FSO channel conditions provide enhanced secrecy.