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IEEE photonics journal

Enhancing Secrecy Capacity in FSO Links via MISO Systems Through Turbulence-Induced Fading Channels With Misalignment Errors

Rubén Boluda-Ruiz, Sezer Can Tokgoz, Antonio García-Zambrana, Khalid A. Qaraqe

Published Apr 17, 2020• 28 citations
Publisher

Abstract

It has recently been proved that the free-space optical (FSO) communication links are susceptible to interceptions. Due to this reason, the optics community shows a special interest in studying these high-speed links in greater detail from a physical layer security (PLS) point of view. Therefore, in this paper, we propose, for the first time, enhancing the average secrecy capacity (ASC) in FSO links via multiple-input/single-output (MISO) systems. It is well-known that the fading effects in FSO channels can be significantly mitigated by exploiting spatial diversity techniques at the transmitter end. Thus, we develop a new asymptotic closed-form solution at high signal-to-noise-ratio (SNR) to accurately compute the ASC for MISO based FSO communication systems with equal gain combining (EGC) reception through generalized misalignment and atmospheric turbulence-induced fading channels. As a key feature, we investigate the impact of the eavesdropper's orientation along with its location in the pointing error model. We can conclude that the influence of the eavesdropper on recollecting radiated power is diminished considerably by increasing not only the normalized beam width at the receiver end, but also by increasing the number of laser sources. Numerical results are tested by exact Monte Carlo simulations.

DOI
10.1109/jphot.2020.2987378

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It has recently been proved that the free-space optical (FSO) communication links are susceptible to interceptions. Due to this reason, the optics community shows a special interest in studying these high-speed links in greater detail from a physical layer security (PLS) point of view. Therefore, in this paper, we propose, for the first time, enhancing the average secrecy capacity (ASC) in FSO links via multiple-input/single-output (MISO) systems. It is well-known that the fading effects in FSO channels can be significantly mitigated by exploiting spatial diversity techniques at the transmitter end. Thus, we develop a new asymptotic closed-form solution at high signal-to-noise-ratio (SNR) to accurately compute the ASC for MISO based FSO communication systems with equal gain combining (EGC) reception through generalized misalignment and atmospheric turbulence-induced fading channels. As a key feature, we investigate the impact of the eavesdropper's orientation along with its location in the pointing error model. We can conclude that the influence of the eavesdropper on recollecting radiated power is diminished considerably by increasing not only the normalized beam width at the receiver end, but also by increasing the number of laser sources. Numerical results are tested by exact Monte Carlo simulations.
Lately, it has been demonstrated that free-space optical (FSO) communication systems are not interception-free. For that reason, the scientific community is paying special attention to study these links from a physical layer security point of view. In this work, some potential eavesdropping attacks are studied for multiple-input/single-output (MISO) FSO communication systems with equal gain combining (EGC) reception over gamma-gamma (GG) and misalignment fading channels. A novel asymptotic solution at high signal-to-noise-ratio (SNR) is obtained to compute the average secrecy rate over turbulence channels. The robustness of FSO technology is once again proven by employing multiple laser sources at the transmitter side and, hence increasing the secrecy rate and obtaining a performance close to the capacity with no eavesdropper.
When the beam waist at the receiver is significantly larger than the receiver size, free-space optical (FSO) links may be vulnerable to some optical tapping risks at the physical layer. In this paper, we conduct a new framework for the analysis of the secrecy performance in terms of the secrecy outage probability (SOP) of FSO systems affected by gamma-gamma (GG) turbulence-induced fading channels with pointing errors. As a key feature, we evaluate the SOP in the presence of an external eavesdropper with generic location and orientation. For that reason, a new misalignment error model is proposed to consider a non-orthogonal optical beam with respect to the photodetector plane at the eavesdropper's receiver, where the effective area is determined by a rotated ellipse. New approximate and asymptotic solutions at high signal-to-noise-ratio (SNR) for the secrecy performance are obtained in closed-form, which are verified by exact Monte Carlo simulations. By using the developed expressions, we analyze in greater detail some effects such as the SNR of the eavesdropper's channel, the normalized beamwidth at the receiver-side, and the location and orientation of the eavesdropper on the secrecy performance for different turbulence conditions.
Today, free-space optical (FSO) communication in the presence of an eavesdropper is considered as an important topic in the current literature. In this paper, security issues are investigated for FSO wiretap channels. Novel closed-form solutions, both approximate and asymptotic, for the average secrecy capacity (ASC) are derived over gamma-gamma (GG) atmospheric turbulence channels with pointing errors. As a significant aspect, misalignment errors at the eavesdropper's receiver follow the Rice distribution in order to take into consideration the spacing between the legitimate receiver and the eavesdropper's receiver, which is modeled as a nonzero boresight displacement. It is concluded that a greater normalized beam width is required to reduce the loss produced by such attacks in the high signal-to-noise-ratio (SNR) regime. Analytical results are further verified by Monte Carlo simulation results.
Despite the high directivity of optical beams, free-space optical (FSO) communications may be compromised by the presence of an external observer, particularly when the beam footprint at the receiver side is greatly wider than the receiver size. In this paper, a new framework is developed for the analysis of the secrecy outage probability (SOP) and the probability of strictly positive secrecy capacity (SPSC) in FSO systems over gamma-gamma (GG) fading channels with nonzero boresight pointing errors. As a key feature, we include the eavesdropper's location in the pointing error model corresponding to the wiretap channel model. In this way, new approximate and asymptotic solutions for both the SOP and the probability of SPSC are obtained in closed-form, which are validated by Monte Carlo simulations. Using the developed expressions, we can study some effects such as the impact of the eavesdropper's location and the beam width on the secrecy outage performance.
This paper investigates the physical layer security (PLS) of multi-aperture coherent free-space optical (FSO) systems with heterodyne detection over atmospheric turbulence-induced fading channels, while accounting for residual phase distortions after Zernike polynomial-based modal compensation. Unlike prior work focused primarily on intensity modulation/direct detection (IM/DD)-based FSO systems, this study considers coherent multi-aperture reception. Based on a Rician approximation of the turbulence-induced fading envelope, we derive an analytical approximation for the secrecy outage probability (SOP), a tractable semi-closed-form expression for the SOP lower bound, and a closed-form approximation for the strictly positive achievable secrecy rate probability. Moreover, high-SNR and low-SNR asymptotic analyses are presented to characterize the asymptotic secrecy behavior. The results show that the secrecy diversity order equals the number of receive apertures at the legitimate receiver, whereas, in the high-SNR regime at the legitimate receiver, the eavesdropper’s channel parameters affect the leading-order coefficient but not the decay exponent. Numerical results validate the proposed analytical approximations and asymptotic expressions and demonstrate that increasing the number of receive apertures at the legitimate receiver improves secrecy performance under various turbulence conditions.
The use of free-space optical (FSO) systems as secure transmission media has recently attracted research efforts worldwide. However, their secrecy performance may be compromised by the presence of an adversarial eavesdropper. In addition, misalignments between transceivers could severely affect the legitimate FSO channel and increase eavesdropping risks. This paper, for the first time, offers a complete framework to analyze the impact of an eavesdropper's location on the secrecy performance of terrestrial FSO systems under generalized misalignments and atmospheric turbulence conditions. Particularly, the probability density functions (PDFs) of the eavesdropping channel are newly developed and presented in closed-form expressions. Capitalizing on the derived PDFs, secrecy performance metrics in the physical-layer security (PLS) and intensity modulation/direct detection (IM/DD) continuous-variable quantum key distribution (CV-QKD) systems can be analytically analyzed, incorporating all combined effects of the atmospheric turbulence, transceiver misalignments, receiver noises, and the eavesdropper's location. Monte-Carlo (MC) simulations are also implemented to corroborate the analytical results.