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IEEE Transactions on Communications

Secrecy Analysis of FSO Systems Considering Misalignments and Eavesdropper’s Location

Phuc V. Trinh, Alberto Carrasco‐Casado, Anh T. Pham, Morio Toyoshima

Published Sep 11, 2020• 57 citations
Publisher

Abstract

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.

DOI
10.1109/tcomm.2020.3023465

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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.
This paper explores the secrecy performance of the recently proposed intensity modulation/direct detection (IM/DD) terrestrial free-space quantum key distribution (QKD) system, by using a Gaussian-beam propagation model and considering the combined effects of atmospheric turbulence and legitimate transceivers’ misalignment. Secrecy performance metrics including quantum bit error rate (QBER) and ergodic secret-key rate are newly derived in closedform expressions, taking into account all combined effects of turbulence- and misalignment-induced fading channels, the eavesdropper’s location relative to the legitimate receiver, and receiver noises. To satisfy security constraints, the system designs based on the intensity modulation depth and beam waist of the Gaussian beam at the transmitter, and dualthreshold (D-T) selection at the receiver, are comprehensively discussed under turbulence and misalignment conditions as well as different eavesdropper’s locations. Monte-Carlo (M-C) simulations are also implemented to verify the analytical results. Remarkably, this paper also offers the first framework in the literature to evaluate the secrecy performance of free-space optical (FSO) systems considering the eavesdropper’s location under the effect of misalignment between legitimate transceivers.
In this paper, the secrecy performance of the physical layer security (PLS) of the hybrid free-space optical/radio frequency (FSO/RF) communication system is analyzed. The transmission protocol of the considered system is performed under the eavesdropper's attempt to overhear the RF link between the transmitter and legitimate receiver of the hybrid system. The FSO link is characterized by Málaga-M distribution while the RF links are modeled by Nakagami-m distribution. The two practical eavesdropping modes considered in this paper include: colluding and non-colluding. Exact closed-form expressions for the system's secrecy outage probability (SOP), the asymptotic of the SOP (SOP∞), the probability of strictly positive secrecy capacity (SPSC), the average secrecy capacity (ASC), and the asymptotic of the ASC (ASC∞), are specifically derived under the influence of both eavesdropper modes. Our derived analytical expressions present an efficient tool to investigate the impact of some channel parameters on the secrecy performance, namely the fading severity of the RF links, atmospheric turbulence severity, pointing error of the FSO link, number of eavesdroppers, and the power of the eavesdropper links. The results show that the increase of the eavesdroppers' number under both modes profoundly degrades the considered system secrecy performance. The accuracy of the numerical results obtained is validated by Monte-Carlo simulations.
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.
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.
In this paper, we numerically study the secrecy performance of the free-space optical (FSO) system by considering different eavesdropping scenarios. More precisely, we considered three possible eavesdropping scenarios for Eve : 1) Eve is between Alice and Bob ; 2) Eve and Bob are in the same receiving plane; 3) Eve is behind Bob . We adopt the Málaga ( M )-distribution channel to model atmospheric turbulence due to the presence of link blockage while considering the non-zero boresight pointing error and path loss. To do so, we obtain a novel probability density function (PDF) and cumulative distribution function (CDF) under different eavesdropping scenarios, based on which we derived the secrecy outage probability (SOP) analytical expressions as well as their asymptotic expressions at a high SNR regime. We verified the results using Monte Carlo simulations, which showed that the parameters related to atmospheric turbulence and pointing errors, as well as the location of the eavesdropper, have different effects on different eavesdropping scenarios.
In this article, we study the physical layer security of free-space optical (FSO) communications under different eavesdropping scenarios. More specifically, the secrecy performance of FSO communication employing intensity modulation/direct detection detection is analyzed for the well-established Málaga channels. Three different realistic scenarios of eavesdropping are considered by assuming different placement locations for the eavesdropper in the paper. Novel expressions for the average secrecy capacity (ASC) and secrecy outage probability (SOP) are derived for the considered scenarios, and useful insights are also provided through asymptotic analysis. The results show: (1) When the eavesdropper is placed near the transmitter, atmospheric condition imposes a less significant impact on secrecy performance; (2) Certain level of correlation can potentially enhance the secrecy performance for FSO communications; (3) The correlation imposes opposite impacts on the ASC and SOP of FSO communications; and the secrecy performance metrics exhibit a non-monotonic impact with the increase of correlation; (5) When the correlation of the FSO links is too small or too large (i.e., the correlation parameter around 0 or 1), the correlation plays a more significant impact on secrecy performance; and (6) The asymptotic slope of the SOP is 0.5 for all eavesdropping scenarios under practical FSO channels.