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IEEE Access

Secrecy Outage Analysis of Coherent FSO Systems With Multiple Receive Apertures Over Atmospheric Turbulence-Induced Fading Channels

Chungnyeong Lee, Seong Ho Chae, Hoojin Lee

Published Jan 1, 2026• 0 citations
Publisher

Abstract

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.

DOI
10.1109/access.2026.3732881

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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.
This work is concerned with the secrecy performance of a free-space optical (FSO) communication system over Fisher-SnedecorF\mathcal {F}F-distribution turbulence fading channel. Considering the influence of pointing error, atmospheric turbulence and different detection technique (i.e., the intensity modulation with direct detection and heterodyne detection), we derive the exact closed-form expressions for the average secrecy capacity (ASC), the secrecy outage probability (SOP) and the strictly positive secrecy capacity (SPSC) in terms of Meijer’s G-function. Furthermore, the asymptotic expression for SOP is analyzed as the electrical signal-to-noise ratio approaches infinity. Monte Carlo simulations are provided to validate analytical results.
Nowadays, the hybrid automatic retransmission request (H-ARQ) protocols are prevalent in fifth-generation (5G) and beyond communications. The utilization of H-ARQ protocols is extended to free-space optical (FSO) communication systems to achieve a reliable link. In this paper, we investigate the secrecy performance of chase combining (CC) H-ARQ protocol in the presence of an eavesdropper. We present the closed-form expressions of the average secrecy capacity (ASC) and secrecy outage probability (SOP) of FSO systems over atmospheric turbulence-induced fading channels and different rounds of H-ARQ transmission. We also derive the conditional SOP and secrecy throughput of the considered FSO system. The expressions are valid for weak to strong atmospheric turbulence, intensity modulation direct detection, and different number of H-ARQ rounds. We have also validated our proposed results using Monte Carlo simulations.
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.
To cope with the scarcity of radio frequency (RF) counterparts, free-space optics (FSO) based on satellite communications technologies have recently gained a lot of interest. Firstly, this investigates the security of space-to-ground intensity modulation/direct detection FSO satellite communications in various effects from the physical world, such as propagation loss, beam misalignment, cloud attenuation, and fading caused by atmospheric turbulence. Next, A wiretap channel composed of a genuine broadcaster Alice (i.e., the satellite), a legitimate user Bob, and an eavesdropper Eve is considered over turbulent channels characterized by the Fisher-Snedecor F\mathcal{F}F distribution. In addition, the closed-form expressions of the secrecy performance metrics, including average secrecy capacity, secrecy outage probability, and strictly positive secrecy capacity, are derived in our study. Finally, the numerical findings indicate that the satellite altitude of 600 km should be used to achieve a secrecy outage of roughly 50% for the whole turbulence regime. In addition, high levels of cloud liquid water content (CLWC) could improve the secrecy performance as it is shown that, the average secrecy capacity increases by about 12% when the CLWC increases from 1 mg/m31\mathrm{~mg}/\mathrm{m}^{3}1 mg/m3 to 2 mg/m32\mathrm{~mg}/\mathrm{m}^{3}2 mg/m3.
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.
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.