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.
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.
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.
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.
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.
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.
The emergence of an array of new wireless networks has led researchers to evaluate the prospect of utilizing the physical properties of the wireless medium in order to design secure systems. In this paper, the physical layer secrecy performance of a mixed radio frequency-free space optical (RF-FSO) system with variable gain relaying scheme is investigated in the presence of an eavesdropper. We assume that the eavesdropper can wiretap the transmitted confidential data from the RF link only. It is further assumed that the main and eavesdropper RF links are modeled as generalized Gamma (GG) fading channel, and the free space optical (FSO) link experiences Málaga turbulence with pointing error impairment. Our primary concern is to protect this confidential information from being wiretapped. Besides pointing error, the atmospheric turbulence and two types of detection techniques (i.e. heterodyne detection and intensity modulation with direct detection) are also taken into consideration. Utilizing amplify-and-forward (AF) scheme, the novel mathematical closed-form expressions for average secrecy capacity, lower bound of secrecy outage probability, and strictly positive secrecy capacity are derived. As both the links (RF and FSO) undergo generalized fading channels, the derived expressions are also general. We present a unification of some existing works utilizing the proposed model to better clarify the novelty of this work. Finally, all the derived expressions are justified via Monte-Carlo simulations.
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.