Underwater wireless optical communication (UWOC) possesses significant advantages, such as high bandwidth, low latency, and low power consumption, making it a key technology for building information networks in marine environments. However, due to the scattering effect of seawater, some photons carrying information inevitably scatter out of their predetermined paths, leading to the possibility for information leakage. Therefore, we propose a physical-layer security analysis model for UWOC systems based on the wiretap channel model. The model evaluates the security of the communication system by calculating the capacity difference between the legitimate channel and the eavesdropping channel in the UWOC system. Specifically, the model first constructs the three-dimensional intensity distribution of scattered photons in the underwater channel via Monte Carlo simulations and experimental measurements. Then, it calculates the capacities of both the legitimate and eavesdropping channels based on the decoding results. Finally, the three-dimensional distribution of secrecy capacity is derived to assess the security of the communication system. In this work this model is used to analyze the security of the UWOC system in clear seawater environments. The results show that the secrecy capacity of the system is zero within a certain range near the transmission path, demonstrating that scattered photons can cause information leakage. We recommend that, in practical applications, monitoring the non-signal transmission area near the transmitter is essential to ensure communication security. This research provides a solution for analyzing the quantitative security of UWOC, which can strongly support the design of UWOC systems and encoding/decoding schemes.
Underwater wireless optical communication (UWOC) possesses significant advantages, such as high bandwidth, low latency, and low power consumption, making it a key technology for building information networks in marine environments. However, due to the scattering effect of seawater, some photons carrying information inevitably scatter out of their predetermined paths, leading to the possibility for information leakage. Therefore, we propose a physical-layer security analysis model for UWOC systems based on the wiretap channel model. The model evaluates the security of the communication system by calculating the capacity difference between the legitimate channel and the eavesdropping channel in the UWOC system. Specifically, the model first constructs the three-dimensional intensity distribution of scattered photons in the underwater channel via Monte Carlo simulations and experimental measurements. Then, it calculates the capacities of both the legitimate and eavesdropping channels based on the decoding results. Finally, the three-dimensional distribution of secrecy capacity is derived to assess the security of the communication system. In this work this model is used to analyze the security of the UWOC system in clear seawater environments. The results show that the secrecy capacity of the system is zero within a certain range near the transmission path, demonstrating that scattered photons can cause information leakage. We recommend that, in practical applications, monitoring the non-signal transmission area near the transmitter is essential to ensure communication security. This research provides a solution for analyzing the quantitative security of UWOC, which can strongly support the design of UWOC systems and encoding/decoding schemes.
Although underwater wireless optical communication (UWOC) receives much interest lately, security issues associated with it get little attention. In this work, it is the first attempt to investigate the physical layer security (PLS) performance of the vertical UWOC system with perfect and imperfect channel state information (CSI). Specifically speaking, the communication between two legitimate peers in the presence of an external eavesdropper is studied from the information-theoretic security perspective. Assuming that turbulence-induced fading over the vertical UWOC links is respectively subject to cascaded lognormal (LN) and Gamma-Gamma (GG) distributions for weak and moderate/strong turbulence conditions, and the angular pointing error is randomized by the Beckmann distribution, the composite cascaded statistical fading models are derived with the comprehensive effects of path loss, underwater turbulence, angular pointing errors, and channel estimation error. On the basis of these models, analysis frameworks of the probability of strictly positive secrecy capacity (SPSC), secrecy outage probability (SOP), and average secrecy capacity (ASC) are further obtained for this UWOC system, which are confirmed by Monte Carlo (MC) simulations. Furthermore, the effects including the number of layers, the level of channel estimation error, the link distance, the location of the eavesdropper, the quality of the main and eavesdropping channels on this system are analyzed for different water conditions. The presented results give valuable insights into the practical aspects of deployment of UWOC networks.
Abstract This work investigates physical-layer security of underwater wireless optical communication systems under covert non-line-of-sight (NLOS) eavesdropping caused by scattered photons. A first-order single-scattering model is developed for the near-transmitter NLOS eavesdropping link by incorporating seawater absorption, scattering, beam divergence, receiver field of view, and Eve’s geometry. Turbulence-induced fading on the legitimate line-of-sight link is also considered. Based on photon-counting detection, secrecy capacity, interception probability, and secrecy outage probability are analyzed. Results show that stronger scattering enlarges the high-risk region of NLOS information leakage, while stronger Bob-link turbulence degrades the secrecy performance of the system. A comparison with multiple-scattering Monte Carlo photon tracing for Jerlov IB and Jerlov II waters shows close agreement in the dominant spatial photon-count distribution.
With the expansion of human activities into the ocean, there is an urgent need for information transmission technology with higher data rates and more security. On the other hand, satellite communication plays an important role in the maritime area, but is also vulnerable to eavesdropping because of its broadcast nature. Therefore, in this study, the physical-layer security for a hybrid satellite and underwater wireless optical communication (UWOC) system is considered. Specifically, a satellite source first broadcasts information to an overwater relay via an RF link, and then the relay converts the electrical signal to an optical one and transmits it to an underwater destination via a UWOC link, while an eavesdropper is overhearing this transmission. We derived the closed-form expressions for several system performance metrics, i.e., secrecy outage probability (SOP), and average secrecy capacity (ASC). Finally, Monte-Carlo simulations verify the accuracy of those results.
We analyze a secure two-hop mixed radio frequency (RF) and underwater wireless optical communication (UWOC) system using a fixed-gain amplify-and-forward (AF) relay. The UWOC channel is modeled using a unified mixture exponential-generalized Gamma distribution to consider the combined effects of air bubbles and temperature gradients on transmission characteristics. Both legitimate and eavesdropping RF channels are modeled using flexible α−μ distributions. Specifically, we first derive both the probability density function (PDF) and cumulative distribution function (CDF) of the received signal-to-noise ratio (SNR) of the mixed RF and UWOC system. Based on the PDF and CDF expressions, we derive the closed-form expressions for the tight lower bound of the secrecy outage probability (SOP) and the probability of non-zero secrecy capacity (PNZ), which are both expressed in terms bivariate Fox's H-function. To utilize these analytical expressions, we derive asymptotic expressions of SOP and PNZ using only elementary functions. Also, we use asymptotic expressions to determine the optimal transmitting power to maximize energy efficiency. Further, we thoroughly investigate the effect of levels of air bubbles and temperature gradients in the UWOC channel, and study nonlinear characteristics of the transmission medium and the number of multipath clusters of the RF channel on the secrecy performance. Finally, all analyses are validated using Monte Carlo simulation.
The advantages of physical layer security (PLS) have led researchers to apply this in various communication scenarios, such as radio frequency (RF) communication, free space optical (FSO) communication, and underwater optical wireless communication (UOWC). This paper analyzes the PLS, specifically the effective secrecy throughput (EST) over the mixture exponential generalized gamma (mEGG) UOWC channel link in the presence of a passive eavesdropper. To achieve this goal, firstly the closed-form expression of the secure outage probability (SOP) of the system is derived, and using that SOP expression, the final EST expression is found considering the underwater turbulence (air bubble and temperature gradient) as well as two different detection scenarios (heterodyne detection (HDD) & intensity modulation/direct detection(IM/DD)) in terms of Meijer-G functions. Moreover, the average secrecy capacity (ASC) of such a scenario is also exploited in this work. Finally, the expressions are verified through Monte-Carlo simulation.
In this work, the effective secrecy throughput (EST) which explicitly captures both the reliability and secrecy features of wiretap channels for an underwater wireless optical communication (UWOC) system has been optimized by considering the generalized transmit laser selection (GTLS) scheme. Specifically, the instantaneous channel state information (CSI) of the main channel and eavesdropper's channel is assumed to be unavailable at the transmitter, which is more practical for the underwater application. The fading of salinity induced oceanic turbulence channels is described by the Weibull distribution considering angular pointing error. The GTLS scheme is first introduced into the UWOC eavesdropping system to improve the quality of the main channel. Based on these models, the analytical closed-form expression of the EST with a constraint on the maximum allowed secrecy outage probability (SOP) is mathematically derived in terms of Meijer's G function and Gauss hypergeometric function. Then, an optimization problem with the objective of maximizing the constrained EST of the proposed UWOC system while ensuring wiretap code rate constraints are formulated. To solve this problem, a whale optimization algorithm (WOA) based wiretap code rates optimization method is effectively developed to acquire the maximum constrained EST, which is validated by the exhaustive search (ES) method. Simulation results suggest that the maximum constrained EST of this UWOC system can be significantly improved by the GTLS scheme. Besides, the maximum allowed SOP has a pronounced impact on the maximum constrained EST. The presented results could offer valuable insights into the physical layer security (PLS) performance enhancement of UWOC systems.