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IEEE Journal on Selected Areas in Communications

Covert Performance of Bidirectional Air–Water Cross-Boundary Optical Wireless Communication Systems

Qingqing Hu, Bin Chen, Chen Gong, Murat Uysal

Published Nov 25, 2025• 1 citation
Publisher

Abstract

Optical wireless communication (OWC) has been recognized as a promising solution for establishing high-speed, low-latency data connectivity between air and water. During the propagation of optical beams across dynamic water surfaces, random scattering and refraction can result in information leakage issues, significantly increasing the probability of an adversary listener (Willie) detecting the signal transmission. In this paper, we investigate the covert performance of bidirectional air-water cross-boundary OWC systems under the detection of arbitrary underwater/aerial Willie for the first time. To characterize the detection accuracy at Willie and communication quality between a legitimate transmitter (Alice) and a legitimate receiver (Bob), the covert performance metrics including the covertness outage probability and covert throughput are computed based on the theoretical derivations of channel gain under dynamic wave, surface refraction, and particle absorption and scattering. The numerical simulation reveals the variation patterns of covert performance metrics with respect to multiple environmental and system parameters, which shows that wave intensity and transmitted beam’s divergence angle have opposite effects on detection performance of Willie locating at different positions, and both the optical wavelength beyond blue-green band and strong ambient light can increase Willie’s detection uncertainty. Furthermore, it can be seen that the upper bound on covertness outage probability reaches 1 and the achievable covert throughput approaches 0 in some cases, while covert threat regions exist in both atmospheric and underwater areas within several meters, which indicates that covert transmission strategy and system parameters are required to be further designed and optimized to enhance communication security.

DOI
10.1109/jsac.2025.3637016

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Optical wireless communication (OWC) has been recognized as a promising solution for establishing high-speed, low-latency data connectivity between air and water. During the propagation of optical beams across dynamic water surfaces, random scattering and refraction can result in information leakage issues, significantly increasing the probability of an adversary listener (Willie) detecting the signal transmission. In this paper, we investigate the covert performance of bidirectional air-water cross-boundary OWC systems under the detection of arbitrary underwater/aerial Willie for the first time. To characterize the detection accuracy at Willie and communication quality between a legitimate transmitter (Alice) and a legitimate receiver (Bob), the covert performance metrics including the covertness outage probability and covert throughput are computed based on the theoretical derivations of channel gain under dynamic wave, surface refraction, and particle absorption and scattering. The numerical simulation reveals the variation patterns of covert performance metrics with respect to multiple environmental and system parameters, which shows that wave intensity and transmitted beam’s divergence angle have opposite effects on detection performance of Willie locating at different positions, and both the optical wavelength beyond blue-green band and strong ambient light can increase Willie’s detection uncertainty. Furthermore, it can be seen that the upper bound on covertness outage probability reaches 1 and the achievable covert throughput approaches 0 in some cases, while covert threat regions exist in both atmospheric and underwater areas within several meters, which indicates that covert transmission strategy and system parameters are required to be further designed and optimized to enhance communication security.
Covert communications hide the information transmission from a watchful adversary (Willie) while ensuring a satisfactory decoding performance at a legitimate receiver (Bob). In this paper, we propose a covert transmission scheme based on narrow signal waveband and nonlinear suppression effect of avalanche photodiodes (APDs) for water-to-air (W2A) optical wireless communication (OWC) systems. Narrow signal waveband is selected based on a given wavelength selection pattern which is negotiated by the transmitter and receiver. Willie’s detection performance is investigated under APD characteristics and ambient radiation. Under the covertness constraints on Willie’s detection performance, the covert throughput between Alice and Bob is maximized by optimizing the power and blocklength of transmitted signals. Except for the case of Alice with knowledge of Willie’s position and optical filter passband-width, we explore a more practical scenario where the knowledge is not available to Alice. The optimal passband-width of Willie’s filter is calculated by taking into account both overlapped waveband and detection performance, and a covert region is established to limit Willie’s detection range. Numerical and experimental results indicate that strong ambient radiation and narrow signal waveband can enhance the signal covertness significantly.
This paper investigates the physical-layer security for water-to-air (W2A) visible light communication (VLC). The impacts of surface fluctuation and underwater scattering on the channel gain are theoretically analyzed, which is verified in a laboratory W2A-VLC link. The statistical properties of wave slope under experimental conditions are measured by image processing and the probability density function of the slope can be approximated by a logistic distribution. We observe that dynamic waves can cause information leakage and experimental results indicate that the probability of secure communication increases from 90.39% to 96.18% as the eavesdropper’s elevation increases from 1.15 m to 1.42 m, which are approximately consistent with the numerical results. To investigate the parameters affecting the secrecy performance, several secrecy performance metrics are numerically calculated under different wave intensities, eavesdropper’s locations, divergence angles of LED, underwater scattering coefficients and wavelengths of transmitted beam. It is seen that the secrecy performance is degraded as the wave intensity increases, while random scattering can mitigate the water surface fluctuations and reduce the probability of information leakage.
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.
This paper investigates the covert communication in the scenario of counter detection. In traditional covert communication design where the uncertainty of the position of the Warden Willie typically reduces covert performance. To overcome this limitation, we leverage the sensing ability of the ISAC system in a new way to reduce the channel uncertainty between Alice and Willie, thereby enabling less conservative power allocation and significantly improving the achievable covert throughput. In the first stage, the dual functional sensing and communication (DFRC) base station Bob sends joint signals to estimate the azimuth of Willie and transmits information to Alice at the same time. In the second stage, Alice uses the obtained azimuth information of Willie to adjust transmit power while meeting the covert constraints. We build a mathematical model of azimuth angle estimation error and connect it with the Cramér-Rao Bound (CRB) to change it into distance error. This connects sensing accuracy with channel state information (CSI) uncertainty in a linear way, allowing Alice to adopt a higher transmit power while still satisfying the covert constraint, and thus improving the achievable covert throughput. To increase the covert throughput of the user, we use a layered solving strategy to decouple the original problem. First, we use S-procedure to change the constraint on possible distance error into a linear matrix inequality (LMI) constraint and then find the upper limit of transmit power on the user side. Second, we combine it with the uplink communication link rate constraint to find the closed form expression of the resource allocation strategy in vector space. Finally, we use one dimensional search to find the optimal time slot allocation strategy. Simulation results show that the proposed two-stage protocol effectively improves the achievable covert throughput by reducing position uncertainty through sensing.
Covert communication enforces the transmitted signal to be indistinguishable from other signals under wireless environment, making it difficult for an eavesdropper to detect and intercept the message. The most common approach within covert systems is the augmentation of artificial noise (AN), ensuring reliable transmission. However, this strategy becomes problematic when the eavesdropper's position is uncertain, as continuous amplification of interference power can precipitate a substantial decline in covert communication capacity. In this paper, we investigate the covert communication between legitimate transmitter (i.e., Alice) and full duplex receiver (i.e., Bob) in the presence of the location uncertainties of the eavesdropper (i.e., Willie). We first derive the sufficient condition for achieving covert communication through robustness analysis. Then, we derive the effective throughput to measure the covert communication capacity. Afterward, by using joint optimization technology, the optimal AN power of Bob and the number of antennas for Alice are jointly designed to maximize the effective throughput subject to a covertness constraint. Theoretical analysis indicates that reducing the AN power while increasing the number of antennas helps to improve the system capacity, the effective throughput between Alice and Bob is inversely proportional to the distance between them. Finally, the validity of our analytical approach is corroborated by simulation results.
Much of security research focuses on preventing an adversary from deciphering a message's content, but there are a number of applications that motivate the more challenging goal of “covert” communications: transmitter Alice conveying information to legitimate receiver Bob while preventing a capable and attentive adversary Willie from detecting the presence of the message. Willie detects Alice's transmission by looking for deviations from the background environment; hence, covert throughput can be increased if Willie has uncertainty about that background environment, as might be caused by user mobility or time-varying jamming. Here, we consider how the dynamics of the background environment impact the throughput of covert communications. Our previous work has shown that a background environment varying a constant number of times per codeword duration can greatly increase the covert throughput. In this paper, we consider the covert throughput for all potential rates of variation of the background. Neither extreme of variation is favorable to Alice: (1) when the background environment varies too quickly relative to the codeword length, Willie can average out the variation and hence limit Alice's covert throughput; (2) when the background environment varies too slowly, Willie can estimate the current environment and again limit Alice's covert throughput. We analyze a detector at Willie that provides an upper bound (converse) to the covert throughput of Alice for all potential rates of variation. Next, we provide an explicit construction for a scheme for Alice that provides a lower bound (achievability) on the covert throughput at each rate of variation.