Synchronization is a fundamental problem in communication system. In this paper, we consider a scenario where an unmanned aerial vehicle broadcasts information to several underwater unmanned vehicles. Beacon signals are adopted to achieve global time synchronization. We conduct experiments in a laboratory water tank and an outdoor deep water pool under different background radiation intensities. The experimental results demonstrate that spatial optical filter can mitigate the impact of background radiation. Additionally, via analyzing the experimental data, it can be concluded that the number of consecutive unsynchronized frames in air-to-water visible light communication system follows a geometric distribution. Then, we analyze the probability of data slot overlaps due to offset accumulation. We calculate the optimal guard interval for two-user scenario to maximize throughput via semi-analytical method, which is extended to n-user scenario via numerical approaches. Finally, the practical feasibility of the transmission model is validated via comparing the results of semi-analytical model and experiments.

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.

In this work, we develop a theoretical transmission model for optical wireless communication systems with receiver nonlinearity. To provide a framework for analyzing transmission rates, we derive lower and upper bounds on channel capacity under average and peak optical power constraints in both single-input single-output and multiple-input multiple-output systems using entropy power inequalities and dual expression. Numerical results verify the tightness of our derived capacity bounds and highlight the significant relationship between channel capacity and receiver nonlinearity.

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.