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.

Considering the scattering characteristics of wireless ultraviolet (UV) channels, the double-scatter assumption is revealed to well characterize the scattering-enhanced channel over a range of less than 100 m. Based on the cuboid scatterers with representative shapes, we present a ray-tracing algorithm under scattering enhancement conditions and give the probability density function of the free path in a non-uniform atmosphere. Then, we propose a semi-analytical (SA) link gain model with scattering enhancement, via generating a set of coordinates for a series of conical integral paths using a rotation matrix. On this basis, a one-dimensional integral expression for the single-scatter of the proposed SA link gain model is obtained, and the expression for the double-scatter is further derived. The link gain evaluation results show that the presence of scattering region can improve the received power of the non-line-of-sight UV system at a receiver. For all considered transceiver geometry, the results of the proposed SA model match well with the counterparts of Monte-Carlo (MC) based numerical calculation, with a gap of around 2 dB for both narrow and wide beam. In terms of computational efficiency, the execution time of the proposed SA model for the single-scatter and double-scatter is significantly reduced to 0.065% and 9.69% of those of the MC simulations, respectively. We further establish experiments using a fog-filled quartz cubic cavity in an outdoor environment to verify the correctness of the proposed model. The link gain from the numerical evaluation assuming double-scatter can improve the agreement with the experimental results by 1 dB compared with that of the single-scatter assumption.

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.