Optimal Guard Interval for Air-to-Water Communication System Under Strong Background Radiation

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.