Title: Performance Enhancement via Real-time Image-based Beam Tracking for WA-OWC with Dynamic Waves and Mobile Receivers

Abstract: Intensified underwater activities have driven the escalating demand for reliable, flexible, and high data-rate underwater communication links. Optical wireless communication (OWC) emerges as the most promising technology for short- to medium-range communication, facilitating the real-time high-speed transmission of information from undersea to an aerial vehicle which can subsequently relay the information to a terrestrial station. However, establishing a robust water-air link confronts two primary challenges: (i) beam wandering due to the time-varying refraction when the light beam passes through the undulating ocean surface and (ii) the drone's movement when it hovers above the ocean surface. This paper experimentally demonstrated a real-time imaged-based beam tracking system to mitigate beam misalignment due to dynamic waves and receiver movement over a 0.14-m underwater and 1.83-m free-space OWC channel. Experimental results evince a notable reduction in the standard deviation of the received light spot offset from the receiver. Moreover, the tracking system can proficiently accommodate receiver velocities of up to 150 cm/s while maintaining a paltry packet loss rate (PLR) below 10%. By addressing the combined effects of dynamic waves and moving receivers, the proposed beam tracking system successfully enables a 70% reduction in PLR and an order of magnitude decrease in bit error rate (BER). This results in a substantial 17-fold surge in maximum throughput, from 50 Mbps to 850 Mbps. The experimental results validate the feasibility and effectiveness of the beam tracking system for vanquishing the detrimental effects in the complex water-air OWC (WA-OWC) channel and supporting high-speed data transmission.