Real-time beaconless MEMS mirror-based beam steering and tracking system for mobile underwater optical wireless communication

Underwater optical wireless communication (UOWC) provides high-speed and low-latency links for underwater applications, but the narrow divergence of laser beams makes reliable beam steering and tracking challenging for mobile underwater terminals. This paper presents a real-time microelectromechanical system (MEMS) mirror-based beam steering and tracking system for mobile UOWC. Unlike beacon-assisted alignment approaches that require an additional active optical source, the proposed system achieves beam acquisition and tracking through optical feedback from the communication link itself. A robust coarse acquisition strategy combining rotating raster scanning, sliding-window density detection, and spatial clustering is developed to identify valid optical returns through spatio-temporal signal aggregation rather than isolated intensity peaks. The proposed approach is implemented in an integrated waterproof optical terminal incorporating laser-diode transmission, a two-axis MEMS mirror, photomultiplier-tube-based optical feedback sensing, real-time control implemented on a field-programmable gate array, and communication modules. Experimental results demonstrate a minimum average acquisition time of 2.01 s, tracking errors within ±10 mm at 0.25 m/s and ±15 mm at 0.50 m/s. A 50 m swimming-pool experiment with a remotely operated vehicle-mounted terminal further validates reliable UOWC over 50 m at 10 Mbps, 25 m at 50 Mbps, and 20 m at 100 Mbps. These results support robust beam steering and tracking against misalignment for mobile UOWC under the tested swimming-pool conditions.