Design and implementation of underwater optical wireless communication prototype system supporting automatic beam alignment

We implement a prototype of the underwater optical wireless communication (UOWC) system, incorporating an adaptive optical beam alignment mechanism to ensure the seamless alignment of the optical beam between the transmitter and receiver. In this system, the transmitter, utilizing a laser diode as an optical source, leverages the camera's object recognition capability to ascertain the position of the receiver, which may experience movement and shaking due to various external factors. However, the underwater condition poses challenges to camera visibility. To address this issue, a green light-emitting diode (LED) is installed on the receiver to make it easier for the transmitter to locate the position of the receiver within the camera image. The detected LED position is converted into angular data, enabling the operation of a servo motor to adjust the transmitter's beam pointing, via a 2-axis servo motor pan-tilt system. Moreover, in the proposed UOWC prototype system, we employ the timer interrupt algorithm integrated with a real-time operating system and the direct register access method for general-purpose input/output on the NVIDIA Jetson Nano. This strategic combination facilitates the reduction of errors within the timer interval cycle between the transmitter and receiver, optimizing communication reliability through precise control of timer interrupts. To assess the system's performance, extensive experimental tests are conducted in a large water tank. (c) 2024 The Authors. Published by Elsevier B.V. on behalf of The Korean Institute of Communications and Information Sciences. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).