## 5 Conclusions  
In this paper, a new control structure for the AUV motion based on real-time commands using the developed UWOC system is studied. In this control structure, control inputs for the AUV attitude from outside of the water are transferred to the AUV for motion control, while its posture data and video images from the AUV camera are sent to the control station outside of the water via the UWOC system. The UWOC system capable of two-way communication was constructed for wireless optical communication between the AUV and other underwater or over-water equipment, and the LEDs of the AUV transmitter were arranged in a circular form to increase the light-emitting area. A PIN photodiode was used for the AUV receiver. The UWOC speed of 1 Mbps equivalent to 21.2 FPS with VGA (640 \(\times\) 480) resolution for video image streaming was implemented between the AUV and the control station over the water. Also, an LPF filter was used to block noise from ambient light, and the RZ modulation technique was successfully applied to prevent signal errors caused by the LPF. Followed by the identification of the damping coefficient, a method for estimating the UWOC communication distance was proposed with reference to the received light intensity and sensor area. For attitude control, AHRS and a depth sensor were integrated, four thrusters were installed, and a PID controller was designed to regulate the AUV's motion. The experiment was conducted in an indoor water tank to verify communication performance and attitude control. The experiment was successful in controlling the roll and pitch attitude of the AUV. The roll and pitch motion were effectively controlled within 5\({}^{\circ}\) and 2.5\({}^{\circ}\) in real time, respectively. Despite the influences of the magnetic field around the indoor water tank and the electric field inside the AUV on the AHRS sensor, the yaw angle was adequately controlled within 3.7\({}^{\circ}\). Particularly, the error in depth was less than 2 mm even at the shallow depth of the water tank, indicating an effective control. While controlling the AUV, the video data from the underwater image with 1 Mbps equivalent to 21.2 FPS with VGA (640 \(\times\) 480) resolution and the roll and yaw motion data of the AUV were successfully transferred to the over-water control station. Ultimately, this experiment demonstrated that it was possible to monitor images of the underwater AUV from the ground in real-time and to control the attitude of the AUV in real time through the developed UWOC system. Future work will evaluate the developed UWOC system in real marine environments with varying levels of turbidity to assess its practical performance. It will also focus on enhancing communication algorithms and refining filtering schemes to maintain reliable connectivity under dynamic and unpredictable water conditions. In parallel, more robust and low-power hardware components will be designed and fabricated to withstand long-term conditions in harsh marine environments. Additionally, efforts will be made to  
Figure 22: Picture of the video images transferred from AUV. calculate the exact bit error rate (BER) and increase it so that we can verify the reliability of the communication between the land and underwater modules. Conceptualization, S.K. and H.C. ([PERSON]); Data curation, R.Z. ; Formal analysis, D.J. ([PERSON]), R.Z. and H.C. ([PERSON]); Funding acquisition, D.J. ([PERSON]) and H.C. ([PERSON]); Investigation, D.J. ([PERSON]); Methodology, D.J. ([PERSON]), R.Z. and H.C. ([PERSON]); Project administration, D.J. ([PERSON]) and H.C. ([PERSON]); Software, D.J. ([PERSON]) and H.C. ([PERSON]); Supervision, H.C. ([PERSON]); Validation, H.C. ([PERSON]); Visualization, R.Z. ; Writing--original draft, D.J. ([PERSON]) and H.C. ([PERSON]); Writing--review and editing, D.J. ([PERSON]), S.K. and [PERSON]. ([PERSON]). All authors have read and agreed to the published version of the manuscript.