## 3 Underwater Communication Experiment and Discussion
### BER Testing of the System
When the bubble intensity further increases and the BER exceeds the error correction capability of RS coding, the system's BER dramatically increases. These experiments demonstrate that MF and BEDF effectively enhance system reliability, particularly in complex environments. The performance of the fully digital AGC system was then evaluated in simulated clear seawater, and the results are shown in Figure 15. Under different bubble strengths, the design of the AGC module leads to a significant reduction of the BER, and the effect is more obvious as the bubble strength increases. Without AGC adjustment, the system's BER increases faster between 4 and 5 L/min bubble strength, indicating that it exceeds the error correction limit of RS coding. The BER is even higher than the FEC at 5 L/min, with a BER of 1.85 \(\times\)\(10^{-2}\), which makes it difficult to realize reliable communication. The BER of the system decreases to 6.8 \(\times\)\(10^{-5}\) after gain control, demonstrating a significant improvement in its anti-turbulence capability. Finally, the corresponding experiments were analyzed under three different water quality conditions: clear seawater (I), coastal seawater (II), and turbid harbor water (III). Clear seawater, characterized by a high amount of dissolved particles, contrasts with coastal seawater, which has higher turbidity due to increased dissolved and suspended particles, and turbid harbor water, which exhibits the highest turbidity and severely limits light propagation due to strong absorption and scattering effects. The total link loss coeffi  
Figure 14: BER test of device with different bubble strengths. cients, attributed to underwater absorption and scattering, are 0.151 m\({}^{-1}\), 0.339 m\({}^{-1}\), and 2.195 m\({}^{-1}\) for clear seawater, coastal seawater, and turbid harbor water, respectively [51]. To simulate coastal seawater, natural seawater was collected from the coastal area of Dushu Lake in Suzhou. The seawater was filtered to remove large particles while retaining its natural constituents. Fine sediment particles (such as clay and silt) and organic matter (such as algae and detritus) were then added to the filtered seawater to adjust the concentration and achieve the desired water quality level. The proportions of these additions were carefully controlled to ensure that the total attenuation coefficients, calculated using Equation (6), were 0.147 m\({}^{-1}\), 0.333 m\({}^{-1}\), and 2.114 m\({}^{-1}\), respectively, which approximately simulate the three water qualities. The communication was tested at distances ranging from 2 m to 10 m under three simulated water qualities, and the results are illustrated in Figure 16. With the increase in transmission distance, the system's BER increases approximately linearly. When the transmission distance reaches 8 m in class III water quality, it is difficult for the receiver to detect the laser signal and the communication link is broken. The results show that when the received light intensity is within the detectable range of the APD, the BER of the system is lower than the FEC in the three types of water quality, which satisfies the demand for reliable communication in a wide range of underwater environments. Figure 11:0 BER test of the system at different communication distances. Figure 11:1 Impact of the AGC module on system BER.