## 3 Underwater Communication Experiment and Discussion
### BER Testing of the System  
In order to meet the system's demand for real-time BER detection, this study designed a BER test module at the digital end and verified the effects of AGC, filtering, and RS coding on the system's BER based on this design. At the transmitter, a pseudo-random sequence (PRS) of 10,000,000 bits was generated using a linear feedback shift register (LFSR), and used as the original data for transmission testing. At the receiver end, the corresponding random sequence was generated in the same manner and compared with the received data to calculate the system's BER. Firstly, the BER of the system was evaluated under various SNRs. Sampled signals at various SNRs are illustrated in Figure 12. As the SNR decreases, the received signal  
Figure 11: Effect of bilateral edge detection filtering. Figure 10: Effect of median filtering. waveforms become increasingly distorted. Specifically, when the SNR is reduced from 25 dB to 3 dB, the waveform of the sampled signal progressively deteriorates, but the data stream remains consistent. When the SNR is further decreased to 2 dB, the sampled signal begins to distort, making it difficult to recover the original signal using signal processing techniques. The BER test results are illustrated in Figure 13. At high SNRs, the BER of the three systems remains nearly identical and is well below the forward error correction (FEC) threshold of 3.8 \(\times\) 10\({}^{-3}\). When the SNR is below 15 dB, filtering can reduce the BER by one order of magnitude. Additionally, the incorporation of RS coding further reduces the BER by another order of magnitude. When the SNR is reduced to 3 dB, the BER of the original system becomes higher than the FEC, which fails to satisfy the communication requirements. Then, after the filtering process, the BER is reduced to 2.98 \(\times\) 10\({}^{-4}\), which is reduced by two orders of magnitude. With RS coding applied, the BER is further reduced to 5.38 \(\times\) 10\({}^{-5}\), and the reliability of the system is improved. These results demonstrate that the designed MF, BEDF, and RS coding effectively reduce the system BER. When the SNR is reduced to 2 dB, the sampled signal begins to exhibit errors. At this point, the filtering no longer effectively reduces the BER, which rises above the FEC threshold. The BER reaches 0.0472, exceeding the RS code's error correction capability of 0.0314. Consequently, the system BER increases to 0.0326, and the effectiveness of RS coding diminishes. Figure 12: Sampled signals under different SNRs. Figure 13: BER test of device with different SNRs. To further verify the effectiveness of improving system reliability through filtering and coding, the system's ability to resist underwater turbulence was tested under an SNR of 25 dB. As shown in Figure 7a, a gas pump was used to generate bubbles of varying intensities to simulate underwater turbulence. The gas pump features three flow rate settings and four bubble channels, with a maximum flow rate of 8 L/min, allowing for precise control of bubble intensity. The bubble intensity is calculated from the current flow and number of channels. The experiments were carried out by simulating different turbulence conditions. The results are shown in Figure 14. The system's BER is \(2\times 10^{-8}\) at a bubble intensity of 0 L/min, which represents the actual performance of image transmission under pure seawater. As the bubble intensity increases, the BER of all systems rises approximately linearly. However, the use of filtering and RS coding significantly reduces the system's BER under different bubble intensities. At a bubble intensity of 3 L/min, the original system's BER approaches the FEC. As the bubble intensity increases to 6 L/min, communication becomes unreliable without filtering. Filtering enhances reliability by nearly a factor of two, bringing the system closer to the FEC threshold. With RS coding applied, the system maintains reliable transmission even at a bubble intensity of 7 L/min.