## 4 Experimental Verification  
To verify the performance of this communication system based on DSSS, static underwater experiments were carried out. The experimental equipment included two communication systems, transmitting and receiving electrodes. Since the experimental environment could not simulate an infinite unbounded water area, a 50 m \(\times\) 15 m \(\times\) 2 m swimming pool was chosen based on the actual situation. The measured electrical conductivity of the pool water was 739 \(\mu\)S/cm. As shown in Figure 11a, we experimented with electrode patches of different sizes, as shown in Figure 11b. By examining the potential difference of the receiving electrodes through an oscilloscope, we confirmed that using 100 mm \(\times\) 100 mm \(\times\) 0.2 mm transmitting electrodes and 80 mm \(\times\) 80 mm \(\times\) 0.2 mm receiving electrodes in the experiment results in the maximum received signal-to-noise ratio (SNR). Considering that this system will be installed on underwater robots in the future, the spacing between the transmitting electrodes is set to 60 cm, and the spacing between the receiving electrodes is also set to 60 cm to accommodate the size of underwater robots. The signal received by the receiving electrode undergoes digital signal processing in the FPGA, and the relevant signals are captured using a logic analyzer. Figure 12 illustrates the demodulation, despreading, and decision-making processes performed after the receiver receives the signal. The experimental scene is shown in Figure 11d. Firstly, we verified the impact of electrode angles on the electric field communication system and validated the dipole model. In this specific experiment, while keeping the receiving electrode stationary, we altered the angle between the transmitting electrode and the receiving electrode to evaluate the effect of different angles on communication performance. Experiments were conducted by rotating the transmitting electrode counterclockwise in 5\({}^{\circ}\) increments, and the communication error rate was measured. The experimental results are presented in Figure 13. When the angle was between 0\({}^{\circ}\) and 25\({}^{\circ}\) and between 145\({}^{\circ}\) and 180\({}^{\circ}\), the communication error rate was 100%. At 30\({}^{\circ}\), it was 7.88%, at 35\({}^{\circ}\) it was 18.43%, at 40\({}^{\circ}\) it was 39.26%, at 45\({}^{\circ}\) it was 60.05%, and at 50\({}^{\circ}\) it was 97.67%. As the angle between the transmitting and receiving electrodes approached parallel alignment (90\({}^{\circ}\)), the error rate gradually decreased. Beyond 90\({}^{\circ}\), as the angle approached perpendicular alignment, the error rate gradually increased. At 125\({}^{\circ}\), it was 96.25%, at 130\({}^{\circ}\) it was 61.72%, at 135\({}^{\circ}\) it was 21.58%, and at 140\({}^{\circ}\) it was 7.15%. For all other angles, the error rate was 0%. The experimental results indicate that the highest  
Figure 10: BPSK DSSS receiver structure. communication efficiency is achieved when the transmitting and receiving electrodes are parallel (\(0^{\circ}\), \(180^{\circ}\)), and the error rate reaches 100% when they are perpendicular (\(55^{\circ}\)-\(120^{\circ}\)). According to the theory of electric field communication, the induced electromotive force is the potential difference generated at the receiving electrode by the electric field produced by the transmitting electrode. When both pairs of electrodes are parallel, the potential difference generated is at its maximum, and when both pairs of electrodes are perpendicular, the potential difference is at its minimum. This result is in good agreement with the simplified dipole model and serves to establish the optimal electrode layout for subsequent static experiments. Figure 11: Experimental equipment and experimental setting. Figure 12: Receiver signal visualization.