## 3 Simulation of Underwater Electric Field Communication
### CSMA/CA Channel Backoff Algorithm Design and Simulation Analysis
#### 4.2.4 Channel Detection
To achieve a miniaturized and integrated underwater electric field wireless communication system, a communication system combining digital and analog circuits was designed. The transmitting end of the communication system includes a transmitter and a transmitting electrode. The transmitter system can be divided into PC, FPGA, analog-to-digital converter, and power amplifier. The PC upper computer sends raw information to FPGA through the UART communication interface, and combines the received binary signals into decimal information in FPGA for channel encoding. Subsequently, the encoded data are converted into binary signals and frequency shift keying modulation is performed in FPGA. The continuous phase digital signal generated by modulation is converted into a continuous phase analog frequency band signal through a high conversion rate DAC circuit. Finally, the converted analog frequency band signal needs to be power amplified, and the amplified frequency band signal is emitted into the water through a pair of electrodes, forming an electrostatic field in the water. In this article, an underwater electric field communication system based on 2 FSK was built, and data transmission between underwater AUVs was completed, laying the foundation for AUV multi-node communication. The main process is shown below. Firstly, AUV1 sends communication data through Raspberry Pi, which undergoes 2 FSK modulation, analog-to-digital conversion, and power amplification before being transmitted to the underwater channel through the transmitting electrode. Then, the AUV2 receiving the electrode processes the received communication data through differential amplification, bandpass filtering, variable gain amplification, and digital-to-analog conversion into the FPGA for processing, completing the signal's DAC drive, adaptive gain control, and 2 FSK envelope demodulation. Finally, it is uploaded to the Raspberry Pi to complete the entire communication process. The communication process of AUV is based on 2 FSK, as shown in Figure 12.

Figure 11: Schematic and physical diagram of the 2 FSK underwater electric field communication system. (**a**) Schematic diagram of the experiment. (**b**) Physical system of underwater electric field communication. To verify the feasibility of underwater dynamic communication for AUVs, this section details the experiments conducted on AUV underwater electric field dynamic communication. As the AUV swarm moves during the process of tracking and capturing, it is necessary to achieve dynamic mobile communication for the underwater electric field. Therefore, this study experimented on underwater electric field dynamic communication, as shown in Figure 13, where the transmitting and receiving electrode plates and the 2 FSK underwater electric field communication system are installed on two AUVs. The communication distance is changed by moving the two AUVs. In this process, the receiving end of the PGA amplification circuit adjusts the amplification factor through the FPGA program to amplify the received signal voltage to the appropriate level to meet the

Figure 12: Communication process of AUV based on 2 FSK. Figure 13: AUV based on 2 FSK communication system. demodulation requirements. The PC Raspberry Pi remote control software is used to view the demodulation information, as shown in Figure 14.

The point-to-point dynamic communication experiment of the underwater electric field was conducted in a water tank with dimensions of 50 m in length and 25 m in width. The purpose of the experiment was to verify the effectiveness of underwater electric field communication for mobile AUVs by adjusting the relative distance between AUV Node 0 and AUV Node 1.