## 1 Introduction  
The oceans cover three-quarters of the earth's surface and possess abundant resources. Especially with the scarcity of terrestrial resources, marine development has been attracting emerging attention in recent years [1, 2]. Underwater wireless communication (UWC) technology plays a significant role in marine resource exploration, aquaculture development, military defense tasks, ecosystem monitoring, and maritime search and rescue [3, 4]. UWC refers to the method of data transmission in the unguided water medium. The most commonly used methods are acoustic communication, underwater electromagnetic wave communication, and underwater optical wireless communication (UOWC) [5]. Compared with the other two technologies, the UOWC technology has apparent advantages in data rate, transmission delay, device size, and power consumption, making it an attractive and practical solution for short- to medium-distance and high-speed communication in underwater environments [6, 7]. The blue-green wavelength (450-550 nm) light has a window of low absorption in seawater, which is the premise of the UOWC system application [8]. Laser diodes (LDs) and light-emitting diodes (LEDs) are commonly utilized as light sources in typical UOWC systems. In general, the LD light source is more suitable for high-speed and long-distance transmission due to its excellent collimation and high modulation bandwidth. However,the acquisition and tracking issues of the laser beams will limit the mobility of the UOWC system in some scenarios, such as regional communication applications for autonomous underwater vehicles (AUVs) [9; 10]. For example, when AUVs collect data from underwater fixed sensor nodes, the precise alignment increases the burden on the AUV's navigation and attitude control systems. Especially for AUV swarm application scenarios, as shown in Figure 1, when they accomplish collective tasks, such as data collection from sensor networks or seabed mapping, a high-mobility and wide-coverage UOWC system is urgently needed [11; 12]. Moreover, for the real-time ocean observing system, the UOWC also plays an important role in information relay in natural hazards monitoring and the collection of biogeochemical parameters for seawater [13]. In comparison, the LED light source has a larger divergence angle with reduced alignment requirements, which is more suitable for underwater mobile networks and has been widely researched in recent years. In addition, the LED provides significant advantages in terms of power consumption, temperature sensitivity, system costs, and easy compatibility with underwater lighting equipment [14]. The Woods Hole Oceanographic Institution of the United States of America developed a low-power UOWC system with LEDs and the IrDA protocol to achieve a 5 m transmission in the seawater, proving the feasibility of LEDs in underwater communication systems [15]. [PERSON] et al. designed two prototypes of AquaOptical I and AquaOptical II. A bidirectional transmission with a data rate of several Mbps based on LEDs was achieved over a transmission distance of 50 m [16; 17]. LEDs have also been used in commercial underwater modems, such as the Sonardyne BlueComm, which can operate over 150 m at a maximum rate of 10 Mbps [11; 18]. However, the modulation bandwidth of the traditional commercial LED source is only approximately 3 MHz due to its large RC time constant, which limits the data rates of the system [19]. Several approaches have been explored to improve the \(-3\) dB modulation bandwidth and realize faster data transmission [20]. The equalization technique is one of the bandwidth enhancement approaches. In [21], the \(-3\) dB modulation bandwidth of the proposed UOWC system was extended over 700 MHz by utilizing a hardware single-stage equalizer. With a cascaded pre-equalization circuit, a bandwidth of 366 MHz and data rates of 1.3 Gbps were achieved [22].