## 1 Introduction
Regarding channel coding, forward error correction (FEC) codes, particularly RS codes and low-density parity-check (LDPC) codes, have been extensively employed in UWOC systems. RS codes have high error correction capability, but their high computational complexity may lead to large delays [31]. LDPC codes excel in performance close to the Shannon limit, but their FPGA implementation is difficult [32]. Additionally, Turbo codes and convolutional codes have been utilized in UWOC systems and their implementation complexity is also much higher than RS codes, so there is a need to explore better hardware implementations for scenarios with high BER requirements [33; 34]. In hardware implementation, FPGAs have emerged as the preferred platform for UWOC systems due to their flexibility, high performance, and parallel computing. Unlike communication systems based on signal generators achieving Gbps rates, FPGA-basedimplementations offer enhanced real-time performance and miniaturization, significantly improving practical applicability [35; 36]. Considering the power supply constraints in underwater environments, research efforts should focus on low-power design and resource optimization in hardware implementation. The development of novel power supply solutions that enable self-powered systems is crucial to meet the requirements for prolonged underwater operations [37]. Furthermore, the performance of optical components, including light source types, receiver sensitivity, and beam divergence angle, significantly influences the communication distances and data rates of UWOC systems [38; 39; 40; 41]. The above analysis indicates that, despite significant advancements in the development of UWOC systems, the complex characteristics of underwater environments, particularly optical turbulence, suspended particulate matter, and multipath propagation effects continue to present substantial challenges to achieving reliable communication and extending operational range [42; 43]. Even when the transmission wavelength is selected within the blue-green spectral window, the optical signal still undergoes considerable attenuation and multipath fading due to absorption and scattering effects. Previous studies, however, have been limited in their specificity regarding hardware implementation design and have not effectively addressed the noise and interference inherent in underwater environments. Furthermore, these studies have paid insufficient attention to optimizing resources, energy consumption, and delay in FPGA implementations, ultimately impacting the practicality of the systems. In 2022, [PERSON] et al. [35] reduced the burr interference generated by the threshold judgment during OOK demodulation by moving-average filtering, and achieved a transmission distance of 20 m and a BER of \(10^{-6}\) at 5 Mbps in clear seawater. In 2024, [PERSON] et al. [36] suppressed light intensity flicker caused by underwater turbulence by employing a board-level AGC circuit and combined it with the Altera Reed Solomon IP core for error correction, and achieved a 22 Mbps rate with a BER of \(2.467\times 10^{-4}\) in a 12 m underwater channel. While these studies have addressed certain aspects of the existing challenges, limitations persist in hardware resource optimization, environmental adaptability, and optical signal attenuation mitigation, indicating substantial potential for further improvement. To address these challenges, this paper proposes a series of innovative solutions: firstly, a fully digital AGC system is designed to effectively resist light intensity flicker; secondly, BEDF and MF are used to eliminate burrs and spike noise, respectively, which significantly improves the system's ability to resist turbulence and noise interference; in addition, the RS coding IP is implemented in hardware, which is optimized to significantly reduce the resource loss and decoding delay; finally, a dedicated collimated beam expanding system is designed to significantly reduce the beam divergence angle and extend the communication distance.