# Recent Progress on Underwater Wireless Communication Methods and Applications
## 2 Main Methods of Underwater Wireless Communication
### Underwater Wireless Optical Communication  
Underwater Wireless Optical Communication (UWOC) has recently garnered significant attention for its ability to facilitate high-speed, low-latency, and high-bandwidth underwater communication. Operating within the blue-green light spectrum (450-550 nm), UWOC overcomes the limitations of traditional acoustic and radiofrequency methods. This makes it particularly suitable for applications such as oceanographic research, underwater monitoring, and marine resource exploration [59; 60]. Unlike acoustic signals, which are hindered by long propagation delays and low data rates, or radiofrequency signals, which suffer from severe attenuation in water, UWOC offers a high-performance and energy-efficient solution, especially for short--to medium--range communication [61]. The core of UWOC technology lies in its use of laser diodes (LDs) or light-emitting diodes (LEDs) as transmitters, coupled with advanced modulation techniques. These techniques include OFDM, Non-Return to Zero On-Off Keying (NRZ-OOK), and Optical Code Division Multiple Access (OCDMA), all of which are designed to boost data throughput, efficiency, and system robustness [61; 62]. LDs, with their high energy concentration and modulation bandwidth, are ideal for long-distance and high-speed communication scenarios [63; 64]. LEDs, on the other hand, with their broad beam dispersion and cost advantages, are better suited for short-range or broadcast applications where link alignment is less critical [65; 66]. These characteristics endow UWOC systems with remarkable versatility, enabling them to meet the diverse needs of underwater sensor networks, AUVs, and real-time monitoring systems [59; 63]. Recent advancements in UWOC systems have demonstrated their potential for high-speed data transmission across different distances. One study developed a UWOC system using a 450-nm laser diode paired with a silicon avalanche photodetector. At 80 mA and with 51.3 mW optical power and a 1 GHz modulation bandwidth, the system achieved data rates of 2 Gbps over 12 m and 1.5 Gbps over 20 m, maintaining BER within forward error correction (FEC) limits. This highlights its suitability for high-speed, short-range applications [67]. Another system, utilizing a 520-nm green laser diode with NRZ-OOK modulation, reached 2.70 Gbps over 34.5 m, along with 4.60 Gbps at 2.3 m and 3.48 Gbps at 20.7 m. Predictions indicate potential for 0.15 Gbps at 90.7 m and 1 Gbps at 62.7 m, suggesting its capability for long-distance, real-time UWOC applications [62]. Optical OFDM has also seen progress, with a 405-nm OFDM signal transmitted via FPGA achieving 1.45 Gbps over 4.8 m with BER within FEC limits [61]. This implies that further optimization could enhance transmission distance and system resilience in complex underwater conditions. A mid-range application study showcased a system with a 520-nm laser diode and NRZ-OOK modulation, achieving 500 Mbps over 100 m with BER below FEC thresholds. Using reflective mirrors in a 10-meter water tank, predictions suggested communication ranges of 146 m at 500 Mbps and 174 m at 100 Mbps, demonstrating the system's feasibility for practical long range deployment [63]. Additionally, coherent detection techniques have been proven effective for robust high-speed UWOC under challenging conditions. An experimental demonstration of an 8-Gbps quadrature-phase-shift-keying (QPSK) system using a 532-nm signal and coherent heterodyne detection showcased superior performance under scattering environments, achieving low BERs even in high-scatter conditions. This approach highlights the potential of phase modulation and coherent detection in achieving high data rates with enhanced system reliability [64]. Despite these advancements, UWOC systems face challenges from light absorption, scattering, and underwater turbulence, which degrade performance. Environmental factors such as salinity, temperature variations, and particulates significantly impact signal quality.