Sustainable Marine Structures

Key areas such as marine resource exploration, real-time monitoring of ecological environments, and national defense security systems urgently require reliable underwater information transmission capabilities as a foundation. Underwater acoustic communication (UAC), leveraging its unique advantages as the most effective method for long-range data transfer in aquatic environments, has become an indispensable enabling technology for supporting these core applications. This review systematically examines recent advancements in UAC technology and their critical role in enabling modern marine initiatives. The analysis covers key developments in both non-coherent and coherent communication systems, including single-carrier and multi-carrier modulation schemes such as OFDM. It highlights their respective advantages in terms of robustness and high-data-rate transmission. The significant impact of challenging underwater channel characteristics, notably severe multipath fading, time-varying Doppler shifts, limited bandwidth, and environmental noise, is discussed alongside corresponding mitigation strategies. Furthermore, the integration of machine learning for sophisticated channel estimation, adaptive equalization, and intelligent system optimization is explored as a promising frontier. Emerging technologies like spread-spectrum, full-duplex, and covert UAC are also evaluated for their potential in specialized and high-stakes applications. The paper concludes by identifying persistent challenges, including regulatory constraints, physical-layer security issues, interoperability across platforms, and energy efficiency demands. Finally, it outlines future research directions aimed at developing more intelligent, secure, and efficient next-generation underwater networks.