A Comprehensive Review of Maritime Wireless Propagation: Salinity, Humidity, and Evaporation Duct Effects on 5G/6G Networks

Maritime and coastal communication systems are becoming increasingly critical for shipping, offshore industries, naval operations, and emerging applications such as the maritime Internet of Things (IoT), autonomous vessels, and coastal 6G networks. Unlike terrestrial environments, radio wave propagation over sea channels is strongly influenced by unique environmental factors, including salinity-dependent dielectric properties of seawater, high relative humidity, evaporation ducts, and sea surface roughness. These conditions alter reflection, scattering, and absorption mechanisms, leading to significant deviations from conventional terrestrial propagation models. Traditional methods such as the Free-Space Path Loss (FSPL), Two-Ray Ground Reflection (2-RGR) model, and ITU-R recommendations provide useful baselines but fail to fully capture the combined effects of salinity, humidity, and refractivity structures. This review consolidates state-of-the-art research on maritime wireless propagation by classifying existing models into theoretical, empirical, measurement-based, and machine learning-driven approaches, highlighting their strengths, limitations, and validation scenarios. Special attention is given to the role of evaporation ducts in extending communication ranges, the impact of salinity on seawater dielectric constants, and humidity-induced attenuation at high-frequency bands. The review also identifies open research challenges such as the scarcity of large-scale maritime datasets, limited validation at millimeter-wave and terahertz frequencies, and the need for hybrid AI-physics models. The insights presented here aim to support the development of more robust propagation models and adaptive techniques for future 5G/6G maritime networks.