# Wireless Channel Models for Over-the-Sea Communication: A Comparative Study
## 7 Challenging Issues and Research Directions
### Challenging Issues  
Due to the fluctuant and uncertain nature of the marine environment, it is quite challenging to develop a robust wireless channel model for over-the-sea wave propagation. Challenges that need to be addressed in the study of this area are briefly discussed in this subsection.  
**Path loss model:** Path loss can be caused by reflection, diffraction, scattering, refraction, etc. As the sea surface is an unrest surface, scattering of waves is large, and when developing a path loss model, refraction phenomena should be carefully considered. Popular models are two-ray, FSL, and three-ray path loss models. Researchers have to carefully select a model. For example, if someone intends to design a wireless channel that will serve for long-distance wave propagation, a three-ray path loss model may be the best option. On the contrary, the two-ray path loss model works fine for short-distance wave propagation. It is also possible to develop a new path loss model by modifying some parameters in the existing ones.  
**Real-life implementation:** The ongoing trend of research in this area is to opt for real-life implementation of the developed channel models. For example, the work presented in [8] was implemented in the Aegean Sea, Greece. This requires a good amount of hardware and deployment costs, which is a great challenge for researchers. There are works, such as [13; 14], that do not present real-life implementation scenarios and; therefore, are not reliable regarding how mathematical models will perform in real life.  
**Lack of simulation tool:** There is no simulator specifically designed for the wave propagation in the sea surface environment. If someone wants to conduct research in this field using simulation, they must either make a custom simulator or use any traditional one with a custom implementation.  
**Barriers to the wave propagation:** Coastline hills and the surrounding ships are great barriers to the propagation of waves over the sea surface, which causes significant signal attenuation. Considering this fact while developing a channel model is difficult.  
**Fluctuating sea environment:** The marine environment varies greatly. Atmospheric pressure, humidity, air temperature, surface pressure, etc. change at different times of the day in the same sea surface area. In addition, different oceans (e.g., Pacific, Atlantic, etc.) have different environments, and important parameters change for each of the mentioned ocean surfaces. For example, the height of the evaporation duct on different sea surfaces varies greatly. Therefore, it is almost impossible to develop a universal channel model with common parameters.  
**Lack of statistical data:** To develop a channel model for the surface of the ocean, a huge amount of sea state statistics is needed, such as temperature, pressure, and evaporation duct heights. This kind of data acquisition is often voluntary and not available for different oceans in the world. Therefore, this type of channel model is very difficult to develop due to the lack of data. In [14], a fading margin calculation scheme was introduced based on the years of statistical data gathered on evaporation duct heights in the South China Sea. However, it was possible to propose such mathematical formulations only because the data were available.  
**Frequently changing weather:** Spontaneous changes in the weather are very frequent on the sea surface. Any disaster can drastically affect the propagation of waves.