# Wireless Channel Models for Over-the-Sea Communication: A Comparative Study
## 7 Challenging Issues and Research Directions
### Future Research Directions  
In this subsection, we shed some light on the future research directions of the wireless channel models for over-the-sea communication.  
**Antenna height:** Antenna height can be really crucial for over-the-sea wave propagation, due to different kinds of surface ducts. Usually, a vertical distance more than 100 m is thought to fall under the elevated duct region, not in the evaporation duct region. Channel models vary in different kinds of surface ducts. Therefore, a channel model meant to facilitate an elevated duct can have a height more than 100 m. On the other hand, the ideal height for wave propagation in an evaporation duct starts from 7 m. Therefore, while designing a new channel model, careful consideration should be given to choose the optimal antenna height.  
**Sea-surface smoothness:** A sea surface can be considered as a smooth and perfect reflector. A channel model based on this assumption is quite different from the assumption that the sea surface is rough and cannot act as a perfect reflector. A perfect channel model should consider the roughness of the sea surface. In the existing works, very few channel models consider the sea surface roughness. They avoided it because of the complex calculations needed to find the refractive profile for a rough sea surface. A future channel model can address this issue.  
**Type of links:** This is a very important issue to be taken care of in future researches. Channels can be modeled based on LoS, nLoS, or bLoS propagation links. Propagation loss models and channel evaluation parameters (such as mean excess delay, RMS delay spread, etc.) vary based on the type of links considered in the channel model. Therefore, a thorough study is needed regarding the region of interest in the sea. For example, if anyone wants to design a channel model for over-the-sea communication that involves a lot of islands and rocks, which may work as obstructions to links, bLoS links should be considered with higher frequencies (35-94 GHz). For short range communication, bLoS propagation links are not necessary. Considering LoS and nLoS propagation is enough.  
**Selecting channel evaluation parameters:** Different channel evaluation parameters can be considered depending on the design and application of the channel models. Usually, mean excess delay, RMS delay spread, channel fade margin, etc. are the common parameters. A future channel model should consider as many channel evaluation parameters as possible to prove the robustness of the design.  
**Appropriate frequency:** Selecting a perfect frequency based on the application, range, and sea type is an important design issue. Long- and short-range transmissions require selecting different frequencies while designing the channel.  
**Comparative study:** The existing channel models did not compare the performance of the designed channel models with the other existing ones. Such kind of comparison helps the readers understand the superiority and robustness of the channel models. It is absent in the current works and can be considered in the future works.