# Wireless Channel Models for Over-the-Sea Communication: A Comparative Study
## 5 Comparison of the Channel Models  
Wireless channel models for over-the-sea wave propagation vary mainly in their allowed frequencies. Except for one channel model, presented in [5], the works covered in this study can be divided based on frequency values. We have chosen different works [5; 8; 9; 10; 11; 12; 13; 14; 15; 16] to present here, in such a way that a wide range of frequencies (from 1.9 to 94 GHz) are covered. The channel models covered are based on real-time measurements, except [5], representing a universal scheme applicable to any frequency and sea state. The eight channel models covered have both advantages and limitations. For example, the measured data in [8] is highly accurate, because they have been collected in a real-life implementation. On the contrary, reliability is low, because the effect of multipath due to fluctuations of the sea waves is not considered. In [11], we can find that the channel model is applicable for long-distance transmission, even beyond 3000 m, but the only channel parameter considered was signal strength. To validate their propositions, the authors of [11] should have conducted more experiments on different parameters. Again, the paper presented in [5] applied a channel model under different carrier frequencies, transmission distances, and sea states, which is a great advantage for understanding and deploying the protocol, but the measured data are not verified using experiments. The paper presented in [10] works for both mobile and fixed antennas, but measurements of specular and diffuse multipath propagation were not carried out. The wireless channel model presented in [21] is incomplete because important channel parameters like mean excess delay, RMS delay spread, etc. were ignored. Apart from this, it was necessary that authors of [21] mention what kind of model was used to calculate the path losses. The authors of [20] presented a channel model that had a high data rate and used small terminal antennas. However, they did not specify, formulate, or assume any kind of propagation loss model. Additionally, only the vertical polarization was used for the incident rays. The proposed channel model in [18] did not consider the atmospheric duct effects. In addition, there was no consideration for the refractive rays while designing the channel model. Table 1 discusses, in detail, the comparison of channel models based on outstanding features, routing metrics, advantages, and disadvantages. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Channel Model** & **Used Frequency** & **Outstanding Features** & **Advantages** & **Limitations** \\ \hline Measurements and & & \(\bullet\) Supports mobile wireless & \(\bullet\) Measured data are authentic & \(\bullet\) Highly dependent on global positioning system (GPS). \\ wideband channel & & vessel-to-vessel & real-time data based on practical & \(\bullet\) Increased delay under non-line-of-sight (nLoS) \\ characterization for over-the-sea propagation & & \(\bullet\) Implements a wideband & \(\bullet\) Propagation environment & \(\bullet\) Implies a wideband \\
As mentioned earlier, the channel models presented in this paper can be distinguished in terms of the propagation scenario (LoS, LoS, nLoS) and the mobility of the transmitter and receiver. In addition, different models considered different types of ducts. Table 2 shows a comparison of channel models based on these parameters. Existing channel models can be also compared with each other based on some other parameters, like antenna type, range, bandwidth, height of the transmitter, and transmission power. Table 3 shows the comparison results of the discussed channel models based on the mentioned parameters. It can be observed that omni-directional antennas have been widely used as antenna type. The height of the transmitters and receivers varied based on the sea environment, technology used, and applications. The highest range has been covered by [13]. It covered 500 km. The lowest range of the analyzed channel models was found to be 2 km ([12,41]).