## 5 Comparative Study of Slant Link Rain Fade Models  
In the previous sections, we discussed the rain fade model prediction process. However, we may be interested in finding out some of the qualitative and quantitative characteristics of these models. Rain attenuation models for telecommunication links that support a wide variety of frequency spectra are a positive force. However, it is recommended to use a maximum 40 GHz frequency, as the next higher frequency band (above 40 GHz) may be used for astronomical and space communication purposes [137]. A 3-40 GHz frequency range is recommended for use in the telecommunication purposed Earth-space link to avoid possible interference with the deep-space communication link [138]. Table 7 presents the properties of slant link rain fade models such as whether they support terrestrial links; whether they consider rain structure including cloud, rainfall rate, frequency, elevation angle, rain layer height; and whether the models take into account the melting layer length within the effective path length. Table 8 shows the frequency spectrum supporting capabilities, the polarization used to evaluate the model, whether the model has a regional climatic parameter, whether the model considers rainfall rate distribution in a non-homogeneous means, and the recorded geographic area that suits the model properties. Table 9 shows whether these models were validated and the sources of rain attenuation related to measured databanks, constraints, and the complexity levels of the models in terms of low, medium, and high levels are classified. To validate the models, different validation tools such as the error figure and goodness-of-fit function, which is also known as relative error probability function, were mostly used [17; 68; 139].Figure 6 shows the frequency range supporting capacity of the studied models. As we can see, the maximum frequency range supporting model is the unified model [89], the second highest is the Crane TC model [94], and the third top model is the GSST model [66]. The lowest frequency band supporting models from lowest to highest bands are Singapore [33], the Breakpoint model [29], the LU model [68], and the Karasawa model [88]. Figure 6: Each“vertical bar” represents the supported frequency range of the studied models. The vertical axis represents frequency in GHz and the horizontal axis represents the models. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Type** & **Ref. ** & \ \begin{tabular}{c} **Used/Validation** \\ **Databank** \\ \end{tabular}  & **Remarks about Validation** & \begin{tabular}{c} **Complexity** \\ **Level** \\ \end{tabular}  & **Constraints** \\ \hline
[32] & \begin{tabular}{c} 62 experiments in the USA, Japan and Europe \\ \end{tabular}  & \ \begin{tabular}{c} Shows better prediction with minimum 2 years \\ rainfall datasets \\ \end{tabular}  & low & \begin{tabular}{c} The operating frequency range \\ is limited to 10–35 GHz \\ \end{tabular}  \\
[33] & \begin{tabular}{c} INTELSAT POR \\ experiment (Singapore) \\ \end{tabular}  & \ \begin{tabular}{c} The results have been validated with CCIR \\ model and measured attenuation \\ \end{tabular}  & low & \begin{tabular}{c} The recommended frequency is \\ limited to 4–6 GHz \\ \end{tabular}  \\
77 satellite links placed & \begin{tabular}{c} \\ in Europe, the USA, Japan, and Australia \\ (CCIR data bank) \\ \end{tabular}  & \ \begin{tabular}{c} Validated by the author \\ \end{tabular}  & low & \begin{tabular}{c} The model is suitable for \\ prediction with an elevation \\ angle ranging from 10\({}^{\circ}\) to 40\({}^{\circ}\) \\ \end{tabular}  \\ EMPI & [66] ITU-R databank & ITU-R databank & ITU-R databank is used to examine the behavior \\  & & of m parameter at different frequencies as well as different sites & low & Not spatial friendly \\  & & & & \\
[68] DBSG3 databank & The RMS value show that the new model & \ \begin{tabular}{c} The RMS value show that the new model \\ provides the smallest rms and STD in all \\ percentages of time except at 0.001\% \\ \end{tabular}  & high &