# Scaling of Rain Attenuation Models: A Survey
## 5 Research Scopes and Challenges
### Experiment Planning  
The primary challenge in scaling method studies is that long-term experiments are necessary. Besides considering the nonuniform distribution of rain, proper approximation techniques are required to be deployed that can represent the rain distribution along with the radio links. In addition, to develop a scaling method, depending on the scaling mechanism, it needs to record attenuation with varying different possible parameters. Long-term experimental data are required to enhance the scaling of rain attenuation further.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Ref.** & **Long-Term** & **Short-Term** & **Innovative Idea** \\ \hline
[14] & & \(\checkmark\) & It proposed a frequency scaling technique based on the DSD distribution. The DSD-based distribution is accurate for comparing the rain rate. However, the DSD distribution is not uniform in every rain event. \\ \hline
[52] & & \(\checkmark\) & This study revealed that the ratio of two averaged attenuations of radio links follows a power rule of the form \({}^{A_{2}}/{A_{1}}=\alpha A_{1}^{\beta}\). \\ \hline
[53] & & \(\checkmark\) & It analyzed the effect of scintillation on frequency scaling, and according to the outcome, scintillation has a significant influence on frequency scaling. \\ \hline
[54] & & \(\checkmark\) & It proposed a frequency scaling scheme for real-time total atmospheric (cloud+scintillation-excluding) conditions. \\ \hline
[21] & \(\checkmark\) & It proposed a method for frequency scaling in clear sky conditions. \\ \hline
[55] & & & It proposed a “statistical factor”-based method for dual-frequency scaling. \\ \hline
[51] & \(\checkmark\) & \(\checkmark\) & It revealed the “hysteresis behavior” of short-term frequency scaling. \\ \hline
[56] & \(\checkmark\) & It considered the concept that the length is frequency-independent [47] and proposed a nonratio-based single-frequency frequency scaling technique. \\ \hline
[57] & \(\checkmark\) & It proposed the experimental results of frequency and polarization scaling for terrestrial links. \\ \hline
[58] & \(\checkmark\) & The validation of existing methods of frequency scaling in Malaysia did not show satisfactory performance close to the measured results. The reason could be that all the existing methods used were for “slant links”. \\ \hline
[6] & \(\checkmark\) & \(\checkmark\) & Proposed a general frequency scaling technique for the Ku/Ka-band, and the predicted attenuation also showed satisfactory performance with “short-term” applications. \\ \hline
[7] & \(\checkmark\) & It used the ITU-R P.618-13 model [60] to calculate vertical and horizontal polarization. \\ \hline
[59] & \(\checkmark\) & It proposed a very simple formula \(A_{30}=C\cdot A_{20}\) to predict rain attenuation at 30 GHz using a constant \(C=1.8185092\) and the attenuation at 20 GHz. The results were not validated by the measured attenuation at 30 GHz. \\ \hline
[13] & & \(\checkmark\) & It used the SAM model-originated specific attenuation parameter (\(\gamma\)) at 20 GHz to estimate rain attenuation at 30 GHz. \\ \hline
[8] & \(\checkmark\) & It proposed a method for frequency scaling, including raindrop-size-related parameters and the rain rate. \\ \hline
[49] & & \(\checkmark\) & It proposed a model that takes into account the attenuation variation due to the variations of the raindrop size. \\ \hline
[11] & – & – & Using the Multi-Exponential CELL (MultiEXCELL; a model to synthetically form rain cells) synthetic model, it proposed a method to predict attenuation within 5\({}^{\circ}\) and 90\({}^{\circ}\), as a function of the link elevation. \\ \hline \hline \end{tabular}
\end{table}
Table 2: Comparison among the scaling of propagation links.