## 8 The SUI Model  
The Stanford University Interim (SUI) model [11; 12] was developed by the IEEE 802.16 working group. This model is purely empirical and was originally designed for fixed-broadband wireless systems operating in the 2.5-2.7 GHz frequency band. While research is available concerning the applicability of the SUI model to 3.5 GHz WiMAX networks [61], no scientific results have been published to date regarding its suitability in the 6 GHz frequency band. Moreover, there is currently no other empirical propagation model of verified accuracy available for use in the 6 GHz band. The SUI model defines the \(PL\) path loss in Decibels (dB) as a function of the distance \(d\), measured in meters from the base station, using the following fundamental equation:  
\[PL(d)=A+10\cdot\gamma\cdot\log_{10}\left(\frac{d}{d_{0}}\right)+X_{f}+X_{h}+S, \tag{44}\]  
Figure 28: Field strength profile along the path from Base Station TestBS_2 to Test Point T36. \begin{table}
\begin{tabular}{l c c c c c c c} \hline \hline  & \multicolumn{3}{c}{**3.6 GHz**} & \multicolumn{3}{c}{**6 GHz**} \\ \cline{2-9}
**dB\({}_{\mu}\)V/m** & \(\mathbf{E_{max}}\) & \(\mathbf{E_{min}}\) & \(\mathbf{Range}\) & \(\mathbf{E_{max}}\) & \(\mathbf{E_{min}}\) & \(\mathbf{Range}\) & \(\mathbf{\Delta_{Range}}\) \\ \hline TestBS\_1 & 76.3 & 9.6 & 66.7 & 76.3 & 7.3 & 69.0 & 2.3 \\ TestBS\_2 & 79.7 & 44.9 & 34.8 & 79.7 & 45.4 & 34.3 & -0.5 \\ TestBS\_3 & 78.7 & 13.3 & 65.4 & 78.7 & 11.0 & 67.7 & 2.3 \\ \hline \hline \end{tabular}
\end{table} 
Table 8: Comparison of results at 3600 MHz and 6000 MHz (ITU-R P452-17). Figure 27: Distribution of field strength values, 6 GHz, ITU-R P452-17. where  
\[A=10\cdot\log_{10}\left(\frac{4\pi d_{0}}{\lambda}\right)\text{ and }\gamma=a-b\cdot h_{b}+\frac{c}{h_{b}}, \tag{45}\]  
with \(d_{0}=100\,\text{m}\) as a constant, \(\lambda\) denoting the wavelength in meters, and \(h_{b}\in[10\,\text{m},\,80\,\text{m}]\) being the base station height above ground level, expressed in meters. The constants \(a\), \(b\), and \(c\) depend on the terrain type. The model distinguishes between three terrain classes: Terrain A, Terrain B, and Terrain C. In the fundamental equation, \(A\) is regarded as the frequency-dependent baseline term, while \(\gamma\) is the attenuation exponent determined by the base station height and terrain characteristics. \(X_{f}\) is a frequency correction factor that extends the applicability of the model to different frequency bands:  
\[X_{f}=6\cdot\log_{10}\left(\frac{f}{2000}\right), \tag{46}\]  
where \(f\) denotes the frequency in MHz, and \(S\) represents the log-normal shadowing loss, modeled as \(S\sim\mathcal{N}(0,\sigma^{2})\), with a standard deviation \(\sigma\in[8.2,10.6]\) depending on the terrain type. The term \(X_{h}\) depends on the height of the receiving antenna above ground level (\(h\), in meters), as well as on the type of terrain:  
\[X_{h}= \begin{cases}-10.8\cdot\log_{10}\left(\frac{h}{2}\right),&\text{if Terrain A or Terrain B,}\\ -20\cdot\log_{10}\left(\frac{h}{2}\right),&\text{if Terrain C.}\end{cases}  \tag{47}\]  
The terrain categories, their characteristics, and the corresponding model constants are summarized in Table 9.  
The fundamental equations of the SUI model clearly show that it does not take into account the terrain profile between the transmitting and receiving antennas. Instead, it relies solely on general morphological characteristics of the environment, applying certain constants based on these terrain categories. All parameters in the model are derived purely from empirical sources, which, on the one hand, allows for some flexibility in adapting the terrain types and frequency dependence to specific scenarios. On the other hand, this approach results in significantly less accurate and more generalized outcomes compared to models that directly incorporate terrain profile information, such as PEM or ITU-R P.1546-6. The model is based on extremely simple mathematical expressions, making it both computationally efficient and cost-effective.