# Reflection Characteristics Measurements of Indoor Wireless Link in D-Band
## 5 Results and Discussions
### Reflection Coefficients  
As can be seen from Figure 6a-e, the measured reflection coefficients fit well with the theoretical model and they are comparable in spite of the small deviation. The MMSE, which are on the order of \(-4\) in Table 7, also back this up. The plexiglass has the lowest MMSE among the five materials which means a better consistency between \(|\Gamma|_{M}\) and \(|\Gamma|_{T}\) \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Material**} & \multicolumn{5}{c}{**Reflection Loss (dB)**} \\ \cline{2-7}  & **20\({}^{\circ}\)** & **30\({}^{\circ}\)** & **40\({}^{\circ}\)** & **50\({}^{\circ}\)** & **60\({}^{\circ}\)** & **70\({}^{\circ}\)** \\ \hline Wood & 17.3 & 19.1 & 20.8 & 26.3 & 27.7 & 14.0 \\ Plexiglass & 12.9 & 14.0 & 15.7 & 19.1 & 25.8 & 14.8 \\ Drywall & 13.2 & 14.4 & 16.1 & 18.7 & 26.6 & 15.8 \\ Concrete & 6.7 & 8.1 & 8.9 & 10.8 & 16.2 & 19.3 \\ Red brick & 10.0 & 10.8 & 13.2 & 18.4 & 21.4 & 15.9 \\ \hline \hline \end{tabular}
\end{table} 
Table 6: The Average Reflection Loss [\(P_{ref}\) (dBm) \(-\)\(P_{\text{LoS}}\) (dBm)] at Six Incident Angles in D-band. \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Material**} & \multirow{2}{*}{\(\mathbf{\epsilon_{rz}}\)} & \multirow{2}{*}{\(\mathbf{\epsilon_{rz}}\)} & \multirow{2}{*}{\(\mathbf{h_{rms}}\) (\(\mathbf{\mathbf{\mathbf{\mathbf{\mathbf{\mathbf{\mathbf{\mathbf{\mathbf{\mathbf{\mathbf{\mathbf{\mathbf{\mathbf{\mathbf{bmbm{ }}}}}}}}}}}}}\)\)}} & **MMSE\({\mathbf{\epsilon_{zve}}}\) \\ \hline Wood & 1.60–1.89 & 1.75–1.92 & 76.4–80.0 & 3.79 \(\times\) e\({}^{-4}\) \\ Plexiglass & 2.581–2.602 & 2.56–2.69 & 8.2–14.5 & 3.04 \(\times\) e\({}^{-4}\) \\ Drywall & 2.595–2.602 & 2.51–2.69 & 96.4–99.2 & 5.47 \(\times\) e\({}^{-4}\) \\ Concrete & 6.2–7.0 & 5.94–6.26 & 264.3–269.0 & 7.25 \(\times\) e\({}^{-4}\) \\ Red brick & 3.75 & 3.49–3.76 & 321.3–325.0 & 6.43 \(\times\) e\({}^{-4}\) \\ \hline \hline \end{tabular}
\end{table} 
Table 7: Collection of Estimated Parameters and MSE. Figure 6: Comparison between measured and Fresnel reflection coefficients for five materials [24; 25; 26; 27; 30; 31], (**a**) Wood, (**b**) Plexiglass, (**c**) Drywall (**plaster), (**d**) Concrete board, (**e**) Red brick. whereas the concrete with the highest MMSE shows a worse consistency. Both the measured and theoretical reflection coefficients are a decreasing function for the angles smaller than the Brewster angle, whereas they increase rapidly with angles larger than the Brewster angle. When the angle rises up to 90\({}^{\circ}\), the reflection coefficient will be 1 because the main transmission path is not a reflection but an LoS path. This conforms to the properties of parallel polarized wave [17; 20]. The trend is more clear in insets (i) of Figure 6a-e, which demonstrate the specific fitting situations. The Brewster angles of different materials at a certain frequency vary greatly. The wood's Brewster angle is close to 50\({}^{\circ}\), while that of the concrete board is almost 70\({}^{\circ}\). The remaining three materials have Brewster angles of about 60\({}^{\circ}\). The difference is caused by the dielectric properties of materials. Additionally, at a certain angle smaller than the Brewster angle, the theoretical reflection coefficients decrease slightly from 110 GHz to 170 GHz, but some measured results don't follow it. For example, the measured reflection coefficient of plexiglass at 120 GHz is smaller than that at 130 GHz and the measured reflection coefficient of red brick at 170 GHz is larger than that at 160 GHz. This is attributed to the limited resolution of devices and environmental interference. Figure 7a-e shows the relationship between the measured reflection coefficients and the frequencies at different incident angles. It can be seen that the reflection coefficients demonstrate an overall downward trend with slight fluctuations from 110 GHz to 170 GHz at a certain angle, and the trend is flat.