# Reflection Characteristics Measurements of Indoor Wireless Link in D-Band
## 4 Experimental Setup  
Figure 3 demonstrates the experimental setup of the reflection measurement in the D-band indoor wireless link. The narrowband signal in D-band is generated from the synthesized continuous wave generator. After up-conversion via two cascaded frequency multipliers (i.e., \(\times\)2 and \(\times\)6 respectively), the narrowband D-band signal is transmitted to the free space by the standard horn antenna with a gain of 25 dBi, and then reflected by different types of materials. The reflected signal is received by the other standard horn antenna (HA), which is the same as the HA at the transmitter side. Figure 2: The flow chart of dataset processing. In the receiving end, the received signal is amplified by a low noise amplifier (LNA) with a gain of 35 dB, and then down-converted in a mixer. The local oscillator (LO) signal is generated by a continuous wave generator and up-converted by a twelve times frequency multiplier. After down-conversion, the intermediate frequency (IF) signal passes through a signal combiner and is finally measured by a signal analyzer to show its spectrum. Therefore, the characteristics of the reflected signal including frequency and power can be obtained. In order to analyze the reflection characteristics in the full D-band range, seven D-band signals with a frequency interval of 10 GHz are generated for the measurements. The detailed hardware parameters are demonstrated in Table 4. \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Specifications** & & & & **Values** & & \\ \hline Center frequency (GHz) & 110 & 120 & 130 & 140 & 150 & 160 & 170 \\ LO frequency (GHz) & 9.06 & 9.90 & 10.73 & 11.56 & 12.40 & 13.23 & 14.06 \\ IF frequency (GHz) & & & & 1.2 & & & \\ Tx/Rx antenna gain (dBi) & & & & 25 & & & \\ Tx/Rx azimuth HPBW & & & E plane: 9\({}^{\circ}\)/H plane: 10\({}^{\circ}\) & & & & \\ Tx/Rx polarization & & & Horizontal & & & & \\ Tx/Rx caliber (mm \(\times\) mm) & & & \(17.5\times 13.6\) & & & & \\ Tx/Rx projection diameter (mm) & & & 19.1 & & & & \\ LNA gain (dB) & & & 35 & & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 4: D-band Reflection System Specifications. Figure 3: Transmission in free space. (**a**) LoS propagation measurement, (**b**) Reflection measurement. For the reflection loss measurements, in order to ensure that Tx and Rx antennas are located in the far field region of the antennas, the distances to the reflective materials should be greater than Fraunhofer distance, i.e.,  
\[d>\frac{2D^{2}}{\lambda}, \tag{9}\]  
where \(D\) represents the horn antenna aperture size [40]. According to (8), the maximum Fraunhofer distance in this system is about 0.27 m, so the distances \(d_{1}\) and \(d_{2}\) are both set as 0.5 m. This distance is a compromise between far-field conditions and illumination of the reflective materials by the antennas [41]. Additionally, (4) indicates that the incident angle has an influence on the reflection coefficient and further affects the received power. Therefore, for the narrowband signals at different frequencies, six angles changing from 20\({}^{\circ}\) to 70\({}^{\circ}\) are selected as incident angles for measurements, as shown in Figure 4a [40]. Both the angles and distances are measured by a high precision angle ruler in Figure 4b to minimize the calibration error. In fact, different materials have different properties. In order to explore the reflection characteristics of an indoor wireless link in D-band, five kinds of materials, i.e., wood, plexiglass, drywall (plaster), concrete board, and red brick, are selected as the reflection media. The measurement scenarios are shown in Figure 5.  
Table 5 demonstrates the sizes of materials and they are much larger than the first Fresnel zone radii in D-band. Based on the experiments above, the dataset of the received power in reflection measurements can be collected.