# A New Propagation Channel Synthesizer for UAVs in the Presence of Tree Canopies
## 5 Experimental Data Validation  
Now, we may present the validation against an experimental dataset obtained during measurement trials with the use of a blimp as a UAV. The trials were taken in July 2015 at Fleming Square in Prague, Czech Republic, and the same measurement setup as in [19] was utilized. This means that a 3D positioner attached to the UAV enabled instant pointing of the transmit (Tx) antennas towards the receiver (Rx) based on the corresponding GPS coordinates which, for Rx were \(50^{\circ}6^{\prime}16.97^{\prime\prime}\)N and \(14^{\circ}23^{\prime}31.62^{\prime\prime}\)E in the WGS84 coordinates. The two Tx antennas, one left-handed (LHCP) and the other right-handed (RHCP) circularly polarized, enabled to transmit two unmodulated continuous wave signals with a stable output power of 27 dBm. These signals were distinguished by a frequency offset of 200 kHz and, thus, were transmitted at 2.00106 GHz and 2.00086 GHz. The Rx utilized a dual-polarized patch antenna placed on a tripod at a height of 1.5 m above ground level with its main lobe oriented towards the zenith. Its four parallel channels enabled to simultaneously record both the co-polarized and cross-polarized components of the transmitted signals with a sampling rate of 10 kHz, i.e., four samples were obtained every 0.1 ms. The whole measurement setup provided a sufficient dynamic range covering the fast fading observed during the measurements. For the maximum distance between the UAV and Rx of about 500 m and Rx noise floor of \(-126\) dBm, the resulting dynamic range was about 65 dB, which was not exceeded during the measurements. The selected scenario is shown in Figure 8, together with the UAV during the flight. The position of trees were obtained from Google Maps [20] and are shown from the top view together with the selected flyover of the UAV in Figure 9a and also in Figure 9b in more detail, including the incident plane wave directions and Rx position. We clearly see that, due to the spacing between the trees, LoS propagation conditions are achieved for more than half of the UAV flyover, whereas a blockage by one tree canopy occurs for the rest of the route. Figure 8: The selected measurement scenario at Fleming Square in Prague, the Czech Republic: (**a**) location of the receiver marked by the magenta dot; (**b**) UAV during a flight, the Tx antennas placed on the 3D positioner are marked by the magenta circle. As the deciduous trees in the alley are dense, higher specific attenuation than about 1 dB/m can be expected at 2 GHz. This was reflected in the synthesizer by using the physical parameters of the trees shown in Table 1, which differ from [2; 4; 5; 6] in the number density values. As a consequence, the resulting specific attenuation was obtained as 2.9 dB/m, which corresponds to the experimentally-obtained attenuation values as identified further in this section. Before we can start analyzing the synthesizer's output, we can summarize the simplifications applied during the simulations:  
* As the change of the UAV position in elevation and azimuth w.r.t. the Rx was less than 30\({}^{\circ}\), thus, not very significant from the scattering characteristics point of view, only one value of \(K\) and \(\sigma^{\alpha\dagger}(-\hat{\mathbf{k}}_{\mathrm{inc}},\hat{\mathbf{k}}_{ \mathrm{inc}})\) was calculated. * Only two trees influencing the coherent field calculations the most were considered within the combined PO/MST part of the simulator, namely the third and fourth tree from the right in the bottom row of trees in Figure 9b. * the UAV speed was kept below 10 m/s, respecting the data sampling rate of 10 kHz, the spatial distance between two adjacent samples is below 1 mm, which is too dense. To characterize the fast fading thoroughly, a sufficient step may be longer, e.g., about 0.1\(\cong\) 15 mm. Following this, we provide an output for every 10 th experimental data sample, which translates into a step in the UAV distance below \(\sim\) 0.07\(\lambda\).