# A New Propagation Channel Synthesizer for UAVs in the Presence of Tree Canopies
## 5 Experimental Data Validation
The total attenuation is about 10 dB, which approximately corresponds to the maximum path through the canopy (about twice the radius, i.e., 3.6 m) multiplied by the resulting specific attenuation of 2.9 dB/m. Here, however, the experimental data may contain deep fades (e.g., around 23 s), which can be generally assigned to individual branches in the canopies or ground reflections disregarded by the simulation. In Figure 10, fewer changes in the synthesizer's output can be noticed, for example, between the time of 15 s and 20 s. This is, however, simply a consequence of plotting the synthesizer's output against the time axis within which the incidence direction does not change constantly, but follows the change in the UAV position, i.e., it is dependent on its speed and direction, which are not constant during the flight. Then, slower changes in the incidence direction are responsible for sparser time series when plotted against the time axis. The detail shown in Figure 10 enables to observe a very important characteristic of the resulting time series, which is their continuity. There are no abrupt changes present in the synthesizer's output, which is an important feature of site-specific simulations where the resulting outputs respect the electromagnetic field continuity within a particular scenario. To provide more insight into the synthesizer's output, Figure 11 provides the coherent and incoherent scattered field time series as generated inside the synthesizer. Clearly, the coherent scattered field is responsible for the attenuation by the tree canopy. On the other hand, the mean value of the incoherent scattered field is independent on the Tx position as the overall scattered power is about the same for all the incident directions and its instant magnitude is changing following the phase differences between Tx and each voxel in a canopy. We may also note that the incoherent scattered power is about 15 dB below the coherent one, which is in agreement with [4; 5; 6; 7], and, thus, it hasa stronger impact on the resulting field time series in the shadowed areas, as can be seen in Figure 10 by the increase of the signal levels deviation. Apart from the above presented narrowband channel characteristics, the synthesizer is also capable of directly providing wideband characteristics in the form of the channel impulse response (CIR). This can be obtained by assuming that the coherent and incoherent scattered field contributions originate from the center point of each canopy. Based on the total distance between Tx, canopy center and Rx, the time delay is calculated. Normalization with respect to power and time delay of the unobstructed direct signal between Tx and Rx, which may (LoS) or may not (NLoS) be present, can be applied. To demonstrate the differences between LoS and NLoS propagation conditions, Figures 12 and 13 show the corresponding CIR characteristics for the experimental scenario from Figures 8 and 9. Referring to Figures 10 and 11, the CIR plots were generated for times of flight 5s (LoS) and 25 s (NLoS). In Figure 12, we clearly see that the direct signal (in black), actually present at Rx, is dominant. Its normalized power and time delay are 0 dB and 0 ns, respectively. The coherent scattered fields from the two closest tree canopies (in blue) have negligible effects on the resulting received signal as their level is at least 30 dB below the direct component. On the other hand, the level of incoherent scattered fields (in red) is generally higher. As already noted, this is the consequence of the fact that outside  
Figure 11: The coherent (red) and incoherent (magenta) scattered field as obtained inside the synthesizer. Faster changes in the incoherent scattered field result from more significant changes in the incidence direction within the particular time intervals. Figure 10: The resulting received signal time series as provided by the synthesizer (black line) together with the actual experimental data (cyan line).