*2.3. Simulation Results*

In order to perform wireless channel characterization considering blockage, the implemented simulation scenario considers the location of a static transmitter and a mobile terminal with a thick tree in the propagation path, as depicted in Figure 2. The transmitter source (TX), operating at 2.4 GHz is located at a height of 2 m and is highlighted by a red circle. Material properties and simulation parameters are detailed in Tables 1 and 2, respectively. The simulation scenario has been implemented with boundary conditions defined by air in order to avoid unwanted reflection components. Note that the parameters regarding the radio signal communications (transmitted power level, antenna type and operation frequency) have been chosen in order to fit the parameters used by the real devices. It is important to mention that the 3D-RL simulation tool has the option to include the effect of diffraction phenomenon. For the current analysis, the diffraction has been activated since it is a relevant phenomenon in the proposed case under study. Simulation results have been obtained for the complete scenario volume. For the sake of clarity, as a specific example, propagation losses for the linear TX-RX radials defined by the yellow dashed lines depicted in Figure 2b,d have been considered for height cut planes of 1, 2 and 3 m.

**Table 2.** Ray Launching simulation parameters.


**Figure 2.** Created scenario for the 3D Ray Launching simulations. (**a**) General view with the oak tree; (**b**) Upper view with the oak tree; (**c**) General view with the pine tree; (**d**) Upper view with the pine tree.

### 2.3.1. Path Loss Model

Once the simulation results have been obtained for the scenario, path loss was analyzed considering relative mobile receiver displacement. Results have been obtained considering both oak and pine tree models, for an operating frequency of 2.4 GHz and with increasing distance from the tree location. From the results it can be observed that field scattering owing to the tree location can be divided in two zones: a diffraction dominant zone within the tree vicinity and a free-space zone, depicted in Figure 3. Consequently, at larger distances relative to the tree location, power decay follows free space conditions, owing to dominant line-of-sight (LOS) condition. When the distance from the receiver to the tree location is decreased, the observed response in terms of power decay corresponds to dominant multipath or scattering conditions. In this case, a linear variation of opposite trend to the free-space in received signal is observed, corresponding to a diffraction dominant zone in which relevant tree blockage attenuation recovers, subsequently following a free-space component trend.

The results obtained follow a similar trend as those described in [14], in which a scenario with low elevation is presented. In that case, the propagation path corresponds to an air-to-ground radio link with blockage owing to an isolated tree. The transmitter is located over both the tree and a ground mobile receiver, considering several tree species at frequencies within the X band (8–12 GHz) and Ku band (12–18 GHz). The results presented in [14] identified an OLoS region as well as two different scattering zones: a diffuse scattering-dominant region within the tree trunk vicinity, in which only a statistical distribution function model was followed by the signal level; and a colliding region in which prevalence of tree crown diffraction is observed, modelled considering knife-edge diffraction loss with a correction of the tree height. Beyond this second scattering zone, the signal recovers the power decay corresponding to the OLoS model.

In the presented model, the region within the vicinity of the tree canopy corresponding to diffuse scattering phenomena has not been clearly identified. For the oak tree, this may be due to the absence of air gaps in the simulation model adopted for the mass of leaves which would turn the propagation media into a multi dispersive material. In the case of the pine tree, the simulation model already includes a sufficient volume of air gaps offering a more realistic approach. An experimentally derived model based on anechoic chamber measurements is presented in [14]. For actual trees the mass of leaves is not homogeneous as in the simulation. A diffuse scattering is created, given by random nature of fading owing to interaction with the leaves. It is worth noting that signal attenuation as observed in Figure 3a, corresponding to the oak tree case, strongly decreases, which is in principle given by the plausible existence of a diffuse scattering zone.

**Figure 3.** Simulation path loss for a single tree, (**a**) Oak tree; (**b**) Pine tree.

Experimental measurements later shown in Section 2.4 have corroborated this fact. Figure 3b depicts the results considering the pine, which are similar with lower definition, given mainly by lower attenuation considering air gaps in the pine model vs. the homogeneous model employed in the case of the oak tree.
