# A New Propagation Channel Synthesizer for UAVs in the Presence of Tree Canopies
## 1 Introduction  
In the present days, the spreading usage of Unmanned Aerial Vehicles (UAVs) spans a broad area of applications, both civil and military. This results in the need for their proper operation within different environments. Apart from the command and control link, remote collection of data from various user devices placed on UAVs, such as cameras or radar, is of great interest as well. As corresponding data rates are quite high, links operating in the L-band and above are required. However, apart from the distance-dependent free-space path loss generally affecting the received signal power, radio wave propagation conditions are, in these bands, strongly influenced by the scenario between the UAV and its ground terminal as the line-of-sight (LoS) propagation conditions may not be always held. This may include, for example, signal attenuation by nearby objects causing slow signal fading or multipath effects due to scattering and reflections resulting in fast fading. If such signal attenuation results in an insufficient link's signal-to-noise ratio (SNR), one may utilize beyond-LoS (BLoS) relayed data links, e.g., by satellites or other UAVs. It is also highly probable that the link of interest will be influenced by the presence of vegetation, such as trees in the form of forests in rural areas or alleys in urban areas. This may generally happen in the close vicinity of both the UAV and its ground terminal, thus directly affecting the BLoS relays as well. This can be, e.g., the case of the scenarios from [1]. To ensure proper link operation under such conditions, it is first necessary to thoroughly characterize such influence. Only after that, the corresponding link can be dimensioned properly. From a very simplified point of view, radio wave propagation through trees can be addressed by using a so-called specific attenuation in dB per distance through the canopy in meters, as given for short paths through woodland in Rec. ITU-R P.833 for frequencies from 30 MHz up to 30 GHz [2]. On the other hand, for an isolated tree and terrestrial paths with low elevation angles, [2] suggests to use the radiative transfer theory, applied in [3], and also account for ground reflections and diffraction around the canopy. For slant paths, generally applicable for UAV scenarios, [2] recommends applying multiple scattering theory (MST) described in detail in Section 2.2. Unfortunately, the use of MST introduces enormous requirements in terms of computation time and the software implementation complexity [4; 5; 6]. Recently, the authors have developed in [7] a new model addressing scattering from tree canopies which significantly lowers the corresponding requirements by replacing MST coherent field calculations by utilizing the physical optics (PO) approach based on [8]. In this way, near-field effects close to the canopy are modeled properly and smooth transitions between LoS and shadowed areas behind the canopy are achieved [7]. In this paper, we follow the model developed in [7] and, by adding the incoherent scattered field obtained by MST, we build a new propagation channel synthesizer for UAVs operating in close vicinities of tree canopies, which enables the generation of site-specific time series of received signal levels. This means that the output of this synthesizer can directly be utilized to obtain first- and second-order characteristics of the received signal for a particular scenario, i.e., respecting the positions of UAV, the terminal and trees, including the physical parameters of the canopies, such as dimensions and types of branches and leaves. It should be noted that the synthesizer's output is multiplicative to other propagation effects. Thus, the resulting attenuation in dB can be summed with the attenuation obtained for any other propagation effect. In this way, the synthesizer relates to the SNR graphs of the link by directly influencing the corresponding signal levels. The paper is organized as follows.