## 3 Air-to-Air Channel Models
### Experimental Campaigns  
Wideband A2A channel measurements were carried out in [37], transmitting a 20-MHz signal at 250 MHz, with an output power of 10 W. Two rotor-propelled aircraft were employed for this purpose, carrying the Tx and the Rx. Three different flight altitudes were selected (600, 1600, and 2600 m), while the trajectories included different environment types, such as water, urban, forest, and grassland. It was found that the received-power-delay profile (PDP) comprised a direct LOS component, as well as a specular reflection from the ground, which was strongest when flying above a water terrain (like). The measured delay spreads were found between 0.356 us and 0.979 us, depending on the terrain and the flight altitude. Furthermore, the observed Doppler spread was in the range of 10.66-23.23 Hz; the highest value occurred when the flight path was over the forest at low altitudes (600 m). In general, delays in the reflected component in the PDP and the Doppler spread conveyed the distance between the two aircraft. Finally, additional low-power components were observed in the Doppler profile at frequencies up to \(\pm\)128 Hz. These peaks were independent of the terrain type and the flight altitude and were caused by reflections on the rotor blades, which were related to the number of rotor-blade revolutions. This indicates that apart from the UAVs' relative motion, additional Doppler spread can be induced by the vehicles' structures themselves, such as those of rotor-propulsion systems. Three different scenarios between two small UAVs (drones) for low altitudes (up to 50 m) were investigated for an IEEE 802.11n wireless link in [36]. The measurements were carried out at 2.4 GHz, with an output power of 20 dBm, and the received-signal strength was recorded at the Rx drone. The first scenario involved the two drones hovering at 35 m above the ground, and the fading properties were examined as a function of their separation. The results verified that the free-space-loss model accurately described the signal attenuation caused by the distance. The second scenario investigated the effect of the antenna directivity, in which the Rx drone hovered 25 m above the ground and rotated many times around the yaw axis. A significant variation in the received signal was observed, reaching up to 17 dB, which was associated to the drones' antenna-radiation pattern. The final scenario was focused on examining the multipath effects in conjunction with the drones' flight altitudes. The two drones were separated by about 50 m and hovered at 10, 25, and 40 m, respectively, above the ground. It was observed that at lower altitudes, there was a strong multipath from specular ground reflections, whereas the direct LOS component dominated at higher elevations. In this context, the authors proposed an extension of the Rician channel model in which the strength of the multipath power \(\sigma\) was modeled exponentially according to the following expression:  
\[\sigma=\alpha h^{b}+c, \tag{5}\]  
where \(a\), \(b\), and \(c\) are constant parameters obtained by fitting (5) to the measurements, and \(h\) denotes the flight altitude in meters. The constant values were \(a=212.3\), \(b=-2.221\), and \(c=1.289\). Therefore, the K-factor (direct to multipath ratio) of the Rice distribution can be described by  
\[K=\frac{\rho^{2}}{2{(\alpha h^{b}+c)}^{2}}, \tag{6}\]  
where \(\rho\) is the signal strength of the direct component. The obtained K-factor values for different altitudes were between 3.5 and 10 dB, indicating that the LOS component becomes dominant and stronger at higher altitudes, whereas the multipath due to the ground reflection affects the channel less.