## 3 Air-to-Air Channel Models
### Experimental Campaigns
The scenarios involved three different flight altitudes (6, 12, and 15 m above the ground) and 14 separation distances between the two UAVs, from 6 up to 40 m. The measuring equipment incorporated both Tx and Rx, arrays of 36 \(\times\) 8 antenna elements, providing an angular coverage of 90\({}^{\circ}\) and 64 beam directions in the azimuth plane, which entailed a beam spacing of 1.4\({}^{\circ}\). Two empirical models were proposed and compared: the CI model, which was previously described by (10), and the fixed intercept (FI) model, in which the path loss, in decibels, is expressed as  
\[PL=a+10\beta\log_{10}(\frac{d}{d_{0}})+X_{e}, \tag{8}\]  
where \(a\) is the floating intercept and \(\beta\) is the slope, which can both be yielded from fitting (8) to the measured data. The results showed that both models provide accurate prediction, with path-loss exponents equal to 2.25 and 2.33, and shadowing factors equal to 3.56 and 3.52 dB, for the CI and FI model, respectively. It should be pointed out that the FI model has no physical reference and simply fits to the measured data. Using the CI model, the impact of the flight altitude on the path loss was examined further. It was observed that both the path-loss exponent and the shadowing factor exhibited comparable values as the height increased. This indicated that the height did not influence the path loss during the UAV flight. Finally, the path-loss variation was examined by taking into consideration the beam misalignments. In this context, the FI model was fitted to the measured data for beam pairs that were not perfectly aligned. It was observed that the path loss increased progressively as the antenna misalignment between various beam pairs reduced the beamforming gain. For example, from perfect alignment between the best beam pairs (zero degrees) up to the highest misalignment of 4.2\({}^{\circ}\), the floating path loss increased by almost 11 dB. A large-scale statistical model for an A2A channel was proposed in [40], in which measurements were carried out at 5.8 GHz, employing two small-sized drones (hexacopters), which carried the equipment. The coupling effect of the antenna mounted on the drones was initially investigated by using an anechoic chamber. The antenna-radiation patterns were measured in the presence or absence of the air vehicles. The results showed that the coupling effect induced by the drones was insignificant and that the antenna patterns remained unaffected in both the azimuth and elevation planes. Furthermore, the measurements took place in a rural area with many trees in its terrain, inducing increased ground reflections. During the measurements, the Rx was stationary, whereas the Tx moved along a predefined route for distances between 25 and 425 m, and at a constant flight altitude of 50 m above the ground. The CI model, given in (10), was proposed to describe the  
Figure 8: Path loss versus distance for different flight altitudes. The produced path-loss samples included the measured and the additional shadowing factor. large-scale characteristics of the A2A channel. The path-loss exponent was determined by establishing the time (obtained by averaging the power samples in the time domain) and frequency (obtained by summing the power samples in the frequency domain) and performing a time-frequency analysis (a short-duration Fourier transform was applied, after which an ensemble average was taken) of the measured data in terms of frequency-swept measurements using a 7-MHz bandwidth. The path-loss exponents were 1.817, 1.903, and 1.773, respectively, when applying each of these methods. Therefore, the final path-loss model is expressed as  
\[PL= \begin{cases}76.7-18.17\log_{10}(d/d_{0})+13.26+X_{\sigma},&\text{Time}\\ 76.7-17.73\log_{10}(d/d_{0})+0.5138+X_{\sigma},&\text{Time}-\text{Frequency},\\ 76.7-19.03\log_{10}(d/d_{0})-9.573+X_{\sigma},&\text{Frequency}\end{cases}  \tag{9}\]  
where \(d_{0}\) is the reference distance selected (28 m) for the specific scenario (resulting in a free-space loss of 76.7 dB).