## 3 Air-to-Air Channel Models
### Experimental Campaigns
Applying (6) in the probability-density function (pdf) of the Rician distribution, considering a signal fade between 0 and 14 dB, the distribution of the signal variation for different Rx altitudes between 10 and 40 m is depicted in Figure 7. It is evident that as the Rx elevates, the \(\sigma\) decreases exponentially, which entails a stronger LOS component (i.e., the pdf and the K-factor increase), implying that the multipath power from the ground reflections diminishes. Figure 7 also demonstrates that at higher flightaltitudes, the effect of the ground terrain declines and the established wireless link preserves a stronger unobstructed direct component. Path-loss measurements at 2.4 GHz between two small drones were carried out in [38]. Two different scenarios were examined. In the first case, the two drones were separated by about 10 m and then flew at a constant speed of 0.1 m/s, from the ground up to a height of 50 m. In the second scenario, the distance effect was investigated, in which the two drones hovered at 20 m above the ground, after which one remained steady and the other flew about 35 m away at a constant speed of 0.1 m/s. Based on the measurement results, a model similar to the close-in (CI) [9] was proposed (see also (13)), although the path-loss exponent was found to be associated with the flight altitude \(h\) according to \(n=al^{b}+c\), where \(a=2.6\), \(b=-0.53\), and \(c=2\), yielded by finding the best fit for the measured data. Moreover, apart from the shadowing factor, an additional fading parameter was recommended, also with a zero-mean Gaussian random variable. It was found to be related to the altitude, according to the linear relationship \(X_{h}=\gamma h+\delta\). The constant parameters were yielded from the best-fitting \(\gamma=-0.09\) and \(\delta=4.7\). The additional fading parameter implies that at higher altitudes, the standard deviation of the extra Gaussian distributed fading declines (in fact, it reduces linearly). A similar tendency can be observed in Figure 7, although in this specific scenario, the standard deviation reduced exponentially with the flight altitude. Therefore, the CI model can be rewritten as  
\[PL(d,h)=20\log_{10}(\frac{4\pi d_{0}}{\lambda})+10n\log_{10}(\frac{d}{d_{0}})+ X(0,\sigma)+X_{h}(0,\gamma h+\delta), \tag{7}\]  
where, according to the measurement results, the standard shadowing factor \(\sigma\) = 1.9 dB. Applying (7) at 5.8 GHz, incorporating the additional fading parameter, the path-loss variation is illustrated in Figure 8. The drone separation is assumed to vary between 10 m and 1 km, with flight altitudes between 1 m and 50 m. In Figure 8, it is clear that as the flight altitude decreases, the shadowing factor becomes more intense, with a much greater magnitude. The ground-terrain effect accounts for this observation, as also explained previously, and increased fading is expected at lower altitudes. This tendency diminishes as the flight altitude increases, and at heights above 35-40 m, the fading magnitude is drastically limited (almost flat), as can be observed in Figure 8. Furthermore, the path-loss exponent varies from 2.6 down to 2.3 for flight altitudes in the range of 1 m to 50 m. In other words, higher losses are expected at lower heights. This is more severe at altitudes below 15 m, where the path loss increases at a higher rate. For example, at a separation distance of 800 m, the path loss increases by almost 50 dB (from 100 dB up to 150 dB) when the flight altitude changes from 50 m down to 1 m. However, the path-loss variation relative to the drone height reduces down to 25 dB for smaller Tx-Rx separations (e.g., 200 m). Figure 7: Probability density function of the Rician fading for various drone-flight altitudes. Additional experiments at millimeter-wave frequencies for the air-to-air channel were carried out in [39]. Two small hexacopters were employed to perform the measurements at a carrier frequency of 60.48 GHz, with a bandwidth of 2.16 GHz.