## 3 Air-to-Air Channel Models
### Experimental Campaigns
The second constant values were the power offset relative to the free-space model. The shadowing factors were 0.52, 0.54, and 0.69 dB for the time, time-frequency, and frequency methods, respectively. The path-loss exponent was lower than that of the free space, which was attributed to the distinctive nature of the A2A channel and the drones' characteristics. According to the results, the best fit is provided by the time-based method. Finally, Table 2 summarizes all the related experimental campaigns regarding the A2A channel. For the sake of completeness, additional measurement campaigns are listed [41; 42; 43; 44; 45; 46]; however, these are not analyzed further in the text. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Ref.** & **Signaling** & **Frequency** & **Equipment** & **Delivered Research** \\ \hline
[37] &
\begin{tabular}{c} BW = 20 MHz \\ P\({}_{\text{Tx}}\) = 10 W \\ \end{tabular} & 250 MHz & Aircraft Cessna C-208B & Channel-impulse response, PDP, delay \\  & & & 228-101 & spread, Doppler \\ \hline
[36] & \
\begin{tabular}{c} IEEE 802.11n \\ P\({}_{\text{Tx}}\) = 20 dBm \\ \end{tabular} & 2.4 GHz & Small drones &
\begin{tabular}{c} Received-signal strength (RSS), \\ antenna-directivity effects, \\ Rice-model extension \\ \end{tabular} \\ \hline
[38] & \begin{tabular}{c} IEEE 802.11n \\ \end{tabular} & 2.4 GHz & \
\begin{tabular}{c} Small drones \\ (DJI Mavic 2 Zoom) \\ \end{tabular} &
\begin{tabular}{c} RSS, path loss, path-loss exponent, and \\ shadow fading depend on flight altitude \\ \end{tabular} \\ \hline
[39] & \begin{tabular}{c} IEEE 802.11 ad \\ BW = 2.16 GHz \\ \end{tabular} & 60.48 GHz & \
\begin{tabular}{c} Hexacopters \\ (DJI M600) \\ \end{tabular} &
\begin{tabular}{c} Path-loss models (FI and CI), \\ beam misalignments \\ \end{tabular} \\ \hline
[40] & \begin{tabular}{c} BW = 7 MHz \\ \end{tabular} & 5.8 GHz & \
\begin{tabular}{c} Hexacopters \\ DJI Matrice 600 Pro \\ \end{tabular} &
\begin{tabular}{c} Path loss based on time, frequency, and \\ time–frequency analysis \\ \end{tabular} \\ \hline
[41] &
\begin{tabular}{c} BW = 20 MHz \\ P\({}_{\text{Tx}}\) = 10 W \\ \end{tabular} & 250 MHz & Aircraft Cessna C-208B & Delay and Doppler characteristics, \\  & & & Aircraft Dornier & surface-scatter characterization \\ \hline
[42] & \
\begin{tabular}{c} IEEE 802.15.4 \\ P\({}_{\text{Tx}}\) = 30 dBm \\ \end{tabular} & 2.45 GHz & Hexacopters &
\begin{tabular}{c} Path-loss characterization (FI model with \\ path-loss exponent 2.05), packet reception rate \\ \end{tabular} \\ \hline
[43] & \begin{tabular}{c} IEEE 802.15.4 \\ P\({}_{\text{Tx}}\) = 60 mW \\ \end{tabular} & 2.4 GHz & \
\begin{tabular}{c} Fixed Wing \\ (0.5 m wing span) \\ \end{tabular} &
\begin{tabular}{c} RSS, Path loss (FI model with path loss \\ exponent 1.92), packet loss characterization \\ \end{tabular} \\ \hline
[44] & \begin{tabular}{c} IEEE 802.15.4 \\ P\({}_{\text{Tx}}\) = 10 mW \\ \end{tabular} & 2.4 GHz & \
\begin{tabular}{c} Delta-wing UAV \\ (0.8 m wing span) \\ \end{tabular} &
\begin{tabular}{c} RSS, path loss (FI model with path-loss \\ exponent 0.93), packet error characterization \\ \end{tabular} \\ \hline
[45] & \begin{tabular}{c} BW = 7 MHz \\ P\({}_{\text{Tx}}\) = 30 dBm \\ \end{tabular} & 5110 MHz & \
\begin{tabular}{c} Fixed Wing \\ (2.8 m wing span) \\ \end{tabular} &
\begin{tabular}{c} RSS, PDP, time delay \\ Model: LOS + multipath \\ \end{tabular} \\ \hline
[46] & \begin{tabular}{c} BW = 20 MHz \\ P\({}_{\text{Tx}}\) = 20 dBm \\ \end{tabular} & 2.375 GHz & \
\begin{tabular}{c} Manned aircraft \\ \end{tabular} &
\begin{tabular}{c} Channel-impulse response, PDP, delay spread \\ Two-ray model: direct + specular diffuse \\ \end{tabular} \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Summary of previous experimental campaigns for the A2A channel.