## 5 Open Problems and Future Opportunities for UAV Wireless Networks
### Channel Characteristics  
The proper channel model in wireless communication networks is most important factor to accurately evaluate and estimate the network performance [15]. In particular, the channel characteristics of UAV wireless networks are quite different from those of the traditional ground-based networks due to the dominance of air-to-air and air-to-ground links [72]. Furthermore, compared to the air-to-air link, which composes of strong LoS \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline  & **Objective** & **Algorithm/Analysis** & \multicolumn{5}{c}{**Design Considerations of UAV Networks**} \\ \cline{3-8}  & & & **Trajectory of UAV** & **Speed of UAV** & **Environment/Scenario** & **Channel Model** \\ \hline \multirow{4}{*}{[54]} & Enhance a & Develop & \multirow{2}{*}{Fixed (along the roadway)} & \multirow{2}{*}{Fixed} & \multirow{2}{*}{VANET} & \multirow{2}{*}{-} \\  & connectivity and & mathematical & & & & \\  & reduce an & model for & & & roadway) & & \\  & end-to-end packet & UAV-assisted & & & & \\  & delivery delay & vehicular network & & & & & \\ \hline \multirow{4}{*}{[55]} & Investigate two data & \multirow{4}{*}{Develop} & \multirow{2}{*}{Fixed} & \multirow{2}{*}{Fixed} & \multirow{2}{*}{Animal Monitoring} & \multirow{2}{*}{-} \\  & collection schemes & mathematical & & & & \\  & in terms of average & model considering & & & & \\  & energy & dynamics of WSN & & & & \\ \hline \multirow{4}{*}{[56]} & Maximize the data & \multirow{4}{*}{Formulate data} & \multirow{4}{*}{Flexible} & \multirow{2}{*}{-} & Wireless sensor & \multirow{2}{*}{-} \\  & collection utility & maximization & & & networks & \\  & problem & & & & & \\ \hline \multirow{4}{*}{[57]} & \multirow{4}{*}{Minimize the total & \multirow{4}{*}{Minimize the total & \multirow{4}{*}{traveling salesman}} & \multirow{4}{*}{Flexible} & \multirow{2}{*}{Fixed} & Large-scale wireless & Air-to-ground \\  & mission time for & & & & & \\  & gathering data & propose cooperative & & & & \\  & trajectory planning & & & & \\  & algorithm & & & & \\ \hline \multirow{4}{*}{[67]} & \multirow{4}{*}{Maximize the energy efficiency} & \multirow{4}{*}{Derive the \(\,\)propulsion energy & Fixed (circular \(\,\)trajectory)} & \multirow{2}{*}{Variable} & \multirow{2}{*}{-} & Large scale LoS \\  & & & & & & \\  & & & & & & \\  & & & & & & \\ \hline \multirow{4}{*}{[68]} & Cover the area with & \multirow{4}{*}{Formulate disk \(\,\)covering problem} & \multirow{2}{*}{Flexible (find stop \(\,\)points of UAV)} & \multirow{2}{*}{Fixed} & \multirow{2}{*}{\
\begin{tabular}{c} D2D \\ communications in \\ heterogeneous \\ networks \\ \end{tabular} } & Air-to-ground \\  & & & & & & channel model [61] \\ \hline \multirow{4}{*}{[69]} & Maximize the & \multirow{4}{*}{Approximate algorithm to \(\,\)ult allocation and UAV \(\,\)trajectory} & \multirow{2}{*}{Flexible} & Maximum speed of UAV & \multirow{2}{*}{-} & Large scale LoS \\  & & & & & \\ \hline \multirow{4}{*}{[70]} & Maximize the & \multirow{4}{*}{
\begin{tabular}{c} Consider relaxed \\ spectral and energy \\ efficiency \\ \end{tabular} } & \multirow{4}{*}{Fixed (circular \(\,\)trajectory)} & \multirow{2}{*}{Variable} & Urban & Large scale LoS \\  & & & & & pathloss model \\ \cline{1-1}  & & & & & \\ \cline{1-1}  & & & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 3: \(Cont.\)communication channels, an air-to-ground link is prone to experience the blockage, thereby requiring an accurate air-to-ground channel model to construct optimal UAV wireless networks. However, the channel characteristics of air-to-ground link vary with the UAV altitude, elevation angle between UAV and ground node, environment, and so on. Therefore, comprehensive measurements in various environments considering the effects of key factors is essential for air-to-ground channel model. Clearly, it is challenging to find a generic air-to-ground channel model. Recent studies provide valuable insights into air-to-ground channel characteristics that are useful to derive a generic air-to-ground channel model.