## 4 Summary and Lessons Learned
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline  & \multirow{2}{*}{**Objective**} & \multirow{2}{*}{**Algorithm/Analysis**} & \multicolumn{5}{c}{**Design Considerations of UAV Networks**} \\ \cline{4-8}  & & & **UAV Altitude** & \multicolumn{2}{c}{**Interference**} & \multicolumn{2}{c}{**Environment/Scenario**} & \multicolumn{1}{c}{**Channel Model**} \\ \hline  & & & & & & & \\ \hline \multicolumn{8}{c}{Use Case 1: UAV BS} \\ \hline \multirow{3}{*}{[21]} & Maximize the number of covered users & Jaccard dissimilarity metric, Artificial intelligence algorithm & Fixed & - & Disaster & - \\ \hline \multirow{3}{*}{[22]} & Cover the area with the minimum of UAVs & Develop computer program for postinfining terrestrial and aerial BS & Variable & Within the overlapped area & Disaster in heterogeneous networks & - \\ \hline \multirow{3}{*}{[23]} & Maximize the coverage of UAV & Mathematical analysis (downlink coverage analysis) & Derive an optimal altitude & Interference /interference-free conditions & Urban & Air-to-ground channel model [61] \\ \hline \multirow{3}{*}{[24]} & Maximize the coverage with minimum number of UAVs & Circle packing theory & Numerically updating the altitude & Within the overlapped area & Urban & Air-to-ground channel model [52] \\ \hline \multirow{3}{*}{[25]} & Improve the throughput coverage & Genetic algorithm & Fixed & Within the overlapped area & General public safety & Large scale LoS pathloss model (\(\bar{\rho}_{LoS}(\theta)=\mathrm{L}\,\forall\theta\)) \\ \hline \multirow{3}{*}{[26]} & Improve the throughput coverage & Brute force algorithm (ehaustive search) & Fixed & Interference-limited condition & Disaster in heterogeneous networks & Large scale LoS pathloss model \\ \hline \multirow{3}{*}{[27]} & Maximize the network & Mean-shift and successive convex optimization & Find an optimal altitude & Co-channel interference & - & Air-to-ground channel model [61] \\ \hline \multirow{3}{*}{[28]} & Maximize the spectral efficiency & Two distributed algorithm (UAV repositioning) & Fixed & Interference leakage (signal-to-leakage ratio) & 49 cells & Air-to-ground channel model [61] \\ \hline \multirow{3}{*}{[29]} & Maximize the spectral efficiency & K-means clustering and a stable marriage approach & Find an optimal altitude & Co-channel interference from all UAVs & Disaster & Air-to-ground channel model [61] \\ \hline \multirow{3}{*}{[30]} & Improve capacity and coverage & \multicolumn{2}{c}{Two-stage} & \multicolumn{2}{c}{} \\  & & problems(i,e, & & & & \\ \cline{3-6}  & & decision problem -\(>\) cooperative & Assume an optimal altitude & From all UAVs except the serving UAV & High demand area in heterogeneous networks & Large scale LoS pathloss model (\(\bar{\rho}_{LoS}(\theta)=\mathrm{L}\,\forall\theta\)) \\ \cline{3-6}  & & problem (network & & UAV & networks & (\(\bar{\rho}_{LoS}(\theta)=\mathrm{L}\,\forall\theta\)) \\ \hline \multirow{3}{*}{[31]} & Optimize the overall network delay & Entropy (neural decision tree) & From all UAVs except the serving UAV & Heterogeneous networks & Large scale LoS pathloss model (\(\bar{\rho}_{LoS}(\theta)=\mathrm{L}\,\forall\theta\)) \\ \cline{2-6}  & Improve the spectral efficiency & Formulation of updating the altitude & Numerically updating the UAV & From all UAVs & Areas with high traffic demands & Large scale LoS pathloss model (\(\bar{\rho}_{LoS}(\theta)=\mathrm{L}\,\forall\theta\)) \\ \hline \multirow{3}{*}{[33]} & Investigate the average spectral efficiency & A simple heuristic & Derive an optimal altitude & From all UAVs except the serving UAV & Disaster & Air-to-ground channel model [62] \\ \cline{2-6}  & Average the local balance & Hybrid & & & & \\ \cline{3-6}  & & ARIMA-XGBoost prediction & Fixed & - & Crowded area (i.e., campus) & Small cell pathloss model \\ \hline \multirow{3}{*}{[35]} & Minimize the number of UAV to cover the area & Successive UAV & & & & \\ \cline{3-6}  & number of UAV to placement (i.e., & & & & \\ \cline{3-6}  & cover the area & spiral algorithm) & & & & \\ \hline \multirow{3}{*}{[36]} & Minimize the number of UAV to serve all UEs in the network & Heuristic algorithm & Find an optimal altitude & Within the overlapped area & No terrestrial BS & Air-to-ground channel model [61] \\ \hline \multirow{3}{*}{[37]} & Minimize the number of UAV and deployment delay & UB-K-means algorithm & Determine an altitude & - & Disaster & Air-to-ground channel model [61] \\ \cline{3-6}  & & & & & & \\ \hline \multirow{3}{*}{[42]} & Maximize the capacity & Formulate facility location problem & - & - & Downlink in MANET & - \\ \hline \multirow{3}{*}{[43]} & Maximize the throughput & Propose multi-layer UAV deployment algorithm & Fixed & - & Disaster & Air-to-ground channel model [61] \\ \cline{3-6}  & & & & & & \\ \hline \end{tabular}
\end{table}