## 2 Applications of U-WSN
### Functionalities of UAV as a Communication Node
#### 2.3.2 Data Collection  
In traditional data collection protocols, sensors were usually assumed to be static and densely deployed. Sparse and mobility were not considered because they cannot maintain connectivity of the network. Thus, the nodes may not be connected through multi-hop paths. After the introduction of mobile nodes, how to collect data from the mobile nodes becomes a new challenge. Data collection has been addressed extensively in the literature. In [32], the authors proposed an energy conservation scheme to extend the network lifetime, and the data collecting time is extended accordingly. In reference [24], the authors attempt to make full use of integrating small-scale UAV in a ground wireless network for information collecting and environmental monitoring and surveillance. The cost-reducing and energy-saving were conducted through using small-scale UAVs. In [25], the authors address the data gathering problem of how to efficiently utilize the battery power for the maximizing data collection performance in rechargeable WSN. The rechargeable sensor network can provide enough energy to maintain the system connectivity. In [26], the authors propose the optimal speed control of the UAV which could helps the system to collect data efficiently. Both of them are based on a single UAV. The \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Ref.**} & \multicolumn{4}{c}{**Functionalities**} \\ \cline{2-6}  & **M.C. \({}^{\text{a}}\)** & **D.C. \({}^{\text{b}}\)** & **Loc. \({}^{\text{c}}\)** & **\(N_{\text{a}ay}\)**d** & **Performance Objective** \\ \hline
[4] & Yes & Yes & No & Multiple & Optimization of the photogrammetry process. \\
[3] & Yes & Yes & No & Multiple & Maximization of data delivery. \\
[12] & Yes & Yes & Yes & Multiple & Smart view in smart grid. \\
[13] & Yes & Yes & Yes & Multiple & Civil security and disaster management. \\
[14] & Yes & Yes & Yes & Multiple & Monitoring and mapping optimization of the oil or gas pipelines. \\
[17] & Yes & Yes & No & 1 & Maximization of packet delivery ratio and network fairness. \\
[18] & Yes & Yes & No & 1 & Maximization of packet delivery ratio, system fairness and throughput. \\
[19] & Yes & Yes & No & 1 & Maximization of packet delivery ratio and network fairness. \\
[20] & Yes & Yes & Yes & Multiple & Practical deployment. \\
[21] & Yes & Yes & No & 1 & Optimization of the UAV flying path. \\
[22] & Yes & Yes & No & Multiple & Maximization of data delivery. \\
[23] & Yes & No & Yes & 1 & Optimization of localization \\
[24] & Yes & Yes & No & 1 & Minimization of system energy consumption. \\
[25] & Yes & Yes & No & Multiple & Maximization of data delivery. \\
[26] & Yes & Yes & No & Multiple & Optimization of the speeds of UAVs. \\
[27] & Yes & Yes & No & Multiple & Maximization of data delivery. \\
[28] & Yes & No & Yes & Multiple & Optimization of localization in three-dimension space. \\
[29] & Yes & No & Yes & 1 & Optimization of real-time localization. \\
[7] & Yes & Yes & Yes & Multiple & Maximization of data delivery. \\
[30] & Yes & Yes & Yes & Multiple & Energy efficiency maximization. \\
[31] & Yes & Yes & Yes & 1 & Maximization of data delivery. \\ \hline \hline \end{tabular} a
[FOOTNOTE:a]Footnote a: footnotetext: M.C. is the abbreviation of “Maintain Connectivity”. b  
D.C. is the abbreviation of “Data Collection”. c  
Loc. is the abbreviation of “Localization”. d  
\(N_{\text{a}ay}\) is the number of UAVs. [ENDFOOTNOTE]  
\end{table} 
Table 5: Summarising and comparison of the UAV functionalities in the existing U-WSN. authors in [27] address the data collection issue through multi-UAVs, in which both the packet received ratio and packets tracking ratio were evaluated. However, the fairness and collisions between UAVs are failure to considered in this work. In addition, this is one of the main contributions that the authors in [22] have done. The authors concentrate on the problem of data gathering from scattered sensors through integrating several UAVs in the context of large scale sensor networks.