## 2 Applications of U-WSN
### Functionalities of UAV as a Communication Node
#### 2.3.1 Maintaining Connectivity and Relaying  
Maintain connectivity is one of the key problem in wireless networks. The failures occurrence leads to the disconnect of the networks. The solution of such issues is to provide a reliable multi-hop path to maintain the connectivity through other kinds of nodes, such as mobile vehicles. However, it cannot maintain the wireless network connectivity all the time. That is because the mobility of the mobile sink, based on which a node is within the transmission range of the mobile sink at this moment may out of its range at next moment. In addition, the motion trajectory of the mobile sink is also restricted by the network deployment. Thus, UAV, which trajectory could be predefined or randomly, is introduced to work as mobile sink.  
Extensive investigations have been conducted on maintaining connectivity in the context of U-WSN. Research in [3] address the city-scale video monitoring in WSNs. Multiple UAVs ride buses in a noisy 3-D environment were considered. The UAVs could recharge through moving bus; thus, it could maintain the connectivity of the network. However, their study addressed on static objective nodes. In our previous works [17; 18; 19], we address  
\begin{table}
\begin{tabular}{p{56.9 pt} p{113.8 pt} p{113.8 pt}} \hline \hline
**Category** & **Subcategory** & **Applications** \\ \hline Ground & On-ground & e.g., sensors are deployed on ground for environmental monitoring [7]. \\  & Under-ground & e.g., the nodes are deployed underground for pipelines safety and monitoring [14] \\ \hline Water & On-water & e.g., filming and photographing recreational activity. \\  & Under-water & e.g., use acoustic communications [15] \\ \hline Hybrid & Combination of the above & e.g., sensors are floating on the surface of the sea to monitoring marine disasters [16]. \\ \hline \hline \end{tabular}
\end{table}
Table 4: Classifications of the applications of U-WSN according to the sensor deployment.  
\begin{table}
\begin{tabular}{p{56.9 pt} p{113.8 pt} p{113.8 pt}} \hline \hline
**Category** & **Application Fields** & **Applications** \\ \hline \multirow{8}{*}{Industry Grade} & \multirow{8}{*}{Reconnaissance} & e.g., UAVs are used for aircraft maintenance in airlines. In June 2015 Easylet began testing UAVs in the maintenance of their Airbus A320s. In addition, in 2016, Airbus demonstrated that using UAVs for the visual inspection of an aircraft on Farnborough Airshow. \\  & & e.g., product quality monitoring and Smart-Grid measurements [12]. \\  & Environmental Urban & e.g., agriculture application [4]. \\  & Others & e.g., traffic monitoring, urban surveillance and civilian security [13]. \\ \hline \multirow{4}{*}{Consumer Grade} & \multirow{2}{*}{Recreational Hobby Others} & \multirow{2}{*}{e.g., filming and photographing recreational activity. \(\varepsilon\)., toys. \(\varepsilon\)., journalists using UAV for news gathering.} \\  & & & \\ \hline \hline \end{tabular}
\end{table}
Table 3: Classifications of the applications of U-WSN according to the performance and the capability of the UAV.  
the dynamic wireless networks in which both the mobility of collectors and nodes were considered. The simulation results in [18] present that the moving of UAV can maintaining the connectivity of the wireless network. The sensor nodes that are without the transmission range of the base station could have an opportunity to communicate with the UAV through create connection path to the connected neighbours.  
However, our previous works fail to consider the case that the UAV and the sensor moving in opposite directions. In Section 4.2, we not only study the mobility of the UAV and the sensor, but also consider the case where they move in different directions, and the relationship between them also considered.  
In Table 5, we summarize and compare the use of UAV for maintaining connectivity in wireless networks.