# Routing Protocols for UAV-Aided Wireless Sensor Networks
## 6 Open Research Issues and Challenges
### UAV Path Planning  
One of the major drawbacks of existing routing protocols is the proper path planning of UAVs [56] for data collection over sensor nodes. The major challenges are to minimize the UAV routing path and flight time from the starting point to the destination node while allowing each sensor node to upload data successfully. Furthermore, to reduce the energy consumption and end-to-end delay, rapid path planning is necessary for UAV flight. The routing path should be separated into non-overlapping data-collection intervals.  
\begin{table}
\begin{tabular}{l l l l} \hline \hline
**Protocol** & **Evaluated Performance Metrics** & **Performance Optimization** & **Application Domain** \\ \hline HHA & Path length, number of UAVs & Path optimization & Environment monitoring \\ \hline SN-UAV & Energy consumption & Optimize network lifetime & Environment monitoring \\ \hline UAV-WSN & Energy consumption, response time, and load balancing & Optimize network lifetime & Border surveillance \\ \hline UAV-AS-MS & Energy consumption & Optimization of transmission power & Emergency situation \\ \hline ULSN & Packet delivery ratio, energy consumption, delay, and buffer size & Optimize data transmission & Sea surface pipeline monitoring \\ \hline URP & Deployment time, energy efficiency, and throughput & Reduce the energy consumption in data collection & Crop health monitoring \\ \hline C-UAV-WSN & Packet delivery ratio, energy consumption, and coverage. & Maximize area coverage & Sparse WSNs \\ \hline rHEED & Energy consumption and coverage & Optimize the area coverage & Disaster monitoring \\ \hline UADG & Energy consumption & Optimize data collection and processing & Post-disaster operation \\ \hline DPBA & Packet delivery ratio, energy consumption & Optimize the resource allocation & Data driven \\ \hline EEDCF & UAV travel distance and travel time & Minimize UAV flight time & Deadline-based WSN \\ \hline PCDG & Total number of transmissions and UAV travel distance & Optimize UAV path and data transmission & Large-scale WSN \\ \hline ADCP & Data collection cost & Coverage and data collection & Wildlife application \\ \hline H-UAV-WSN & Position accuracy & Path optimization & Wide area monitoring \\ \hline TADA & Energy consumption & Optimize data transmission & Large-scale WSNs \\ \hline UAV-CDG & Total number of transmissions & Optimize data transmission & WSN \\ \hline LezliS-WSN-UAV & Localization error, energy consumption, and time synchronization & Node location optimization & Distributed WSN \\ \hline LS-UAV-WSN & Packet delivery ratio and delay & Network coverage & Distributed WSN \\ \hline PSO-WSN-UAV & UAV travel time and energy consumption. & Find the optimal topology & Surveillance \\ \hline FSRP & Packet delivery ratio, delay, energy consumption, and the number of alive nodes & Maximum data collection & WSN \\ \hline EFUR-WSN & UAV traveling distance and convergence of algorithm & Optimization of energy consumption and data transmission & WSN \\ \hline \hline \end{tabular}
\end{table}
Table 5: Performance metrics and performance optimization of existing routing protocols for UAWSNs.