## 3 Classifications of Routing Protocols in UAV Networks
### Routing Based on Network Structure
#### 3.1.4 Hybrid Routing Protocol
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Type** & **Ref.** & **Algorithm** & **Main Contribution** & **Advantage** & **Limit** \\ \hline \multirow{8}{*}{Static routing} & \multirow{2}{*}{[36]} & \multirow{2}{*}{LCAD} & \multirow{2}{*}{LCAD-based full-duplex} & Establish flight paths from the ground preparation stage before the UAVs perform their missions. & Fixed route; lacks flexibility when the environment changes \\ \cline{3-5}  & & & & \\ \cline{3-5}  & & & & \\ \cline{3-5}  & \multirow{2}{*}{[37]} & \multirow{2}{*}{DCR} & Special applications in identified UAV deployment environments & Static multipoint routing for improved performance for cluster-based UAV networks & Difficult to adapt to dynamic changes due to network structure requirements with predefined clusters \\ \cline{3-5}  & & & & \\ \cline{3-5}  & & & & UAS provides a distribution path to connect publishers to subscribers. Efficient in networks deploying multiple UAVs & Architecture-dependent; inefficient when publishers/subscribers change greatly \\ \hline \multirow{8}{*}{Proactive routing} & \multirow{2}{*}{[38]} & \multirow{2}{*}{PRP} & Support for applications with real-time requirements & PRP allows fast access to packet routing paths. Fast routing in complex and dynamic environments & Maintaining connectivity becomes difficult in cases where network nodes move rapidly and constantly change positions \\ \cline{3-5}  & & & Application in UAV networks based on Nomadic mobility model & IsH-DSDV optimizes data movement. Suitable for dense network deployment, controlling energy imbalance problem & Energy imbalance due to centralized control; performance degradation in sparse networks \\ \cline{3-5}  & & & & \\ \cline{3-5}  & \multirow{2}{*}{[42]} & \multirow{2}{*}{SROLSR} & Applications in UAV networks & High performance; solving the redundancy problem in MPR selection algorithm & Requires correct network information \\ \hline \multirow{8}{*}{Reaction-based routing} & \multirow{2}{*}{[43]} & \multirow{2}{*}{DSR} & Traffic monitoring application via UAV networks & DSR does not require a fixed network infrastructure but allows the network to self-organize and self-configure. & High route discovery cost; increased latency in large networks \\ \cline{3-5}  & & & & \\ \cline{3-5}  & \multirow{2}{*}{[44]} & \multirow{2}{*}{AODV} & Traffic monitoring application via UAV networks & AODV provides routes based on routing history or initialize a new route request process when no route exists. & High route discovery latency; prone to congestion; no QoS guarantee for real-time applications \\ \cline{3-5}  & & & & \\ \cline{3-5}  & \multirow{2}{*}{[44]} & \multirow{2}{*}{CLEA- AODV} & \multirow{2}{*}{FANET-based UAV system developed to support search and rescue} & Network performance improvement based on: routing with AODV protocol, GSO-based cluster head selection, and Collaborative Media Access Control (MAC) & Complex due to integration of multiple mechanisms; performance depends on GSO parameters \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Prominent routing algorithms based on network structure.