# Position-Monitoring-Based Hybrid Routing Protocol for 3D UAV-Based Networks
## 2 Related Works  
Increasingly significant challenges always exist in wireless communication due to the rapid growth of new emerging applications and service requirements, particularly in the development of fifth-generation (5G) or the emergence of sixth-generation (6G) network architectures, with their improved coverage enhancement and reliable communication [14]. In this case, to support reliable data forwarding in an infrastructure-less wireless network, several studies on the routing protocols have been conducted because routing protocols are the backbone of infrastructure-less wireless communication. Efficient routing protocols play an important role in obtaining the desired outcome of UAV ad-hoc networks. Therefore, researchers have tried to investigate the unique features of UAVs. Recently, many researchers studied routing protocols for UAV ad-hoc networks in different [15; 16; 17] categories, such as topology-based and geographic-based networks. The traditional routing schemes have been deployed in inter-UAV communication or UAV-to-ground user communication scenarios. UAVs integrated with traditional routing schemes offer challenges \begin{table}
\begin{tabular}{c c} \hline \hline
**Abbreviations** & **Definitions** \\ \hline \(D_{n}\) & UAV distance \\ D\({}_{\text{C-}}\) & Centroid distance \\ CB & Betweeness centrality \\ NS2 & Network simulator \\ TCL & Tool command language \\
3D & Three dimensional \\ CBR & Constant bit rate \\ UDP & User datagram protocol \\ \hline \hline \end{tabular}
\end{table} 
Table 1: _Cont._due to 3D communication distance, high mobility and unstable link quality; a number of traditional routing protocols have been investigated in [18]. As for routing information, the author in [19] discussed a routing scheme where all UAVs perform operations using topology-based routing protocols through a predetermined path. Authors adopted a military scenario where UAVs perform critical operations to have secure communication between UAVs [20; 21]. Similarly, in [22], a single UAV is utilized as a relay node to forward data; usually, this type of network is suitable for delay-tolerant networks. Moreover, a swarm of UAVs is considered in [23], where a proactive routing algorithm is used to maintain the efficiency of the UAV network having less packet delay. To optimize the path of a topology-based network, authors in [24] proposed a Q-learning-based routing scheme, where each UAV can only have a table of two next hops. Another topology-based routing scheme in [25] is Optimized link state routing--extended (OLSR-EXT), which is an enhanced version of optimized link state routing (OLSR). OLSR-EXT utilizes the dynamic network topology and maintains reliable communication. Another wireless routing scheme is a hybrid routing protocol that is the combination of multiple algorithms, such as the Zone Routing Protocol (ZRP) [26], which takes features of both proactive and reactive routing protocols. Though UAVs have high mobility, the network's topology changes often; therefore, it is necessary to update network information rapidly. The quick update in the routing information causes overhead due to the frequent transfer of control messages, particularly in large networks, as well as increases the size of the routing table. Thus, topological routing protocols are not recommended for large networks and networks with mobile nodes. Position-based routing reduces the traffic overhead by avoiding the collection and maintenance of unwanted topology information in the routing table. The position routing protocols are based on the exact GPS coordinates of the UAVs in the network. The authors in [27] introduced a location-aided, delay-tolerant routing (LADTR) scheme for disaster post-network scenarios. This routing scheme offers a mechanism to predict the position of each UAV node. If the source and destination have no direct connection, this scheme utilizes the traditional store-carry-forward (SCF) mechanism. Moreover, an adaptive hello interval [28] process is proposed in order to save the energy of UAVs.