# Review of Unmanned Aerial Vehicle Swarm Communication Architectures and Routing Protocols
## 3 Routing Protocols
### Routing Technologies  
Routing technologies provides support for the implementation of routing protocols. The implementation of a routing protocol is usually an improvement on the basic routing technologies or a combination of several routing technologies. UAV swarm Ad hoc network is developed from  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Features**} & \multicolumn{2}{c}{**Centralized Communication**} & \multicolumn{2}{c}{**Decentralized Communication Architecture**} \\  & **Architecture** & **Single-Group** & **Multi-Group** & **Multi-Layer** \\ \hline Multi-hop Communication & \(\times\) & \(\surd\) & \(\surd\) & \(\surd\) \\ UAVs Relay Traffic & \(\times\) & \(\surd\) & \(\surd\) & \(\surd\) \\ Different Types of UAVs & \(\times\) & \(\times\) & \(\surd\) & \(\surd\) \\ Self-configuration & \(\times\) & \(\surd\) & \(\times\) & \(\surd\) \\ Limited Coverage & \(\surd\) & \(\surd\) & \(\surd\) & \(\times\) \\ Single Point of Failure & \(\surd\) & \(\times\) & \(\surd\) & \(\times\) \\ Robustness & \(\surd\) & \(\times\) & \(\times\) & \(\surd\) \\ \hline \hline \end{tabular} Note. “\(\surd\)” = supported, “\(\times\)” = not supported.  
\end{table}
Table 2: Summary of Communication Architectures.  
the traditional Ad hoc network. Therefore, the conventional routing technologies can also be used in the UAV swarm Ad hoc network after analysis, improvement and combination. Figure 6 shows the six common routing technologies of the UAV ad hoc network: store-carry-forward, greedy forward, path discover, single-path, multi-path and predictive routing.  
1. Store-carry-forward technology: when no relay node can be found at a certain time, the current node will store and carry the datagram until it finds the forwarding node. This technology is suitable for intermittent network, but the disadvantage is that it produces a large delay.
2. Greedy forward technology: forwarding principle is to select the neighbor node closest to the destination as the relay node until the datagram is sent to the destination. It can be used in scenarios where the deployment of UAVs is intensive. However, when a node is closest to the destination and there is no path to neighbor node, it will cause a failure. In this case, it should be combined with other technologies to increase the reliability of the technology.
3. Path discover technology: the core is through the flooding of routing request (RREQ), maximizing the accessibility of the path. This technique reduces the likelihood of communication interruptions when the current node loses the destinated geographic location. But the disadvantage is that it consumes bandwidth resources excessively.
4. Single-path technology: characterized by the use of a single path for data transmission. Suitable for extremely limited bandwidth resources. However, it has the disadvantage of poor robustness. There is no alternative path for network failure, so it is easy to lose packets. This technique is rarely used in UAV swarm communication scenarios.
5. Multi-path technology: Multi-path propagation technology can effectively improve the robustness of the link. When one link fails, other links can take over. It is suitable for scenarios where high link reliability is required. The disadvantage is that when the multi-path intersections fail, the network will have a loop which will block the network.
6. Predictive routing technology: The predictive routing technology predicts the future position of a node by its current position, velocity and direction, and further chooses the next optimal hop node. This technology is applicable to scenarios where the positions of nodes change rapidly, so it is widely used in a UAV swarm Ad hoc network.  
Figure 6: The rationales for common routing technologies of UAV ad hoc network. (**a**) Store-carry-forward technology, (**b**) Greedy forward technology, (**c**) Path discover technology, (**d**) Single-path technology, (**e**) Multi-path technology, (**f**) Predictive routing technology.