# Review of Unmanned Aerial Vehicle Swarm Communication Architectures and Routing Protocols
## 3 Routing Protocols
### Geographic/Position-Based Routing Protocols  
Due to the high mobility of nodes in the UAV ad hoc network, it is hard to maintain the routing table. At the same time, traditional routing protocols will cause significant overhead to find the routing repeatedly before sending packets. In response to these problems, researchers have proposed routing protocols based on geographical position information. In this type of protocol, nodes can use location services, such as reactive location services (RLS) [81], grid location services (GLS) [82], or hierarchical location services (HLS) [59]. Geographic/position-based routing protocol is more suitable for highly dynamic networks, including UAV communication networks.  
(1) Greedy Perimeter Stateless Routing (GPSR) is a position-based routing protocol and is originally proposed for mobile ad hoc networks [83]. In a comparative study [84], authors compared the proactive, reactive, and position-based routing protocols in a swarm of UAVs. The results show that the performance of the GPSR protocol is better than others. [PERSON] et al. examined the position-based routing protocols including GPSR for flying Ad hoc networks [85]. They draw a conclusion that the GPSR protocol is applicable for a densely deployed UAV network. However, the generalizability and reliability of this routing protocol have yet to be improved, thus leading to the emergence of many variants.  
Geographic Load Share Routing (GLSR) proposed by [PERSON] et al. [86] is an extension of GPSR. The main contribution is that the GLSR uses \"distance of advance\" to select the appropriate next hop from multiple paths from the source node to the destination node, thus making the path more reliable. As a result, [PERSON]'s team has published a number of articles on the GLSR protocol [87; 88; 89].  
Based on the same principle as GPSR, [PERSON] et al. [90] proposed Mobility Prediction-based Geographic Routing (MPGR). The innovation of this protocol is the use of Gaussian function-based motion prediction method. The result of motion prediction can be used to evaluate the connectivity of the UAV nodes, and furthermore, to select a reliable next hop. In their other study [91], Geographic Position Mobility Oriented Routing (GPMOR) was proposed. UAV nodes can get their own location information via Global Positioning System (GPS). Each UAV periodically exchanges location information with neighbor nodes. GPMOR also uses the \"Gaussian-Markov mobility model\" to predict the movement of the UAVs.  
(2) [PERSON] et al. combines the AODV protocol with Greedy Geographic Forwarding (GGF) technology and further proposed Reactive-Greedy-Reactive (RGR) routing protocol [92]. To obtain the destination address, RGR uses the location information of UAVs. AODV's RREQ / RREP structure and GGF technology complement each other. The protocol first finds the initial path and carries out data transmission according to the way of AODV protocol. When the transmission is cut off accidentally, the strategy will be switched to GGF mode. This protocol improves the packet delivery ratio and reduces the latency. However, due to the high mobility of the UAVs, if the geographic position information of the next hop is not updated in a timely manner, a packet loss fault will occur. Nevertheless, RGR has great application potential for tracking, searching, and multitasking of UAV swarms.  
There are some other geographic/position-based routing protocol. Distance Routing Effect Algorithm for Mobility (DREAM) [93] uses a location table to store the geographical coordinates of nodes. This consumes lesser bandwidth. But it seems that DREAM is more complex than the traditional flooding strategy [94]. [PERSON] et al. [3] proposed the prediction-OLSR protocol, which uses Global Position System (GPS) information to assist routing decisions. At the same time, the expected transmission count is weighted through the relative speed between the nodes.