# An Intelligent Cluster-Based Routing Scheme in 5G Flying Ad Hoc Networks
## 2 Related Work
Cluster heads \(CH_{1}\) and \(CH_{2}\) can communicate with each other in three hops using a route \(CH_{1}-CG_{1,1}-CG_{2,1}-CH_{2}\), whereby links \(CH_{1}-CG_{1,1}\) and \(CG_{2,1}-CH_{2}\) are intra-cluster communications and link \(CG_{1,1}-CG_{2,1}\) is an inter-cluster communication. The CHs can also interact with BS. The CM of a cluster cannot communicate with the CG of another cluster directly; and it must communicate via CGs in inter-cluster communication or VCGs in inter-plane communication. Based on the traditional clustering schemes [2], the underlying clustering scheme in our framework enables the DC of a network plane to gather messages from UAVs periodically in order to form clusters with increased cluster lifetime for ensuring cluster stability. The message contains the three dimensional geographical location, node degree (i.e., the number of neighbors), and mobility metrics of a UAV. The DC selects a UAV with a higher node degree as a CH to increase the CH lifetime. A node with a higher node degree indicates that (a) the mobility of the node and its neighboring nodes is approximately similar, and so it increases connectivity and (b) the node has a larger transmission range. Therefore, the node with a higher node degree is selected as the CH. Using D2D communication, UAVs can communicate with each other in the same cluster, so UAVs can bypass the CHs of their respective clusters via intra-cluster communication. However, D2D communication with nodes from other clusters via inter-cluster communication cannot bypass CHs, which increases the energy consumption of CHs. The DC selects non-clustered UAVs, which are geographically closer to a CH, as the CMs of the CH. The DC selects CGs, which are the CMs with the minimum number of intermediate nodes between two CHs. The DC also selects VCGs, which are the CMs with the highest link expiration time (LET) among the CMs of a cluster, to maintain inter-plane communication. Figure 2: An example of a cluster structure in a traditional clustered network. Solid line represents the connectivity between a node pair, and dash line represents the connectivity between a CH and a BS. Cluster maintenance is performed because UAVs have (a) high node mobility with changing coordinates and the relative speed with neighboring UAVs as time passes, (b) high node density (or ultra-densification), and (c) high heterogeneity with different transmission ranges that may overlap. UAVs can switch between network planes (i.e., switches to either an upper or a lower network plane) based on the relative speed and the number of handovers across different network planes. A handover from one network plane to another can increase clustering overhead in both network planes, and so frequent handover is unfavorable. There are two cluster maintenance mechanisms. First, _cluster merging_ combines two clusters to increase the number of CMs in a cluster (or reduce the number of clusters in a network), contributing to a lower number of handovers across different network planes, a lower interference level among the clusters and network planes, and a higher network scalability. Second, _cluster splitting_ divides a single cluster to reduce the number of CMs in a cluster (or increase the number of clusters in a network), contributing to a higher cluster stability. ### Vertical Routing

Vertical routing selects routes with _lower mobility_ in order to prolong route lifetime for improved QoS as network planes have different mobility levels. Network planes with higher mobility levels have lower stability. The UAVs, which have different characteristics (e.g., the coordinates and relative speed vary with time), require vertical routing to (a) coordinate the UAVs in different network planes whereby UAVs have higher mobility levels in macro-plane, followed by pico-plane and femto-plane, and so the femto-plane provides a higher stability, (b) offload data traffic from macro-plane to pico- or femto-plane, and (c) establish routes with higher stability in the network plane.