# TARCS: A Topology Change Aware-Based Routing Protocol Choosing Scheme of FANETs
## 3 A Topology Change Aware Routing Protocol Choosing Scheme (TARCS)
### Periodically Topology Change Aware (PTCA)  
The purpose of PTCA is to periodically perceive the topology changes of each node and its neighbors within a one-hop transmission range in the network. Since the movement of nodes causes the change of the relative position between the neighboring nodes and in the number of neighboring nodes, thereby causing the network topology to change, the discussion of the topology change begins with the mobility of the node. Figure 5 shows the topology change caused by the change in distance among nodes. Black dots and red dots represent moving nodes, and dashed circles indicate the effective transmission range of nodes. Figure 5a shows the topology in the initial state. Nodes \(n_{1}\) and \(n_{2}\) are within one hop of \(n_{0}\). Node \(n_{3}\) is within one hop of \(n_{1}\), and \(n_{4}\) is within one hop of \(n_{3}\). Node \(n_{0}\) establishes a route through \(n_{1}\), \(n_{3}\), and \(n_{4}\). From this moment on, \(n_{1}\) starts moving down. Figure 5b shows the network topology after time \(T\). At this time, \(n_{1}\) has moved out of the effective range of \(n_{0}\), and the distance between \(n_{0}\) and \(n_{1}\) increases. If \(n_{0}\) still sends data to \(n_{4}\), it can only pass the path: \(n_{0}\to n_{2}\to n_{3}\to n_{4}\). It can be seen that topology changes could be caused by distance changes between nodes. Figure 6 shows the topology change caused by changes in the direction of movement between nodes. Figure 6a is the initial state. From this moment on, \(n_{0}\) will move around \(n_{1}\). After time T, the distance between \(n_{0}\) and \(n_{1}\) is still the same. However, there is no effective path between \(n_{0}\) and \(n_{4}\) due to the change of direction between \(n_{1}\) and \(n_{0}\). As is shown in Figure 6b. It can be seen that changes in the direction of movement of nodes can also cause topology changes. Figure 5: Topology change caused by nodes distance change: (a) topology in the initial state; (b) topology after period \(T\). Figure 6: Topology change caused by nodes direction change: (a) topology in the initial state; (b) topology after time \(T\). where:  
* \(\dfrac{\left|\frac{d_{ij}^{t+T}-d_{ij}^{i}}{d_{ij}^{t}}\right|}{d_{ij}^{t}}\) indicates the change in distance between node \(i\) and node \(j\);
* \(\dfrac{\left|\frac{\theta_{ij}^{t+T}-\theta_{ij}^{t}}{2\pi}\right|}{2\pi}\) indicates the change of direction between node \(i\) and node \(j\);
* \(\dfrac{\left|relv_{ij}^{t+T}-relv_{ij}^{t}\right|}{relv_{ij}^{t}}\) indicates the change of relative rate between node \(i\) and node \(j\);
* \(\omega\) is the weight coefficient: here \(\omega_{1}=\omega_{2}=\omega_{3}=1\). Average topology change of node \(i\) and its neighbors in time \(T\), is define as the \(\mathbf{TCD_{i,nbrs}}(\mathbf{t},\mathbf{t}+\mathbf{T})\). Suppose the number of neighbors \(j\) of node \(i\) is \(n\) at time \(T\) (\(1\leq j\leq n\)), then the average topology change degree of node \(i\) and neighbors in time \(T\) is defined as the topology change degree average value of each pair of node \(i\) and neighbor \(j\), as defined in Equation (2). \[TCD_{i,nbrs}(t,t+T)=\dfrac{\sum_{j=1,j\
eq i}^{n}TCD_{i,j}(t,t+T)}{n} \tag{2}\]  
The whole network topology change in time \(T\), the \(\mathbf{TCD_{nturk}}(\mathbf{t},\mathbf{t}+\mathbf{T})\), is defined as the sum of the topological changes of all nodes in the network if the total number of nodes in the network is \(N\) (N \(>\) n), as shown in Equation (3). \[TCD_{nturk}(t,t+T)=\dfrac{\sum_{i=1}^{N}TCD_{i,nbrs}(t,t+T)}{2} \tag{3}\]  
Experiments have shown that the difference in node mobility affects the network TCD. In addition, TCD has been confirmed to reflect the topology changes around the nodes and the network, and can distinguish several different mobility modes, including RWP and RPGM (with different groups). The first step of TARCS is to perform periodic topology change perception on each node of the entire network to obtain accurate topology change perception results.