# An Efficient Mobility Model for Improving Transmissions in Multi-UAVs Enabled WSNs
## 4 System Model and Proposed Approach  
### System Model  
The proposed approach aims at improving the coverage and reliability of multi-UAV enabled WSN. The article provides a novel self-clustering technique and a novel technique for setting way-points for UAV movements. The network is comprised of UAV, WSN nodes and a base station. The UAVs act as a relay between sensor nodes and the base station. The topology is classified as dense, scarce and scarce but in proximity of the base station. The division is strictly based on network characteristics according to the frequency of transmissions recorded in a particular region. The scarce regions are effectively those regions which are not strategically important. Also decaying charge of sensor nodes leads to the formation of scarce regions. The overall geography can be visualized as a matrix with row-column coordinates serving as the way-points for successive UAV banks. Figure 2 presents a detailed view of the system model.  
The static WSN nodes are randomly deployed over a certain geography of area \(|D|\), which is a subset of the Euclidean space \(R^{d}\), according to the Poisson distribution, where a node \(k\) can transmit anytime. \(k\) belongs to the Set (WSN) of the wireless sensor nodes. Each node \(k\) can transmit messages \(x\) according to Equation (2):  
\[k\in Set(WSN),Set(WSN)=1,2,3,...,n, \tag{1}\]  
\[P\left(x\right)=\frac{e^{-\lambda|D|}\lambda|D|^{x}}{x!}. \tag{2}\]  
Figure 2: System model.  
\(\lambda\) is defined as the likelihood parameter that estimates the average expected transmissions by a set of WSN nodes:  
\[\lambda=\frac{1}{n}\sum_{i=1}^{n}k_{i}, \tag{3}\]  
where \(n\) is the number of WSN nodes, \(k_{i}=0,1,2,3,...,i\) and \(i=0,1,2,3,...,n\) are the observed occurrences of a transmission.  
The overall area is marked in the form of successive checks with the base station placed in any of the square or at the edge of the geography. The base station is capable of accepting direct communication from the WSN nodes as well as the transmissions occurring from the UAVs, acting as a relay between base station and the ground nodes.  
The aerial vehicles that are effectively serving as relays move from one dense region to another accepting the transmissions from the nodes lying in these regions as well as the regions falling in the path between two successive banks of the UAV. The UAV is equipped with two antennas. One omni-directional antenna is responsible for sensing the underlying topology for incoming transmissions and broadcasting the messages regarding the availability of the UAV in a specific region. The other bi-directional antenna provides the channel for incoming and outgoing transmissions. During transmission phase, the omni-directional antenna is used for broadcasting a blocking message for other WSN nodes. The scarce regions that do not directly fall under the UAV antennas facilitate transmission by multi-hopping towards the base station or the cluster head of adjacent dense region. The symbols used in the proposed article are summarized in Table 2. The next subsection describes the proposed approach in detail.