# Efficient Node Deployment of Large-Scale Heterogeneous Wireless Sensor Networks
## 2 Related Work  
Since this paper's main objective is to maximize network coverage, this section starts with the classification of the coverage models followed by the coverage classification in WSNs. Then, it discusses some design issues in WSNs for coverage problems such as the deployment methods, sensor node types, mobility of sensor nodes, and solution approaches. In addition, it describes the algorithms that are mostly related to the proposed solutions presented in [6; 13], and it discusses the differences from our proposal as well. Finally, the proposal is compared to the research accomplished in [6; 13]. Many academics have studied the coverage issue in recent years. The binary and probabilistic models are presented in the literature. The binary model Coverage applies a consistent probability of event detection. Also, the binary coverage probability for a given location p is determined by equation [14]:  
\[Cp(p,s)=\left\{ \begin{array}{ll}1&\quad\text{if }d(s,p)\leq r_{s}\\ 0&\quad\text{otherwise}\end{array} \right. \tag{1}\]  
where sensor node's sensing range is represented by \(r_{s}\), the Euclidean distance is represented by \(d(s,p)\) and the deployment point is represented by \(p\). The probabilistic coverage model, in which the probability of monitoring the events has been included, has to use more higher accuracy. WSNs' coverage may be categorized as [14]:  
1. Area Coverage
2. Target Coverage
3. Barrier Coverage  
Area coverage focuses on actively monitoring the region of interest. Sensors that are placed randomly are shown in Figure 3a. Each circle depicts the sensing range of a sensor node. Conversely, point coverage concentrates on monitoring a region of interest. Target coverage is shown in Figure 3b. Three sensor nodes observe three targets, t\({}_{1}\), t\({}_{2}\), and t\({}_{3}\), whereas all other targets, t\({}_{4}\) and t\({}_{5}\), are monitored by two sensor nodes. Additionally, target coverage saves energy and reduces the number of nodes required. Overall, barrier coverage is important to monitor for unauthorized border intrusion, see Figure 4 for a summary of the barrier coverage classifications. Weak barrier coverage consists of a few uncovered areas or gaps that may lead to the target being undiscovered. Also, the second category (Strong Barrier Coverage) consists of barrier coverage that improves the sensor nodes' ability to track or monitor intruders, thereby increasing security [14]. Figure 3: Coverage of the FoI. (**a**) Area coverage. (**b**) Target coverage [14]. Deployment problems may be deterministic and random deployment. According to Figure 5a, deterministic deployment determines the nodes' coordinates prior to their actual distribution. Sensor nodes' positions are set to reduce network costs, energy consumption, high levels of target monitoring, and sensor network lifetimes. However, many applications disallow the adoption of a deterministic approach because of hostile environments or deployment costs. In these situations, random deployment is used. Randomly, Nodes are spread throughout the Fol, as illustrated in Figure 5b. Therefore, the WSN is expected to function better if sensor nodes are deterministically placed rather than randomly deployed [1; 14]. Figure 4: Barrier coverage of the Fol. (**a**) Strong barrier coverage. (**b**) Weak barrier coverage [14]. Figure 5: Deployment in WSNs (**a**) Deterministic deployment. (**b**) Random deployment [14]. Homogeneous and heterogeneous sensor nodes are two classifications of sensor node types. In homogenous WSNs, sensors are having the same characteristics with respect to sensing range, communication range, energy, and storage. Homogeneous WSNs' coverage and connection have been examined in many of the literatures [15; 16; 17; 18; 19]. Nonetheless, maintaining the homogeneity in WSNs is neither possible nor practical, given certain constraints. The first concern is the cost of WSNs, since the high processing power needed for complicated activities results in a costly sensor node.