ABSTRACT Wireless Sensor Networks (WSNs) consist of several battery powered sensor nodes. The sensing coverage of the Field of Interest (FoI) is an important function of the sensor nodes in connected WSNs. A FoI is said to be covered if each point in the FoI is monitored by at least one sensor node. Due to small size, battery power supply, simple architecture, and light weight Operating System of the sensor nodes, maintaining the desired coverage of the FoI consists various issues and challenges in a connected WSN. This paper surveys the existing work done to address various issues and challenges for solving the coverage and connectivity problems in WSNs. Our discussion emphasis on sensing models, classification of coverage, research issues in WSNs and practical challenges in deployment of WSNs. We review a brief but complete overview of the various solutions of coverage problems in connected WSNs and describing insights into issues and challenges for research in this area. INDEX TERMS Coverage, communication, monitoring, protocol designing, sensor nodes. Currently, Wireless Sensor Networks (WSNs) find their applications in various domains of ubiquitous and pervasive computing. This is because of low-cost, low-power consumption, light weight, and advancement in wireless communication technology [1]- [8] . A sensor node in WSNs is a multi-functional device that consists the following units: sensor unit, which is composed of two subunits namely sensor unit and an analog-to-digital converter unit, data processing and storage unit, data transmission unit, and a power supply unit. The sensor nodes are self-governing entities that autonomously collaborate to sense, collect the sensory data, process the information, and transmit processed data to the base station. The base station processes the data received from sensor nodes and triggers an action for the event monitored [9]- [11] . Monitoring of the targets in the given Field of Interest (FoI) is an important task of the WSNs. Physical stimulus (such as heat, light, sound, or pressure) is also act as the targets in WSNs. A sensor node detects, converts the quantity of the physical stimulus into a recordable signal, and digitalizes to produce sensory data. The monitoring of the targets illustrates how well the given FoI is monitored by the deployed sensor nodes. The communication of the sensory data or information of the target to the sink, is also an important metric to measure the quality of the WSNs [12]- [14] . The sensor nodes in WSNs for monitoring the FoI and communicate the sensory data, are powered by the batteries. It is important to reduce the energy consumption of the sensor nodes during the monitoring of the FoI and connectivity of the WSNs. This is because the batteries of such sensor nodes may not be accessible for recharge and replacement. Targets or physical stimulus in WSNs can be deterministic located inside the FoI, randomly scattered in the FoI, or on the boundary of the FoI. Such targets may change their initial location with time. Based on the requirement of monitoring of the targets, coverage can be classified into three categories: area coverage, point coverage, and barrier coverage [1], [15]-[17]. The objective of the area coverage is to monitor the FoI by a set of deployed active sensor nodes. Next, target coverage, which is also known as point coverage, is to monitoring the given targets on the fixed locations inside the FoI. Finally, the barrier coverage is used to monitor the border of the given FoI. It helps to detect the intrusion or VOLUME 6, 2018 This paper reviews the existing work on the design of WSNs for the desired coverage and connectivity. We mainly focus on area or blanket coverage of the FoI. The next section introduces the sensor networks, components of a sensor node, monitoring of the FoI, and network connectivity. We also introduce the sensor coverage models in this section. Issues in WSNs are discussed in Section III. Here, we also discuss some practical challenges in designing of a WSN, such as fault tolerance and border effects. Section IV describes recent solutions for estimating the coverage of the FoI. Such solutions help to design the cost effective and energy efficient WSNs. Coverage and connectivity problems and solutions are discussed in Section V. Finally, the conclusion of the paper is given in Section VI. In this section, we introduce components of a sensor node in WSNs and definitions used in this work. We discuss the binary and probabilistic sensor coverage models. Table 2 illustrates the notations used in this work. A WSN consists of several sensor nodes, they can communicate with each others. The main components of a sensor node are sensing, communication, processing, and storage units. Fig. 2 illustrates the components of a sensor node in WSNs. Table 1 summarizes the components of sensor nodes [22] . A sensor node in WSNs consists of sensing unit, which interacts with the physical world around the sensor node. Based on the working principle, sensor nodes in WSNs can be classified into the following categories: active and passive sensor nodes. An active sensor node is a monitoring device used for measuring the signal strength emitted by the sensor node that were reflected by the target. The active sensor requires extra power source for generating the signals. A passive sensor measures the signal strength emitted from the physical environment. Different from active sensor nodes, the passive sensor does not require any extra power source for generating the signals. It is, therefore the passive sensor nodes consume less energy than active sensor nodes. Fig. 1
Coverage is to guarantee that each area is monitored by at least one node, and connectivity is to guarantee that any device can communicate with each other REF .
Coverage and Connectivity in WSNs: A Survey, Research Issues and Challenges
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