Sensors_**2009**, \(9\), 7664-7693; doi: 10.3390/s91007664  
_Review_  
**Connectivity, Coverage and Placement in Wireless Sensor Networks**  
**[PERSON] \({}^{1}\), [PERSON] \({}^{2}\), [PERSON] \({}^{3,\star}\), [PERSON] \({}^{4}\) and [PERSON] \({}^{5}\)**  
\({}^{1}\) ARC Special Research Centre for Ultra-Broadband Information Networks (CUBIN), University of Melbourne, Parkville 3010, Australia; E-Mail: [EMAIL_ADDRESS]  
\({}^{2}\) Centre for Advanced Internet Architectures (CAIA), Swinburne University of Technology, Melbourne, Australia; E-Mail: [EMAIL_ADDRESS]  
\({}^{3}\) School of Computer Engineering, Nanyang Technological University, \(639798\) Singapore  
\({}^{4}\) Department of Electronic Engineering, City University of Hong Kong, Kowloon, Hong Kong; E-Mail: [EMAIL_ADDRESS]  
\({}^{5}\) Department of Engineering Science, National Cheng Kung University, Taiwan; E-Mail: [EMAIL_ADDRESS]  
\({}^{\star}\) Author to whom correspondence should be addressed; E-Mail: [EMAIL_ADDRESS]. _Received: 25 August 2009; in revised form: 25 September 2009 / Accepted: 25 September 2009 / Published: 28 September 2009_  
**Abstract:** Wireless communication between sensors allows the formation of flexible sensor networks, which can be deployed rapidly over wide or inaccessible areas. However, the need to gather data from all sensors in the network imposes constraints on the distances between sensors. This survey describes the state of the art in techniques for determining the minimum density and optimal locations of relay nodes and ordinary sensors to ensure connectivity, subject to various degrees of uncertainty in the locations of the nodes. **Keywords:** wireless sensor networks; connectivity; coverage; relay placement; percolation theory Introduction  
This survey provides an overview of wireless sensor network (WSN) connectivity, and discusses existing work that focuses on the connectivity issues in WSNs. In particular, we are interested in maintaining connected WSNs and their connectivity related characteristics including sensor node placement, as well as the construction of a small connected relay set in WSNs. We aim to review extensively the existing results related to these topics, and stimulate new research. Sensor networks have a long history, which can be traced back as far as the \(1950\)'s. It is recognized that the first obvious sensor network was the Sound Surveillance System (SOSUS) [1; 2]. The SOSUS was made up of an array of acoustic sensors that were interconnected by wired cables and were deployed by the US in deep ocean basins during the Cold War to detect and track Soviet submarines. In its early stages, the development of sensor networks was mainly driven by military use, in which sensor nodes were wired together to provide battlefield surveillance. Evolution of technologies has driven sensor networks away from their original appearance. With the emergence of integrated sensors embedded with wireless capability, most of current sensor networks consist of a collection of wirelessly interconnected sensors, each of which is embedded with sensing, computing and communication components. These sensors can observe and respond to phenomena in the physical environment [3]. Such sensor networks are referred to as wireless sensor networks (WSNs). These WSNs provide flexibility in deployment and maintenance, exploit the ability of wireless networks to be deployed in highly dynamic environments and hence enable sensor networks to be potentially used in a wide range of civilian and military applications, including security surveillance (e.g., to alert of terrorist threats), environmental monitoring, habitat monitoring, hazard and disaster monitoring and relief operations, health field applications, and home applications (e.g., smart environments) [3]. The wireless communication in WSNs can be either ad hoc (multi-hop) or single-hop wireless transmission [4].