I. Introduction
Recent advances in low-power integrated circuits have sped up the development of various types of low-cost wireless sensors, which are the building blocks of the wireless sensor networks (WSNs). In WSNs, each sensor node has the capability to sense the environment (e.g., temperature, pressure, light, acoustic) and process the data. In general, WSNs have an ad hoc topology, and each node is capable of relaying the data toward the sink [3]. Since most of the sensor nodes are battery powered, one of the design objectives is to prolong the lifetime of the network [4]. There are various ways to define the lifetime of a WSN. It can be defined as the time at which the first node runs out of energy [5]. This time is equivalent to the time at which the first routing path is disconnected [6]. In [7], the lifetime is defined as the time at which the maximum number of times a certain data-collection function can be carried out. In [8] and [9], it is defined as the time at which a region within the WSN is not covered by any nodes.