Xi Deng and Yuanyuan Yang
Recent years have witnessed the development and proliferation of wireless sensor network (WSNs), attributed to the technological advances of microelectromechanical systems (MEMS) and wireless communications. A WSN is composed of a number of sensor nodes, which are distributed in a specific area to perform certain sensing tasks. A typical sensor node is a low-cost battery-powered device equipped with one or more sensors, a processor, memory, and a short-range wireless transceiver. A variety of sensors, including thermal, optical, magnetic, acoustic, and visual sensors, are used to monitor different properties of the environment. The processor and memory enable the sensor node to perform simple data processing and storing operations. The transceiver makes the sensor node capable of wireless communications, which is critical or WSNs. With many WSNs located in difficult-to-access areas, users usually cannot collect the data in sensor nodes directly. In this case, sensors nodes can transmit the data through the wireless channel to the user or the data sink either directly or by relaying through multiple sensor nodes. Due to the same geographical difficulty, the batteries, as the power supply of the sensor nodes, are difficult to recharge or replace, restricting nodes energy budget. Therefore, energy efficiency is the primary design concern in many WSNs.