I. Introduction
Wireless Sensor Networks (WSNs) have been used for many applications, such as environmental monitoring, military surveillance, and industry automation [1]. Sensors are usually battery powered and the batteries have low capacity due to the limited size required by many applications. The lifetime of the sensors limits that of a WSN [2] [3], while the cost of replacing these “dead” sensors can be very high, especially in harsh environments or areas that human cannot easily reach [4]. With the increasing applications of WSNs, managing the energy consumption of the networks has become one of the most important issues.