I. Introduction
In the last decades, the improvement in electronic device have an enormous impact on development of radio-frequency wireless communication equipment. Therefore, it becomes feasible to build small battery-powered sensor nodes with wireless communication capabilities. Wireless sensor networks (WSN s) consist of a large number of small battery-powered nodes, which need to keep working for several months even a few years without replenishing. Because of the battery dependency of WSN s, energy conservation of the networks by reducing the energy consumption of nodes has become a major topic of research [1]–[2].