I. Introduction
Spectrum shortage emerges due to insufficient use of available spectrum. This report is declared by the Federal Communications Commission (FCC) [1]. The recent study by FCC has shown that most of the allocated frequencies are underutilized in a significant amount. Cognitive radio (CR) which can also be referred as dynamic spectrum access has been proved as a promising technique for spectrum sharing and to increase bandwidth utilization [2]. It allows secondary users (SUs) to dynamically access the licensed spectrum belonging to primary users (PUs) without causing any harmful interference.