cognitive radio is a technology that can solve the wireless spectrum under-utilization problem by allowing secondary users to opportunistically access the licensed channels without causing interference to the communications of the primary users. Cognitive radio can change its transmitter parameters based on interaction with the environment in which it operates [1]. There are two main characteristics of cognitive radios. The first is cognitive capability, which refers to the ability of the radio technology to sense information from its radio environment. Through this capability, the spectrum resources that are not used by primary users can be detected. Consequently, the best spectrum allocation schemes and transmission parameters can be selected. The second is reconfigurability, which enables a user to change the transmitting channel quickly and adaptively according to the radio environment.