Over the last several years the world of wireless communications has undergone some critical changes, which have brought it at the front position of international research and development interest, finally resulting in the introduction of a huge number of innovative technologies and associated products such as Wi-Fi, WiMAX, 802.20, 802.22, wireless mesh networks and software defined radio. Cognitive radio system uses a base of software defined radio technology and makes use of intelligent software packages that enhance their transceivers with the extremely smart properties of self-responsiveness, flexibility and ability to learn.