Date Added: Jul 2010
Due to the rapid improvement of wireless technology, many fixed assigned spectrum, wireless applications, have been developed, such as WiMAX, LTE, WLAN and so on. These fixed spectrum systems have two main problems; one is scarcity of the frequency spectrum and the other one is the variation in temporal and geographical utilization of the assigned spectrum. As a solution for these problems, cognitive radio has been proposed to provide the methodology of dynamic spectrum access. For the Cognitive Users (CUs) to be able to share the spectrum, they have to check whether the Primary User (PU) is occupying the band or not. Because, the interference from the CU toward the primary system is not allowed.