Cognitive Radio: An Intelligent Wireless Communication System: A Survey

Because of the large number of wireless technologies and users, spectrum availability has become an important issue. The regulations of spectrum usage do not accept that unlicensed users should operate in the licensed band. However, the entire licensed spectrum is not used all the time and at all places. So, the efficiency of the spectrum can be increased and unlicensed user can take advantage to communicate using idle spectrum and this is the basic idea behind Software Defined Radio (SDR) and cognitive radio (CR).