Radio spectrum resource is of fundamental importance in wireless communications. Nowadays, the access to radio spectrum is largely based on fixed spectrum allocation principle. With the deployment of more wireless applications/services, most of the available spectrum has been well allocated, thus
many countries are facing the problem of spectrum scarcity.  On the other hand, measurement studies have revealed that most of the allocated spectrum experiences low utilization efficiency. These two facts motivate the introduction of dynamic spectrum access, which allows secondary users to use/share
the same radio spectrum originally allocated to the primary (licensed) users.
So, Cognitive radio which evolves from the Software Defined Radio is the enabling technology for dynamic spectrum access.  The essential components in a cognitive radio system include spectrum sensing, cognitive medium access control and cognitive networking.  That is Cognitive Radio is a transceiver which automatically detects the available channels in wireless spectrum and accordingly changes transmission or reception parameters.  Cognitive radio is an artificial intelligence imposed on Software Defined Radio.