Dear Sir...Cognitive radio concept has been partially implemented and a protocol is designed for using this technology...Kindly refer the specification 802.22 : Wireless Regional Area Networks. You can use this link to download the IEEE paper free of cost from the IEEE website http://standards.ieee.org/about/get/802/802.11.html ..Hope this helps !!!

The following is extract from the attached FCC file:

• There are a number of capabilities that can be incorporated into cognitive radios.  A first is frequency agility, which is the ability of a radio to change its operating frequency, combined with a method to dynamically select the appropriate operating frequency based on the sensing of signals from other transmitters or on some other method.  A second is adaptive modulation that can modify transmission characteristics and waveforms to exploit opportunities to use spectrum. A third capability is transmit power control, which allows transmission at the allowable limits when necessary, but reduces the transmitter power to a lower level to allow greater sharing of spectrum when higher power operation is not necessary.  A fourth capability that a cognitive radio could incorporate is the ability to determine its location and the location of other transmitters, and then select the appropriate operating parameters such as the power and frequency allowed at its location.  Fifth, a cognitive radio could incorporate a mechanism that would enable sharing of spectrum under the terms of an agreement between a licensee and a third party.  

FCC has authorized the use of cognitive radio.

yes see IEEE 802.22 and how they exploit the wide TV band is US for cognitive radio application.