388 CHAPTER 14 Cognitive radio in TV bands measurements carried out in [561] for the frequency bands below 3 GHz and conducted from January 2004 to August 2005, we conclude that, on an average, only about 5.2% of the spectrum is actually in use in the United States in any given location at any given time (please refer to [561] for more detailed information). Interestingly enough, these measurements reveal that heavy spectrum utilization often takes place in unlicensed bands while licensed bands often experience low (e.g., TV bands) or medium (e.g., some cellular bands) utilization. These striking results coupled with recent advancements in radio technology led the FCC to revisit the traditional way of spectrum management. It has been realized that not only is spectrum usage very low in certain licensed bands, but also the scarcity of radio resources is becoming a crisis, hindering the development of many wireless applications, including broadband access (not only in urban/suburban areas but especially in rural/remote areas), public safety, health care, business, and leisure. 14.1.1 Cognitive Radios Cognitive radios (CRs) [7, 60, 551] are seen as the solution to the current low usage of the radio spectrum. It is the key technology that will enable flexible, effi- cient, and reliable spectrum use by adapting the radio's operating characteristics