Cognitive radio (CR) is a newly emerging technology, which has been recently proposed to implement some kind of intelligence to allow a radio terminal to automatically sense, recognize, and make wise use of any available radio frequency spectrum at a given time. The use of the available frequency spectrum is purely on an opportunity driven basis. In other words, it can utilize any idle spectrum sector for the exchange of information and stop using it the instant the primary user of the spectrum sector needs to use it. Thus, cognitive radio is also sometimes called smart radio, frequency agile radio, police radio, or adaptive software radio,1 and so on. For the same reason, the cognitive radio techniques can, in many cases, exempt licensed use of the spectrum that is otherwise not in use or is lightly used; this is done without infringing upon the rights of licensed users or causing harmful interference to licensed operations.
The only difference with SDR (Software Defined Radio) is that a cognitive radio needs to scan a wide range of frequency spectra before deciding which band to use, instead of a predefined one, as an SDR terminal does. One of the most important characteristic features of an SDR terminal is that its signal is processed almost completely in the digital domain, needing very little analogue circuit. This brings a tremendous benefit to make the terminal very flexible (for a multimode terminal) and ultrasmall size with the help of state-of-the-art microelectronics technology.
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