A Cognitive Radio (CR) is a wireless communications device which can adjust its transmission based on information about the local use of the spectrum. A cognitive radio gets information about how the spectrum is being used and makes a decision on how to use the available radio resources to ensure communications with a certain quality of service.
The term cognitive radio was first used by Joseph Mitola III and Gerald Q. Maguire Jr in 1999. They described a Cognitive Radio as a device that can match its own capabilities to external observations (such as available radio resources, prevailing spectrum rules, user needs and preferences, operational costs of a service etc.). A cognitive radio uses this knowledge to adapt itself to provide wireless services most appropriate to the user needs and preferences. The radio can learn from its past actions and experience and incorporate this knowledge in future decisions.
It was a vision of a highly intelligent wireless personal digital assistant with which users travel. Wherever a user goes, the CR device would adapt to the new environment to offer personalized services that satisfy the user's needs.
In this view cognitive radio is a research goal towards which a software defined radio platform evolves. This type of Cognitive Radio is often referred to as a full Cognitive Radio or Mitola radio. It is unlikely that a full Cognitive Radio will be achieved in the next 20 years
Nowadays, the term cognitive radio is more related to means to increase the efficient use of spectrum. The key feature of such a Cognitive Radio is its ability to recognize unused parts of spectrum assigned to conventional users and adapt its communication strategy to use these parts while minimizing the interference that it generates to the conventional users. (See further: CR and dynamic spectrum access)
There are three types of technology foreseen to get information about the spectrum usage: