The nomenclature of cognitive radio, suggested in Reference [219], indicates the concept of a radio that is flexible in terms of its strategy or etiquette so that it can respond to the needs of the users and environment. The fundamental notion of the cognitive radio is that it is aware of its users and environment and makes decisions that maximize the link performance while minimizing adverse effects on other links in its or a legacy network [94].

A significant driver to the investigation of cognitive radios is the observation that allocated spectrum is not always utilized well [6]. Some researchers in cognitive radios focus on the technology used to describe the logic and rules used by the radio [153]. Game-theoretic models for developing dynamic spectrum access techniques have been investigated [161], and punishment techniques have been considered for users that behave poorly [354]. Conversely, some research focuses on information-theoretic investigations [80, 329] that often assume some level of cooperation between cognitive radios. At a deeper level, an aggressive definition would be for a cognitive radio to try strategies and then learn from them how well a particular strategy works, although we will not require that high level of cognition.

As an aside, it has been common in engineering literature to require that a cognitive system be of greater sophistication than an adaptive system. Amusingly, the engineering literature is at odds with the typical usage of these terms. As one can immediately recognize, a system must be cognizant of its environment before it can be adaptive to it. Consequently, typical usage of these terms would indicate that adaptive systems must have greater sophistication than cognitive systems. However, we will not attempt to correct this abuse of the language.