Cognitive radios (CRs) are one of the most researched topics since the introduction of the concept in 1999 [1]. The original definition of the CR concept was ambitious and visionary [1]: CRs have the ability for “… automated reasoning about the needs of the user,” and they are “… radio-domain-aware intelligent agents that search out ways to deliver the services the user wants even if that user does not know how to obtain them” [1]. May be this definition and our intuitive understanding of the word cognitive are enough for now. We can postpone defining the CRs more formally till the next chapter.

In recent years, there have been many efforts toward developing CRs that are, being true to the above definition, user-, and environment-aware. Intuitively, cognitive abilities of a radio must emerge from being able to interpret and react to its RF environment and user’s needs (the performance goals of the radio). Moreover, these cognitive abilities should lead to effective learning. But, of course, we are talking about a radio device. Just as human perception of the external world is gained through the five sensory organs (nose, smell; ears, hearing; eyes, sight; skin, touch; and mouth, taste), a radio’s perception of its external world has to be gained through the single sensory organ a radio is equipped with, that is, its antenna. The sensory input is essentially signals or, in general, some form of electromagnetic radiation, as in Figure 1.1. Hence, the ...

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