The debate around the concept of cognitive radio (CR) has by now broadened its scope to include substantially different technologies and solutions. The identifying features, common to different schools of thought on the subject, are (a) the coexistence on the same spectral resource of both licensed (or primary) and unlicensed (or secondary) terminals and services; and (b) the deployment of “intelligent” radio terminals whose software control processes leverage side information on the radio environment for the achievement of specific goals related to the needs of the user or network. The term CR was coined in 1999 by Joseph Mitola III, who defines a CR as : “A radio that employsmodel based reasoning to achieve a specified level of competence in radio-relateddomains.” Recently, the term CR has been mostly used in a narrower sense. In particular, the Federal Communications Commission (FCC) suggests the following definition : A CR is a radio that can change its transmitter parameters based on the interaction with the environment in which it operates. The motivation behind the interest of both academia and industry on CR is in fact the evidence that (1) current spectrum allocation granting exclusive use to licensed services is highly inefficient , and (2) newwireless communication technologies allow effective spectrum sharing. Among the different debated positions, two main approaches to CR have emerged [3-6]: 1. Commonsmodel: According to this framework, primary terminals are oblivious to the presence of secondary users, thus behaving as if no secondary activity was present. Secondary users, instead, sense the radio environment in search of spectrum holes (portions of the bandwidth where primary users are not active) and then exploit the detected transmission opportunities; 2. Property-rights model (or spectrum leasing): Here primary users own the spectral resource and possibly decide to lease part of it to secondary users in exchange for appropriate remuneration [7, 8].
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