I. Introduction and Motivation
Based on the evidence that fixed (licensed) spectrum allocation entails a highly inefficient resource utilization, cognitive radio prescribes the coexistence of licensed (or primary) and unlicensed (secondary or cognitive) radio nodes on the same bandwidth. While the first group is allowed to access the spectrum any time, the second seeks opportunities for transmission by exploiting the idle periods of primary nodes [1]. The main requirement is that the activity of secondary nodes should be “transparent” to the primary, so as not to interfere with the licensed use of the spectrum.