Contents I. Introduction
The need for efficient utilization of spectrum resources has become a fundamental requirement in modern wireless networks due to the witnessed spectrum scarcity and the ever-increasing demand for higher data rate applications and Internet services. In this context, an interesting proposal has been the development of cognitive radio (CR) networks [2], which can adapt their transmission parameters according to the characteristics of the communication environment. Cognitive radios have been shown to be efficient in increasing spectrum utilization due to their inherent spectrum sensing (SS) capability [3]. In this regard, dynamic spectrum access (DSA), where the secondary users (SU) can opportunistically access the underutilized frequency bands, is the standard solution for the realization of DSA [4], which is envisioned to be an integral part of future communication systems [5]. In order to realize DSA, three strategies have been proposed, namely the underlay, the overlay and the interweave. In the underlay technique, the SUs coexist with a PU provided that the interference level at the PU remains below a certain threshold [6]. In the overlay paradigm, the SUs would be allowed to share the band with PU by exploiting the knowledge of its message and codebook in order to reduce interference. Finally, in the interweave technique, the SU can only access the licensed spectrum of the PU when it is idle [7].
