Today’s wireless networks are regulated by a fixed spectrum assignment policy, i.e. the spectrum is regulated by governmental agencies and is assigned to license holders or services on a long term basis for large geographical regions. However, observations reveal that portions of this spectrum are used sporadically in any given location. Cognitive radio (CR) technology is proposed as one of the enabling technologies for an efficient radio spectrum utilization. The CR paradigm allows CR users to detect, use and share the available spectrum, so that the licensed or primary users (PUs) are unaffected. In particular, a CR user can occupy the band under the condition that it has to vacate the spectrum as soon as the primary user (PU) is detected.