1. Introduction
The limited available frequency bandwidth and the inefficient usage for the radio spectrum necessitate a new communication policy to exploit the unused spectrum holes opportunistically for the existing wireless spectrum resources. Cognitive radio (CR) systems have been proposed as a new approach for wireless communication systems to utilize the existing spectrum resources much more efficiently [1]–[3]. Spectrum utilization can be increased by allowing the unlicensed secondary users (SUs) to utilize the licensed band in the absence of the primary user (PU). In CR terminology, the PU is defined as the user who has legacy rights and higher priority to use the licensed spectrum. On the other hand, an unlicensed SU has lower priority and is allowed to utilize the idle spectrum holes opportunistically without causing harmful interference to the PU [4]–[7].