Over the last decade, the available spectrum in wireless communications systems has been a very rare resource due to the increasing demand. However, licensed spectrum is not fully utilized by licensed users. Therefore, new technologies that focus on enhancing the spectrum usage and energy efficiency of wireless mobile networks have been the major interest of significant works. Cognitive networks has been proposed as an efficient technique for the new communications systems in which the available unused licensed spectrum (also called spectrum holes) are exploited in an opportunistic way. This...