I. Introduction
Cognitive radio (CR) is a promising solution to the spectrum underutilization problem by opportunistically accessing the spectrum of the primary network. There are two main spectrum access mechanisms in CR networks: overlay and underlay. Overlay transmission utilizes the spectrum holes by exclusively using the unoccupied spectrum of the primary user (PU) and vacates on PU reoccupancy. On the other hand, underlay transmission can utilize the spectrum at any time by transmitting in the presence of PUs at low power to avoid causing harmful interference. The overlay and underlay CR approaches have been widely investigated in the literature [1]– [4].