I. Introduction
Rapid growth of wireless communications and conventional fixed spectrum allocation policy have led to the problem of spectrum scarcity [1]. However, according to a survey of spectrum utilization in [2], the licensed spectrum is not used efficiently in both temporal and geographical dimensions. Cognitive radio (CR) which is one of the most promising technologies for the future radio spectrum management, improves spectrum utilization by using these underutilized parts of the spectrum. In CR networks, the secondary users are allowed to communicate, provided that the interferences caused to the primary receivers (PRs) are below a given threshold, called interference temperature limit [3].