Secure Communications with Secondary User Selection in Underlay Cognitive Radio Networks

This paper investigates the effect of multiuser gain provided by the secondary user selection on the secrecy performance of the primary users. We propose a simple secondary user selection scheme and derive a closed-form lower bound of the achievable ergodic secrecy rate of the primary users. In the high signal-to-noise ratio (SNR) regime, asymptotic result shows that the multiuser gain scales logarithmically with the number of the secondary users for a fixed interference temperature. When there is no interference temperature constraint, more multiuser gain can be achieved as the secondary user transmits with a higher power. Theoretical and simulation results show that the interference from the secondary users is helpful and the multiuser gain enhances the security of the primary users greatly.