On user selection in cognitive broadcast channels

This paper considers the cognitive broadcast channels (BC) with user selection, and presents an analytical investigation on the performance of such systems. Specifically, exact closed-form expressions for the outage probability and multiuser interference diversity (MID) gain of the cognitive BC are derived. In addition, closed-form upper and lower bounds for the ergodic capacity of the system are presented. These analytical results not only provide a fast and efficient means to evaluate the performance of the system, they also enable us to gain valuable insights on the impact of key parameters such as peak transmit power, interference temperature constraint, primary transmit power and channel fading parameters on the system performance. Our findings suggest an intuitive result that increasing the number of cognitive users significantly improves the performance of the system, and a rather surprising conclusion that strong primary user interference is beneficial in terms of the MID gain.