  In this paper, we provide an exact expression for the ergodic capacity of the
secondary user, and a unified closed-form expression for its bounds with taking
the consideration of the primary interference at the secondary receiver. In
addition, a simple but accurate approximation of the the ergodic capacity of
the primary user is presented. Moreover, a primary user capacity loss based
power allocation scheme for the secondary user is also proposed. Finally, we
compare the performance of the two power allocation schemes in terms of the sum
capacity. The results are validated through using numerical results and
simulations.
