Exact capacity analysis of spectrum sharing systems: Average received-power constraint

This letter presents an exact closed-form expression for the ergodic capacity of the secondary user in a spectrum sharing environment under an average received-power constraint, considering the effect of the interference from the primary user. Using capacity maximizing water-filling power allocation, we provide the closed-form expressions for the ergodic capacity and the average received-power assuming Rayleigh fading channels. Numerical results show that the primary user interference significantly affects the ergodic capacity of the secondary user. The obtained formulas can be used when predicting whether spectrum sharing is beneficial in a practical channel environment.