Spectrum sharing is a promising technique to enhance spectrum utilization in cognitive radio environments. Cognitive sharing the spectra of licensed users provides unlicensed users with the opportunity to access rigid spectrum resources without degrading the performance of the licensed users. The result of this simultaneous sharing of limited spectrum resources by licensed and unlicensed users is more efficient spectrum utilization. Spectrum sharing with licensed users is achieved by allowing unlicensed users to transmit when the interference power constraint at the licensed users is being satisfied. The interference power constraint protects the performance of the licensed users while at the same time determining the performance of the unlicensed users sharing the licensed spectrum. Therefore, the interference power constraint plays an important role in the spectrum sharing environment. This article provides a guideline for spectrum sharing in the cognitive radio environment. A part of our effort focuses on examining the impact of the interference power constraint and the performance of the unlicensed users under each constraint. We first investigate the representative interference power constraints. Next, we propose a power allocation scheme to satisfy the outage probability of a primary user. The performance comparison is then used to confirm that the proposed scheme not only satisfy the outage probability of primary user, but also is robust to the outdated channel environments.