Outage probability and ergodic capacity of underlay cognitive radio systems with adaptive power transmission

In this paper, we consider a spectrum sharing system in which a primary transmission coexists with a secondary network composed of a secondary base station (SBS) and multiple secondary user receivers (SU-Rxs). The secondary user (SU) can access the spectrum licensed to the primary user (PU) as long as the SU does not cause any harmful interference to the PU. Among all available SU-Rxs, only the best SU-Rx with highest signal-to-interference-plus-noise ratio (SINR) is selected to receive the transmit signals from the SBS. We derive an analytical expression for the outage probability (OP) of the SU-Rx with highest SINR. Moreover, we derive an approximation of the ergodic capacity at the SU-Rx. In order to meet the requirements at the PU receiver (PU-Rx), at the same time, satisfying the quality of the SU link, an adaptive power allocation strategy is developed. The power allocation policy imposes the SU transmitter (SU-Tx) to adapt its transmit power in order to avoid harmful interference at the PU receiver (PU-Rx), with a PU predefined outage constraint.