Power allocation and achievable data rate in spectrum-sharing channels under adaptive primary service outage constraints

In this paper, we focus on a cognitive radio network where adaptive modulation is adopted in primary links. The gap between the primary user (PU)´s received signal-to-noise ratio (SNR) and the lower SNR boundary of the modulation mode that is being used, provides an interference-tolerable zone. Based on this gap, a secondary user (SU) has an increased opportunity to access the licensed spectrum and to determine the transmit power it should use to keep the PU´s quality-of-service (QoS) unaffected. However, since the SU cannot obtain perfect information on the PU´s received SNR, it has to choose an SNR point between the lower and upper boundaries of the PU´s current modulation mode as if this point were the real SNR received by the PU. Considering this issue, in order to quantify the effect of the SU´s transmissions on the PU´s QoS, we define the PU´s service outage probability and obtain its closed-form expressions by taking into account whether the peak transmit power constraint is imposed on the secondary´s transmission or not. Subsequently, we derive the SU´s achievable data rate in closed form for counterpart scenarios. Numerical results provided here quantify the relation between the PU´s service outage probability and the SU´s achievable data rate, which further demonstrate that the higher the peak transmit power a secondary transmitter can support, the better performance the cognitive radio network can achieve.