In this correspondence, an adaptive cooperation diversity scheme with best-relay selection is proposed for multiple-relay cognitive radio networks to improve the performance of secondary transmissions while ensuring the quality of service (QoS) of primary transmissions. Exact closed-form expressions of the outage probability of secondary transmissions, referred to as secondary outage probability, are derived under the constraint of satisfying a required outage probability of primary transmissions (primary outage probability) for both the traditional non-cooperation and the proposed adaptive cooperation schemes over Rayleigh fading channels. Numerical and simulation results show that, with a guaranteed primary outage probability, a floor of the secondary outage probability occurs in high signal-to-noise ratio (SNR) regions. Moreover, the outage probability floor of the adaptive cooperation scheme is lower than that of the non-cooperation scenario, which illustrates the advantage of the proposed scheme. In addition, we generalize the traditional definition of the diversity gain, which can not be applied directly in cognitive radio networks since mutual interference between the primary and secondary users should be considered. We derive the generalized diversity gain and show that, with a guaranteed primary outage probability, the full diversity order is achieved using the proposed adaptive cooperation scheme.

An Adaptive Cooperation Diversity Scheme With Best-Relay Selection in Cognitive Radio Networks