A new cooperative transmission scheme with relay selection for cognitive radio networks

Secondary access to the licensed primary spectrum band at the same time as the primary nodes is generally conditioned on the satisfaction of a Quality-of-Service (QoS) requirement at the primary transmission (such as a Signal-to-Noise-Ratio -SNR- threshold or a primary outage probability threshold). Consequently, at low primary SNR that is below a cut-off value, secondary transmissions are totally blocked. In this paper, we propose a new cooperative scheme for cognitive radio networks, where secondary access to the primary spectrum band is granted whether or not the primary transmission satisfies its QoS requirement thanks to the utilization of secondary relay nodes. Using relay selection and proper power allocation at the secondary nodes, we show that the proposed scheme allows secondary access with low secondary outage performance without degrading the primary outage performance. We also compare the proposed scheme to other ones presented in the literature and we study the impact of the number of available relay nodes and the primary outage threshold value on the primary and secondary outage probabilities.