Incremental Relaying Transmissions with Relay Selection in Cognitive Radio Networks

In this paper, we investigate and evaluate the performance of incremental relaying and relay selection, when used in the context of cognitive radio networks. Assuming that a number of cognitive radio relay nodes N (N >= 2) are co-located with simultaneous primary and secondary transmissions, the "best" relays are chosen to assist the primary and/or the secondary transmission(s) (in case of decoding failure at the destination(s) using the direct link source-destination). The outage probability of both primary and secondary systems is investigated and the associated power allocation problem analyzed. Results show that incremental relaying allows to improve greatly the secondary outage probability with respect to a primary outage probability threshold, compared to the non-cooperative case. Moreover, they suggest that selecting at first the "best" relay to assist the primary transmission before the one that would assist the secondary transmission is more beneficial than choosing at first the "best" relay that would help the secondary transmission. Finally, by proposing an adequate transmit power allocation scheme, we bypass the secondary transmissions' blocking at low primary Signal-to-Noise-Ratio.