  In underlay cognitive radio networks, unlicensed secondary users are allowed
to share the spectrum with licensed primary users when the interference induced
on the primary transmission is limited. In this paper, we propose a new
cooperative transmission scheme for cognitive radio networks where a relay node
is able to help both the primary and secondary transmissions. We derive exact
closed-form and upper bound expressions of the conditional primary and
secondary outage probabilities over Rayleigh fading channels. Furthermore, we
proposed a simple power allocation algorithm. Finally, using numerical
evaluation and simulation results we show the potential of our cooperative
transmission scheme in improving the secondary outage probability without
harming the primary one.
