  In this paper, an adaptive two-way relay cooperation scheme is studied for
multiple-relay cognitive radio networks to improve the performance of secondary
transmissions. The power allocation and relay selection schemes are derived to
minimize the secondary outage probability where only statistical channel
information is needed. Exact closed-form expressions for secondary outage
probability are derived under a constraint on the quality of service of primary
transmissions in terms of the required primary outage probability. To better
understand the impact of primary user interference on secondary transmissions,
we further investigate the asymptotic behaviors of the secondary relay network
including power allocation and outage probability, when the primary
signal-to-noise ratio goes to infinity. Simulation results are provided to
illustrate the performance of the proposed schemes.
