Opportunistic DF–AF Selection for Cognitive Relay Networks

In this paper, we propose and investigate the opportunistic decode-and-forward (DF)-amplify-and-forward (AF) selection scheme for cognitive relay networks. For reference, the opportunistic DF (ODF) scheme is also considered. Different from noncognitive systems and previous works, the transmit power of the source correlates the received signal-to-noise ratio. Our goal is to analyze outage probabilities for ODF-AF and ODF schemes under both transmit power and interference constraints, which are, however, approximated in closed-form expressions since it is difficult to obtain exact outage probabilities. Simulation results are provided to compare both schemes with traditional DF and direct-transmission schemes. They also demonstrate the tightness of approximations and the performance gain provided by the ODF-AF and ODF schemes.