Reduced outage probability relay selection for underlay cognitive networks

In underlay cognitive networks, the unlicensed (secondary) user is allowed to share the spectrum with the licensed (primary) user under the constraint that the interference at the primary user should be below a specific threshold. This constraint forces the cognitive network to apply power control techniques, which typically result in limited transmission power and hence limited coverage area. As a solution to reach the intended far destinations, relaying techniques can be used to improve the performance of underlay cognitive networks in terms of signal-to-noise ratio (SNR). Recently, single best relay selection schemes have been proposed that maintain a desired SNR for the cognitive network while satisfying the interference level at the primary network. However, these schemes suffer from high outage probability especially at low SNR, where the probability of finding a single relay that satisfies both constraints is reduced. Therefore, in this paper, we propose a multiple relay selection scheme, where at each channel realization, we allow for multiple relay selection in case the best relay, among those that satisfy the interference constraint, does not satisfy the desired SNR. In particular, we select the least number of relays whose sum of their individual SNR´s achieves the desired level. This scheme leads to a reduced outage probability especially for low SNR. We derive a closed form expression for the cumulative density function (CDF) of the selected relay(s) SNR for a certain number of relays. Simulation results confirm the analytical results and show that the proposed relay(s) selection algorithm in an underlay cognitive environment improves the outage probability and symbol error rate (SER) in low SNR regions.