Effect of primary spectrum availability on outage probability of cognitive relay systems in secondary spectrum usage

In this paper, we investigate the outage probability of cognitive relaying networks where the relays use primary spectrums in a secondary purpose. The outage probability has been studied and useful conditions of achieving the full diversity have been given based on the assumption that the primary spectrum is always available for secondary usage. However, the available spectrum for cognitive nodes does not always exist practically. The performance then depends on the probability that the primary spectrum is available for the secondary usage. In this paper, we prove that if the probability of spectrum availability for cognitive nodes and the number of primary nodes become large, then the outage probability decreases and the full diversity can be achieved. We show that for a given average number of primary nodes having the spectrum available for cognitive nodes, the outage performance becomes better with a few primary nodes having a high probability of the spectrum availability for cognitive nodes than with many primary nodes having a low probability of the spectrum availability. We also show that if the primary spectrum is not always available for secondary usage, then there is no impact of cooperative detection of available spectrum with multiple relay nodes on the outage performance.