Outage probability analysis of cognitive relay network with four relay selection and end-to-end performance with modified quasi-orthogonal space–time coding

In this study, the authors evaluate the outage probability performance of an amplify-and-forward cooperative relay network where the relays are equipped with cognitive radios. When the number of available relays is more than four the authors use the channel conditions in order to select the best four cognitive relays from a set of M cognitive relay nodes and then they are used for cooperation between the source and the destination nodes. Expressions for outage probability are determined for a frequency flat Rayleigh-fading environment from the received signal-to-noise ratio with perfect and imperfect spectrum acquisition. In addition, a modified distributed quasi-orthogonal space-time block coding scheme with increased code gain distance is considered for use within the proposed cognitive relay network. To utilise the available spectrum opportunities with the modified quasi-orthogonal space-time block code, the code matrix can be adapted to the number of available relays. Simulation results show that the four relay selection improves the system performance. This is confirmed by the outage probability analysis. The simulations also show that the modified code can significantly enhance the performance of the system and improve the reliability of the link as compared with the conventional distributed quasi-orthogonal space-time block coding.