Reactive relay selection in cooperative spectrum-sharing systems

We consider a dual-hop cooperative spectrum-sharing system with multiple relays that are available to opportunistically help the secondary communication system. In this primary/secondary cooperative system, we propose using a reactive relay selection (RRS) technique at the secondary system in which the best relay is chosen based on both the first and second hops transmission conditions. In fact, the best relay is selected as the relay node that not only satisfies a minimum required rate on the first-hop transmission, but can also achieve the highest signal-to-noise ratio (SNR) at the destination node. In this context, we first derive an expression for the cumulative distribution function (CDF) of the received SNR at the secondary destination node while assuming that the secondary transmission is limited by the appropriate interference constraints. Then, we use this CDF expression to obtain a closed-form expression for the end-to-end outage probability of the proposed cooperative system. Finally, illustrative numerical examples are shown and the benefits of using the proposed RRS technique in different channel propagation conditions are discussed.