Abstract
In underlay cognitive radio networks (CRNs), secondary users (SUs) are allowed to co-exist with the primary users at the cost of reduced transmit power in order to satisfy the stringent interference constraint. This obligation reduces the coverage area of the SUs and they have to involve relaying services for assistance. However, selecting the best combination of relays from the available options is more challenging in an energy-constrained environment, since it is affected not only by the interference constraint, but also the selected subset of relays must guarantee some quality-of-service (QoS) at the secondary destination also. This study is focused on an amplify-and-forward-based relay-assisted CRN operating in an underlay scenario and studies three relay subset selection or multiple relay selection schemes taking into consideration outgoing secondary and primary link qualities of each candidate relay. For each scheme, the closed-form expression for probability density function of signal-to-noise ratio at the secondary destination is derived followed by detailed analysis of two QoS metrics, i.e. outage probability and bit error rate. Finally, simulations have been performed to validate the analytical results.