Fundamental Performance Trade-Offs in Cooperative Cognitive Radio Systems

A relay-aided cooperative underlay cognitive radio system with an interference constraint at the primary receiver (PR) is considered. When the primary user (PU) is active, those secondary users (SUs) whose interference at the PR is above a threshold will amplify-and-forward PU's signal to enhance the PU’s performance, and those SUs below this threshold will proceed with their own transmission in an underlay fashion. As the number of relay SUs increases, the PU’s performance will be improved but cause degradation of SU’s performance concurrently. With our overall average bit error rate and the average combined sum achievable rate metrics, we analytically quantify this fundamental performance trade-off in the large number of SUs regime. Both deterministic and random number of SUs scenarios are considered. Monte-Carlo simulations are provided to verify our analytical results.