OPTIMAL ACHIEVABLE RATE TRADE-OFF IN COOPERATIVE COGNITIVE RADIO SYSTEMS

A relay-aided co operative underlay cognitive radio system with random number of secondary users (SU), and an interference constraint at the primary receiver (PR) is considered. When the primary (PU) is active, those 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 me an number of relay SUs increases, the PU's performance is improved but causes degradation of SU system performance. We study the optimal trade-off between the achievable rate of the SU and PU systems in the large mean number of SUs regime. This is then followed by a derivation of the optimal ratio between the me an number of relays and underlay SUs. Monte-Carlo simulations are used to verify our analytical results.