Multi-source multi-relay underlay cognitive radio networks with multiple primary users

The outage behavior of a dual-hop multi-source multi-relay cooperative underlay cognitive radio network is investigated in the presence of multiple primary transmitters and multiple primary receivers. Firstly, with primary users´ (PUs´) outage constraints, an adaptive power control scheme is adopted to obtain maximum allowed secondary transmission power. Then, an SINR-based scheme is proposed for the combined use of cooperative diversity and multiuser diversity, where the best secondary source-relay pair is selected to communicate with the secondary destination. In this context, exact closed-form expressions for the outage probability of reactive decode-and-forward (DF) relaying are derived for maximum ratio combining (MRC) and selection combining (SC) over Rayleigh fading channels, respectively. In particular, we consider the dependence among the received signal-to-interference-plus-noise ratios (SINRs) at the destination caused by multiple PUs. It is shown that although there exists outage floor in the high signal-to-noise ratio (SNR) regime, increasing the number of secondary sources or available relays are efficient ways to decrease the outage floor.