Outage probability of multiuser cognitive relay networks with orthogonal space-time block code transmission

This paper investigates the outage performance of decode-and-forward (DF) based cognitive relay networks (CRNs) in the presence of multiple secondary destinations (SDs) in a spectrum sharing scenario with orthogonal space-time block code (OSTBC) transmission. By employing an opportunistic scheduling algorithm to select one out of L SDs, we derive both the exact closed-form expression of outage probability and asymptotic expression of which in high signal-to-noise ratio (SNR) region over Rayleigh fading channels. Our theoretical analysis is corroborated by Monte Carlo simulation. The result shows that the outage performance of the system will be improved with the increase of both the number of antennas and SDs. When source and relay are equipped with N transmit antennas, the diversity order is N × min (N, L). Under this condition, if the number of antennas N is smaller than L, the diversity order is N², which means the conventional multiuser diversity gain can not be achieved.