Outage Performance of Cognitive Relay Networks with Optimal Relay and Antenna Selection

Cooperative relaying and multiple-input multiple-output (MIMO) transmission technologies exploit spatial diversity to improve the performance of the secondary users in an underlay cognitive radio network. We consider a multiple-relay cognitive MIMO network that is subject to a peak interference power constraint and a peak transmit power constraint. We present an optimal relay and antenna selection scheme, which jointly selects a relay between a secondary source and a destination, a transmit antenna at the source, and a receive antenna at the destination to maximize the end-to-end signal-to-interference-plus-noise ratio (SINR) at the destination. To demonstrate the advantages of our proposed framework, we derive exact closed-form expression for the outage probability of the secondary network under non-identically distributed Rayleigh fading channels. We also derive insightful expressions for the asymptotic outage probability for high SINR. Several important design insights are reached when both fixed and proportional interference power constraints are employed to limit the interference at the primary user's receiver. Under the proportional interference power constraint, the full diversity order is achieved. Under the fixed interference power constraint, the diversity gain is lost. We then consider a practical scenario in which the secondary users have only the mean channel power gains of the interference links to the primary receiver. The secondary source and the selected relay control their transmit powers in order to satisfy an interference outage constraint. Under this scenario, we also provide an expression for the outage probability of the secondary network for the optimal relay and antenna selection scheme. Our analytical results, which are validated with simulations, show the effective impact of the proposed model on enhancing the overall system performance.