Cognitive Multiple-Antenna Network with Outage and Rate Margins at the Primary System

In the common model for spectrum sharing, cognitive users can access the spectrum as long as the target performance in the legitimate primary system is not violated. In this paper, we consider a downlink primary multiple-inputsingle- output (MISO) system which operates under a controlled interference from the downlink MISO cognitive radio, also called secondary system. We derive exact expressions for outage probability of the primary user under Rayleigh fading, when the primary system is exposed to interference from a secondary base station. We treat three different operating modes for the primary base station (BS): space-time coding, antenna selection, and beamforming, each of them with different channel information requirements. We first consider the case in which the primary BS uses a fixed rate and we analyze the outage probability. In high-SNR scenario, we derive closed-form asymptotic formulas for the outage probability. Furthermore, the optimum transmit power in the secondary system is investigated for maximizing the ergodic capacity when there is an outage constraint at the primary system, and simple solutions are proposed. We then consider the case with rate adaptation at the primary BS and introduce a suitable rate margin and a consistent requirement for primary throughput, for which we determine the outage probability. To be able to accommodate the secondary network, a rate margin is assumed at the primary link. We calculate the exact outage probabilities and average throughput of the adaptive-rate transmit-beamforming primary system and the adaptive-rate transmit antenna-selection primary system. The analytical results are confirmed by simulations, in which we analyze the impact of different parameters, such as the number of antennas and the amount of the interference on the system performance.