Distributed Beamforming for Spectrum-Sharing Relay Networks under Mutual Primary-Secondary Interference**This paper was made possible by NPRP grant # 09126-2054 from the Qatar National Research Fund (a member of Qatar Foundation). The statements made herein are solely the responsibility of the authors.

2014 IEEE. In this paper, we consider distributed beamforming for spectrum sharing networks comprising two secondary transceivers, multiple secondary relays and multiple primary transceivers. The aim of this work is to improve the secondary system performance. We assume that the relays that reliably decode the secondary signals participate in the beamforming process. We also assume the presence of mutual interference between the primary and secondary systems, while beamforming is used to suppress the interference inflicted on the primary system. However, the interference inflicted on the secondary system is not mitigated. To examine the impact of this interference on the performance of the secondary system, we derive closed-form expressions for the outage probability and bit error rate (BER) over independent and identically distributed Rayleigh fading channels. Numerical results demonstrate the efficacy of beamforming in making the secondary system performance resilient against the interference caused by the primary system.

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