In this paper, we consider collaborative beamforming for spectrum-sharing two-way relay networks in an effort to improve the performance of the cognitive system and enhance the spectrum ef ciency. In such a joint relaying/spectrum-sharing setting, a pair of secondary transceivers communicates via a set of secondary decode-and-forward (DF) relays in the presence of multiple primary transceivers. Among the available relays, only those that receive the signals reliably participate in the

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... e beamforming process to nullify the interference in icted on primary receivers. Furthermore, the received signals at relays and at secondary transceivers are unavoidably interfered by the signals from primary transmitters. To study the performance of the cognitive system under the effects of these co-channel interferences (CCIs) from the primary transmitters, we derive closed-form expressions for the outage probability and bit error rate (BER) over independent and identically distributed (i.i.d.) Rayleigh fading channels. Numerical results demonstrate the effectiveness of beamforming in compensating the cognitive system performance loss due to the CCIs in addition to mitigating the interference to the primary users.