Cognitive radio technology can effectively improve spectral efficiency by allowing secondary users to access licensed bands without any harmful interference to primary users. Due to harmful channel effects such as fading, or because of far distance between secondary transmitter and receiver, some secondary users who don\\\\\\\'t have any message to transmit at the time, can act as relay nodes to improve secondary users\\\\\\\' connectivity. In this paper, a new algorithm is proposed for cooperative beamforming, power allocation and relay selection in the multiple input multiple output (MIMO) cognitive radio systems where a pair of secondary user communicates with each other assisted by a set of single antenna relay nodes. The objective is to maximize signal to interference plus noise ratio of secondary user subject to guarantee the primary user\\\'s quality of service and power constraints of secondary user and relay node. The transmitter and receiver beamforming vectors of primary user and secondary user are estimated by the proposed two-step iterative algorithm. The performance of the algorithm is evaluated through simulations.

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