In this paper, a new algorithm is proposed for cooperative beamforming in the multiple input multiple output (MIMO) orthogonal frequency division multiple access (OFDMA) cognitive radio (CR) systems. The objective is to maximize the signal to interference plus noise ratio (SINR) of the selected secondary users subject to guarantee the primary user's quality of service over all subchannels. In each subchannel, in addition to the primary user (PU), one secondary user (SU), whose quality of service is the best, is selected to be serviced. The transmitter and receiver beamforming vectors are estimated by a two-step iterative algorithm. The performance of the algorithm is evaluated by computer simulations under different scenarios.

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