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In the present paper one of the main problems of cognitive radios, that is how to allow the secondary (cognitive) users to exploit the unused resources by primary users, is faced by introducing multiple antennas at the cognitive terminals. Under the assumption of perfect Channel State Information (CSI), a transmit beamforming scheme based on a linear algorithm is proposed for the exploitation of the degrees of freedom offered by the spatial diversity. A closed form expression for the achievable rate obtainable by employing two antennas is derived and numerical results regarding the effects of different fading channels are provided.
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