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This work studies a cognitive radio (CR) communication system in which multiple antennas assist the secondary user to achieve transmission diversity, while the constraint of zero interference to the primary user is satisfied. We show that due to the zero interference constraint, the secondary link may occasionally suffer from low signal levels, as if it is in deep fading. Employing space time coding for diversity does not help this situation. We show that a beamformer that is mutually orthogonal in time domain across the antennas is effective in mitigating the low signal effects. Further, N antennas provide an equivalent (N-1) × 1 Rayleigh channel due to the function of this beamformer. The effect of a non-orthogonal beamformer is also analyzed and shown by simulations.