International Journal of Information Technology & Computer Science ( IJITCS )
Two conflicting challenges are how to maintain the interferences generated by the cognitive radio network to the primary network below an acceptable threshold level while maximizing the sum-rate of the cognitive radio network. We present two beamforming methods, modified zero forcing beamforming and transmit-receive beamforming. The zero forcing beamforming is modified by adding the channel gain between the cognitive radio base station and the primary user to meet the two conflicting goals. The orthogonality of transmit beams in MIMO beamforming by Gram-Schmidt method achieves the first goal that the primary user is interference free. To satisfy the second goal, self-interference is reduced by the constrained minimization of the mean output array of cognitive receivers. To reduce complexity of the system, the number of cognitive radio users must be limited. Criteria to select the number of best cognitive radio users should increase the sum-rate of cognitive-radio network. Subspace-based scheduling scheme selects the cognitive radio users orthogonal each other as much as possible so that the self-interference is mitigated. Simulation results are given to evaluate the performances of the proposed methods in forms of symbol error rates and sum-rates.
: beamforming, cognitive radios, constrained optimization
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