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In this paper, the authors develop linear precoding methods for Cognitive Radio (CR) Multi-User Multiple-Input Multiple-Output (MU-MIMO) broadcast systems where unlicensed Secondary Users (SUs) simultaneously use the same spectrum of the licensed Primary User (PU). When the Zero-Forcing Block Diagonalization (ZF-BD) precoder is extended to the CR network, a transmit power boost problem occurs. Therefore, they propose a regularized BD precoder based on the Minimum Mean-Square Error (MMSE) criteria subject to zero interference constraint for the PU. As a result, the proposed MMSE-BD scheme improves the signal-to-interference-plus-noise ratio at each SU's receiver compared to the ZF-BD based method.
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