Beamforming design and sum rate maximization for the downlink of underlay cognitive radio networks

In this paper, we consider a downlink Multiple-Input-Multiple-Output (MIMO) underlay cognitive radio network where unlicensed Secondary Users (SUs) share the Primary Users (PUs) spectrum. We assume that the channels from the Secondary Base Station (SBS) to each receiver are known to the SBS. We propose a beamforming matrix design to ensure that the transmitted signal does not cause harmful interference to the PUs and SUs. Furthermore, we study the optimal power allocation that maximizes the sum rate of the secondary system when the channel is not perfectly known at the SBS. Simulations results show the performance of our system in terms of sum rate of the secondary users in both cases of perfect and imperfect knowledge of Channel State Information (CSI) at the SBS.