Joint user selection and beamforming in interference limited cognitive radio networks

We consider optimal beamforming and user admission in cellular secondary networks where radio access to the licensed spectrum is granted under the premise that the interference level created in the primary network falls below a predefined threshold and Quality of Service (QoS) demands of the secondary users (SUs) need to be fulfilled. In scenarios where the interference constraints are restrictive, or the number of users in the primary network is large, costly user admission generally becomes unavoidable. We propose an iterative transmit beamforming and power allocation technique for interference limited cognitive networks in which a computationally efficient user selection procedure naturally integrates. Our iterative scheme is based on the uplink-downlink duality property of the network. Further, a simple infeasibility detection procedure is proposed from which a computationally attractive heuristic deflation technique for user selection is derived. Simulation results demonstrate that our user selection scheme represents a well-balanced compromise between user selection performance and computational complexity.