Cooperative beamforming in CR systems with asynchronous interferences to multiple primary users

In a cooperative cognitive radio (CR) network when a group of CR users acts as relays for a given user, a cooperative beamforming among these relays can be used to improve the quality of communications. However, this cooperative beamforming can introduce asynchronous interference at the primary receivers due to different propagation delays between different CR relays and the primary receivers. We propose an innovative cooperative beamforming method that maximizes the received signal power at the secondary destination while keeping the asynchronous interference signals at multiple primary receivers below their corresponding target thresholds. Moreover, in this paper, we propose two relay selection strategies in conjunction with cooperative beamforming. The presented numerical results show that the relay selection schemes can further improve the received signal power at the secondary destination.