Cooperative beamforming for CR systems with asynchronous interference to primary user

In a cooperative cognitive radio (CR) network when a group of CR users acts as relays for a given CR user, a cooperative beamforming can be used to improve the quality of communications. However, this cooperative beamforming can introduce asynchronous interference at the primary receiver due to different propagation delays between different CR relays and the primary receiver. In this paper, we propose an innovative cooperative beamforming method that maximizes the received signal power at the secondary destination while keeping the asynchronous interference at the primary receiver below a target threshold. The presented numerical results show that the proposed beamforming method can significantly reduce the interference at the primary receiver and thereby decreases the outage probability up to 75% compared to the zero forcing beamforming method, for a primary user's busy probability of 0.75. This beamforming method is further extended for the case when the channels between the primary receiver and the CR relays are not known perfectly.