Cooperative Beamforming for Cognitive Radio Systems with Asynchronous Interference to Primary User

In a 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 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 compared to the zero forcing beamforming as well as joint leakage suppression method and thereby decreases the outage probability. This beamforming method is further extended for the case when the channels between the primary receiver and the CR relays are not known perfectly. Moreover, in this paper, we propose and investigate two relay selection strategies in conjunction with cooperative beamforming. The presented numerical results show that the relay selection schemes in conjunction with the cooperative beamforming method can further improve the received signal power at the secondary destination.