Cooperative Beamforming for Cognitive Radio-Based Broadcasting Systems with Asynchronous Interferences

In a cooperative cognitive radio (CR) network, cooperative beamforming can enable concurrent transmissions of both primary and secondary systems at a given channel. However, such cooperative beamforming can introduce asynchronous interferences at the primary receivers as well as at the secondary receivers and these asynchronous interferences are overlooked in beamforming design. In order to address the asynchronous interference issue for a generalized scenario with multiple primary and multiple secondary receivers, in this paper, we propose an innovative cooperative beamforming technique. In particular, the cooperative beamforming design is formulated as an optimization problem that maximizes the weighted sum achievable transmission rate of secondary destinations while it maintains the asynchronous interferences at the primary receivers below their target thresholds. In light of the intractability of the problem, we propose a two-phase suboptimal cooperative beamforming technique. First, it finds the beamforming directions corresponding to different secondary destinations. Second, it allocates the power among different beamforming directions. Due to the multiple interference constraints corresponding to multiple primary receivers, the power allocation scheme in the second phase is still complex. Therefore, we also propose a low complex power allocation algorithm. The proposed This work is supported by the Natural Science and Engineering Research Council (NSERC) of Canada, and will be presented in part at the IEEE Wireless Communications and Networking Conference (WCNC’14), Istanbul, Turkey, April 2014. 2 beamforming technique is extended for the cases, when cooperating CR nodes (CCRNs) have statistical or erroneous channel knowledge of the primary receivers. We also investigate the performance of joint CCRN selection and beamforming technique. The presented numerical results show that the proposed beamforming technique can significantly reduce the asynchronous interference signals at the primary receivers and increase the sum transmission rate of secondary destinations compared to the well known zero-forcing beamforming (ZFBF) technique.