Joint Secure Beamforming and User Selection for Multi-user MISO Systems with Confidential Messages

In this paper, we propose a joint secure beamforming and user selection (J-SBS) scheme to improve the secrecy sum rate in multi-user multiple-input-single-output (MISO) broadcast channels with confidential messages (BCC). The user selection mechanism can bring about multi-user diversity which may be also exploited by eavesdroppers in multi-user MISO sysytem. We show how the system secrecy sum rate under the semi-orthogonal user selection (SUS) algorithm will be limited if users collude with each other. To solve this problem, based on the prediction that selected users have mutual orthogonal beamformers, we characterize a new expression of the secrecy rate. By optimizing the secrecy rate we then derive a corresponding suboptimal secrecy beamformer for each user to reduce the interference and information leakage. Moreover, we apply SUS to select suitable beamformers matching with the prediction. Simulation results show the effectiveness of our proposed method.