Secrecy sum-rates for multi-user MIMO linear precoding

We study the multi-user MIMO channel with confidential messages, in which a multi-antenna transmitter has secret communication with an arbitrary number of receivers. We consider the worst case scenario where each user regards the others as cooperating eavesdroppers. We study the effectiveness of simple transmission schemes for maintaining confidentiality among users. We first derive the secrecy sum-rates achievable with generic linear precoding, then we focus on regularized channel inversion (RCI) precoding. For the latter, we discuss the choice of the optimal regularization parameter, finding a region where it lies, and we propose a practical approximation for it.