Mode Selection in MU-MIMO Downlink Networks: A Physical Layer Security Perspective

In this paper, we consider a homogenous multi-antenna downlink network where a passive eavesdropper intends to intercept the communication between a base station (BS) and multiple secure users (SU) over Rayleigh fading channels. In order to guarantee the security of information transfer, physical layer security is employed accordingly. For such a multiple user (MU) secure network, the number of accessing SUs, namely transmission mode, has a great impact on the secrecy performance. Specifically, on the one hand, a large number of accessing SUs will arise high inter-user interference at SUs, resulting in a reduction of the capacity of the legitimate channel. On the other hand, high inter-user interference will interfere with the eavesdropper and thus degrades the performance of the eavesdropper channel. Generally speaking, the harmful inter-user interference may be transformed as a useful tool of anti-eavesdropping. The focus of this paper is on selecting the optimal transmission mode according to channel conditions and system parameters, so as to maximize the sum secrecy outage capacity. Moreover, through asymptotic analysis, we present several simple mode selection schemes in some extreme cases. Finally, simulation results validate the effectiveness of the proposed mode selection schemes in MU secure communications.