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

In this paper, we consider a homogenous multiantenna downlink network, where a passive eavesdropper intends to intercept the communication between a base station and multiple secure users (SUs) 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, i.e., the transmission mode, has a great impact on the secrecy performance. Specifically, on one hand, a large number of accessing SUs will result in high interuser interference at SUs, resulting in a reduction in the capacity of the legitimate channel. On the other hand, high interuser interference will interfere with the eavesdropper and thus degrades the performance of the eavesdropper channel. Generally speaking, the harmful interuser interference may be transformed as a useful tool in antieavesdropping. The focus of this paper is on selecting the optimal transmission mode according to channel conditions and system parameters 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.

I. Introduction

Information security is always a critical issue of wireless communications due to the open nature of wireless channels. Traditionally, information security is realized by using complex cryptography technology. In fact, information theory has proven that secure communication can be guaranteed by only exploiting the characteristics of wireless channels, e.g., fading, noise, and interference, i.e., physical-layer security [1], [2]. The essence of physical-layer security is to maximize the secrecy rate, which is defined as the rate difference between a legitimate channel and an eavesdropper channel [3], [4]. If there are multiple antennas at an information source, by exploiting the spatial degrees of freedom, the legitimate channel rate is increased, and the eavesdropper channel rate is decreased simultaneously; hence, the secrecy rate can be improved significantly. Thus, physical-layer security coupled with multiantenna techniques has considerably received research interest [5]–[8].