Coordination beamforming for secrecy enhancement in the downlink MU-MIMO cellular networks

In a conventional cellular wireless networks, single cell processing (SCP) is usually considered where inter-cell interference is treated as noise. As we know, interference can cause severe information leakage for secret communication, the paper studies the secrecy performance with coordination beamforming (CBf) scheme in the downlink of a two-cell multi-user MIMO (MU-MIMO) cellular network. For CBf, the BSs can jointly design their respective beamformers to control inter-cell interference. Based on regularized channel inversion (RCI) precoding, we derive the concise formulas for the signal-to interference plus noise (SINR) at a legitimate user and the colluding malicious users, and the achievable secrecy sum-rate for the CBf scheme. Furthermore, we obtain their corresponding approximation in large-system regime, in which the number of users per cell K and the number of antenna at BS N tend to infinity, while with their ratio β = K/N being held constant. The analytical results are validated with finite-size system by simulation. Compared with SCP without BS cooperation, simulation results show that the secrecy performance can be significantly enhanced by exploiting CBf scheme.