Secure Transmission Based on Non-Overlapping AOA in Cell-Free Massive MIMO Networks

In this paper, we mainly study the secure transmission method of cell-free massive multiple-input multiple-output (MIMO) system under active eavesdropping. In cell-free massive MIMO system, active pilot attacks will seriously affect the uplink channel estimation and secure transmission. In order to effectively reduce the impact of active pilot attacks on downlink transmission, this paper proposes a channel estimation algorithm based on the non-overlapping angle of arrival (AOA) to improve channel estimation accuracy. First of all, we propose an access point (AP) selection strategy based on AOA information to select APs that provides services for users. Then according to nonoverlapping AOA between legitimate user and eavesdropper, we use discrete Fourier transform (DFT) to distinguish the uplink channels of legitimate user and eavesdropper from the angle domain, thereby eliminating the pilot contamination caused by active pilot attacks. Finally, we obtain the downlink transmission secrecy rate of cell-free massive MIMO system under the multipath channel model. The results show that the proposed channel estimation algorithm can reduce the estimation error by 2–10 dB compared with least-square (LS) estimation, and can increase the secrecy rate of 4 bit/s/Hz at most, which effectively enhances the secure transmission performance of cell-free massive MIMO in strong interference environment.