Nearest APs-Based Downlink Pilot Transmission for High Secrecy Rates in Cell-Free Massive MIMO

Abstract

This paper investigates the downlink (DL) performance of cell-free (CF) massive multiple-input multiple-output (MIMO) systems in the presence of pilot spoofing attacks. Assuming no statistical information of the eavesdropper is available at the access points (AP)s while adopting non-orthogonal pilot sequences in the uplink (UL) training phase, new analytical expressions for the achievable DL rates and information leakage are derived. Besides, we propose a new DL transmission protocol in which the nearest APs to users send DL pilots so that the users can estimate the corresponding DL channel conditions and detect the intended DL transmitted symbols. The performance of the proposed technique is compared with other techniques in the literature, namely, no DL pilots and DL beamforming training. Results reveal that the proposed technique is more robust under pilot spoofing attacks as it significantly limits the information leakage to the eavesdropper and achieves superior secrecy rates.