  In this paper, we study the design of secure communication for time division
duplexing multi-cell multi-user massive multiple-input multiple-output (MIMO)
systems with active eavesdropping. We assume that the eavesdropper actively
attacks the uplink pilot transmission and the uplink data transmission before
eavesdropping the downlink data transmission phase of the desired users. We
exploit both the received pilots and data signals for uplink channel
estimation. We show analytically that when the number of transmit antennas and
the length of the data vector both tend to infinity, the signals of the desired
user and the eavesdropper lie in different eigenspaces of the received signal
matrix at the base station if their signal powers are different. This finding
reveals that decreasing (instead of increasing) the desire user's signal power
might be an effective approach to combat a strong active attack from an
eavesdropper. Inspired by this result, we propose a data-aided secure downlink
transmission scheme and derive an asymptotic achievable secrecy sum-rate
expression for the proposed design. Numerical results indicate that under
strong active attacks, the proposed design achieves significant secrecy rate
gains compared to the conventional design employing matched filter precoding
and artificial noise generation.
