  We consider covert communication, i.e., hiding the presence of communication
from an adversary for multiple-input multiple-output (MIMO) additive white
Gaussian noise (AWGN) channels. We characterize the maximum covert coding rate
under a variety of settings, including different regimes where either the
number of transmit antennas or the blocklength is scaled up. We show that a
non-zero covert capacity can be achieved in the massive MIMO regime in which
the number of transmit antennas scales up but under specific conditions. Under
such conditions, we show that the covert capacity of MIMO AWGN channels
converges the capacity of MIMO AWGN channels. Furthermore, we derive the
order-optimal scaling of the number of covert bits in the regime where the
covert capacity is zero. We provide an insightful comparative analysis of
different cases in which secrecy and energy-undetectability constraints are
imposed separately or jointly.
