  Non-orthogonal multiple access (NOMA) and massive multiple-input
multiple-output (MIMO) systems are highly efficient. Massive MIMO systems are
inherently resistant to passive attackers (eavesdroppers), thanks to
transmissions directed to the desired users. However, active attackers can
transmit a combination of legitimate user pilot signals during the channel
estimation phase. This way they can mislead the base station (BS) to rotate the
transmission in their direction, and allow them to eavesdrop during the
downlink data transmission phase. In this paper, we analyse this vulnerability
in an improved system model and stronger adversary assumptions, and investigate
how physical layer security can mitigate such attacks and ensure secure
(confidential) communication. We derive the secrecy outage probability (SOP)
and a lower bound on the ergodic secrecy capacity, using stochastic geometry
tools when the number of antennas in the BSs tends to infinity. We adapt the
result to evaluate the secrecy performance in massive orthogonal multiple
access (OMA). We find that appropriate power allocation allows NOMA to
outperform OMA in terms of ergodic secrecy rate and SOP.
