  Invoking large transmit antenna arrays, massive MIMO wiretap settings are
capable of suppressing passive eavesdroppers via narrow beamforming towards
legitimate terminals. This implies that secrecy is obtained almost for free in
these settings. We show that this property holds not only for fully digital
MIMO architectures, but also in massive MIMO settings whose transmitters employ
architectures with reduced complexity. The investigations consider two dominant
approaches for complexity reduction, namely antenna selection and hybrid
analog-digital precoding. We show that using either approach, the information
leakage normalized by the achievable sum-rate vanishes as the transmit array
size grows large. For both approaches, the decaying speed is determined. The
results demonstrate that, as the transmit array size grows large, the
normalized leakages obtained by antenna selection and hybrid analog-digital
precoding converge to zero double-logarithmically and logarithmically,
respectively. These analytic derivations are confirmed for various benchmark
architectures through numerical investigations.
