  In this paper, we study the impacts of transmit antenna selection on the
secrecy performance of massive MIMO systems. We consider a wiretap setting in
which a fixed number of transmit antennas are selected and then confidential
messages are transmitted over them to a multi-antenna legitimate receiver while
being overheard by a multi-antenna eavesdropper. For this setup, we derive an
accurate approximation of the instantaneous secrecy rate. Using this
approximation, it is shown that in some wiretap settings under antenna
selection the growth in the number of active antennas enhances the secrecy
performance of the system up to some optimal number and degrades it when this
optimal number is surpassed. This observation demonstrates that antenna
selection in some massive MIMO settings not only reduces the RF-complexity, but
also enhances the secrecy performance. We then consider various scenarios and
derive the optimal number of active antennas analytically using our
large-system approximation. Numerical investigations show an accurate match
between simulations and the analytic results.
