  This paper studies the secrecy performance of multiple-input multiple-output
(MIMO) wiretap channels, also termed as multiple-input multiple-output
multiple-eavesdropper (MIMOME) channels, under transmit antenna selection (TAS)
and BPSK/QPSK modulations. In the main channel between the transmitter and the
legitimate receiver, a single transmit antenna is selected to maximizes the
instantaneous Signal to Noise Ratio (SNR) at the receiver. At the receiver and
the eavesdropper, selection combination (SC) is utilized. By assuming Rayleigh
flat fading, we first derive the closed-form approximated expression for the
ergodic secrecy rate when the channel state information of the eavesdropper
(CSIE) is available at the transmitter. Next, analytical formulas for the
approximated and asymptotic secrecy outage probability (SOP) are also developed
when CSIE is unavailable. Besides theoretical derivations, simulation results
are provided to demonstrate the approximation precision of the derived results.
Furthermore, the asymptotic results reveal that the secrecy diversity order
degrades into 0 due to the finitealphabet inputs, which is totally different
from that driven by the Gaussian inputs.
