  This work investigates the effect of finite-alphabet source input on the
secrecy rate of a multi-antenna wiretap system. Existing works have
characterized maximum achievable secrecy rate or secrecy capacity for single
and multiple antenna systems based on Gaussian source signals and secrecy code.
Despite the impracticality of Gaussian sources, the compact closed-form
expression of mutual information between linear channel Gaussian input and
corresponding output has led to broad application of Gaussian input assumption
in physical secrecy analysis. For practical considerations, we study the effect
of finite discrete-constellation on the achievable secrecy rate of
multiple-antenna wire-tap channels. Our proposed precoding scheme converts the
multi-antenna system into a bank of parallel channels. Based on this precoding
strategy, we propose a decentralized power allocation algorithm based on dual
decomposition for maximizing the achievable secrecy rate. In addition, we
analyze the achievable secrecy rate for finite-alphabet inputs in low and high
SNR cases. Our results demonstrate substantial difference in secrecy rate
between systems given finite-alphabet inputs and systems with Gaussian inputs.
