  Physical layer security is investigated over mixture Gamma (MG) distributed
fading channels with discrete inputs. By the Gaussian quadrature rules,
closed-form expressions are derived to characterize the average secrecy rate
(ASR) and secrecy outage probability (SOP), whose accuracy is validated by
numerical simulations. To show more properties of the finite-alphabet
signaling, we perform an asymptotic analysis on the secrecy metrics in the
large limit of the average signal-to-noise ratio (SNR) of the main channel.
Leveraging the Mellin transform, we find that the ASR and SOP converge to some
constants as the average SNR increases and we derive novel expressions to
characterize the rates of convergence. This work establishes a unified and
general analytical framework for the secrecy performance achieved by discrete
inputs.
