Scalar Gaussian Wiretap Channel: Properties of the Support Size of the Secrecy-Capacity-Achieving Distribution

Abstract: This work studies the secrecy-capacity of a scalar-Gaussian wiretap channel with an amplitude constraint on the input. It is known that for this channel, the secrecy-capacity-achieving distribution is discrete with finitely many points. This work improves such result by showing an upper bound of the order $\frac{\mathsf{A}}{\sigma_1^2}$ where $\mathsf{A}$ is the amplitude constraint and $\sigma_1^2$ is the variance of the Gaussian noise over the legitimate channel.