Abstract-We consider precoder and artificial noise (AN) design for multi-antenna wiretap channels under the finitealphabet input assumption. We assume that the transmitter has access to the channel coefficients of the legitimate receiver and knows the statistics of the eavesdropper's channel. Accordingly, we propose a secrecy rate maximization algorithm using a gradient descent-based optimization of the precoder matrix and an exhaustive search over the power levels allocated to the AN. We also propose algorithms to reduce the complexities of direct ergodic secrecy rate maximization by: 1) maximizing a cut-off rate-based approximation for the ergodic secrecy rate, simplifying the mutual information expression, which lacks a closedform and 2) diagonalizing the channels toward the legitimate receiver and the eavesdropper, which allows for employing a pergroup precoding-based technique. Our numerical results reveal that jointly optimizing the precoder and the AN outperforms the existing solutions in the literature, which rely on the precoder optimization only. We also demonstrate that the proposed low complexity alternatives result in a small loss in performance while offering a significant reduction in computational complexity.Index Terms-Physical layer security, finite-alphabet inputs, precoding, artificial noise, cut-off rate, MIMO communications.

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In this work, we propose an iterative scheme for computing a linear precoder that takes into account the impact of hardware impairments in the multiuser multiple-input singleoutput downlink. We particularly focus on the case when the transmitter is equipped with nonlinear power amplifiers. Using Bussgang's theorem, we formulate a lower bound on the achievable sum rate in the presence of hardware impairments, and maximize it using projected gradient ascent. We provide numerical examples that demonstrate the efficacy of the proposed distortionaware scheme for precoding over a millimeter-wave channel.Index Terms-Multiuser multiple-input single-output, downlink, millimeter wave, nonlinear power amplfier, hardware impairments, Bussgang's theorem, and linear precoding.

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