Optimal Beam Steering Design for Large-Scale mmWave MIMO Wiretap Channel

This paper investigates the optimal secure beam steering design of millimeter wave (mmWave) communications, where an Alice-Bob pair wishes to communicate in secret in the presence of Eve, with each node equipped with large-scale antenna arrays. Owing to the reduced peak-to-average power ratio and hardware cost, beam steering design emerges as an attractive technique in mmWave communications recently. However, from the physical layer perspective, the beam steering design subject to security requirement has not been investigated yet. In this paper, we consider a secrecy rate maximization problem with respect to beam steering design, i.e., analog beam selection of radio frequency (RF) chains and power allocation over the selected RF chains, which turns out to be an intractable mixed integer nonlinear optimization problem. To tackle it, we first determine a set of optimal analog beam candidates, based on which the considered multi-input multi-output (MIMO) wiretap channel is decoupled into a sequence of parallel single-input single-output (SISO) wiretap channels. Then, it is shown that the optimal power allocation over the parallel wiretap channels can be derived in a semi-closed-form. Numerical results illustrate that the proposed design offer better secrecy performance than traditional beam steering design in the presence of wiretapping as long as the channel has more than two propagation paths.