Secure Directional Modulation in RIS-Aided Networks: A Low-Sidelobe Hybrid Beamforming Approach

This letter considers the incorporation of secure directional modulation in reconfigurable intelligent surface (RIS)-aided communication networks. With elaborately selected antenna subsets, secure directional modulation generates randomized signals at undesired directions to assure physical-layer security. One major challenge of introducing RIS is that an additional beam, other than the user-specific beam, is required to align to the RIS. The interplay between the two beams can cause high sidelobe, which is non-trivial to address due to the discrete optimization in antenna subset selection. To tackle this challenge, we propose a novel cross-entropy iterative method to achieve low-sidelobe hybrid beamforming for secure directional modulation in RIS-aided networks. We use maximum sidelobe energy as the objective function to reduce the emission, and deploy Kullback-Leibler divergence as the criterion in the iterative selection of good candidate antenna subsets. Simulation results illustrate that the proposed scheme can reduce the sidelobe energy by 8 dB, and the symbol error rate of the eavesdropper maintains at 0.75 (0.875) for QPSK (8PSK) even when its signal-to-noise ratio increases.