Secure Transceiver Design for Discrete RIS Enhanced Dual-Functional Radar-Communication: A Symbol-Level Precoding Approach

This letter investigates symbol-level security design for a discrete reconfigurable intelligent surface (RIS) enhanced dual-functional radar-communication (DFRC) system. To improve the radar sensing performance, we formulate an optimization problem that jointly addresses the design of space-time transmission waveforms, receiving filters, and discrete reflecting coefficients. The goal is to maximize the minimum radar signal-to-interference-plus-noise ratio (SINR) while ensuring security against eavesdropping. The formulated problem turns out to be intractable due to the practical waveform design restrictions, the discrete RIS coefficient constraints, and the coupling of the optimization variables. To address these issues, we apply the block coordinate descent (BCD) method, incorporating the successive convex approximation (SCA), penalty technique, and the alternating direction method of multipliers (ADMM) algorithm. Simulation results demonstrate the effectiveness of the proposed scheme and show that 2-bit resolution RIS can achieve both satisfactory communication and radar performance close to that achieved with higher resolutions.