Exploiting Active RIS for Covert Communication in mmWave ISAC with Finite Blocklength

Integrated sensing and communication (ISAC) is susceptible to eavesdropping because of wireless signals’ broadcast nature, and thus the enhancement of its security becomes of critical importance. In this paper, we investigate the use of active reconfigurable intelligence surface (RIS) to realize covert communication in a millimeter-wave ISAC network with finite blocklength, where a base station (called Alice) performs the single-target sensing, and meanwhile transmits confidential messages to a communication user (called Bob) under the surveillance of target. Especially, owing to its abilities in modifying radio propagation and resisting multiplicative fading, active RIS can not only boost the desired target return, but also concentrate/suppress the covert signal at Bob/target to prevent detection. We first derive the minimum detection error probability (DEP) of target in a closed-form via the large system analytical method. To further enhance Bob’s covert performance, we then propose a hybrid beamforming design in which the transmit beamforming at Alice and the reflection beamforming at active RIS are jointly optimized to maximize the covert rate, while meeting given constraints on the minimum DEP against target and the sensing signal-to-interference-plus-noise ratio at Alice. To solve this problem, an effective overall iterative algorithm is developed by leveraging the alternating optimization and successive convex approximation methods. Finally, numerical results confirm the covertness superiority of the studied network, driven by the proposed algorithm, over baseline schemes.