Active-RIS-Aided Covert Communications in NOMA-Inspired ISAC Wireless Systems

TL;DR

This work investigates covert communications in a NOMA-inspired ISAC system aided by an active RIS. It introduces two transmission schemes (w/o-DSS and w-DSS), derives a CRB-based multi-target sensing framework, and formulates non-convex joint transmit/reflection beamforming problems aimed at maximizing the covert rate under covertness and sensing constraints. An alternating optimization algorithm with penalty-based rank-one relaxations and a penalized Dinkelbach approach solves the resulting subproblems efficiently, with CVX implementations. Numerical results demonstrate that active RIS yields substantial covert-rate gains and better trade-offs between sensing and communication than passive or no-RIS setups, while the w/o-DSS scheme generally outperforms the w-DSS scheme due to reduced power consumption. The findings offer practical insights into RIS deployment, DSS usage, and the interplay between multi-target sensing accuracy and covert communication performance in realistic NOMA-ISAC networks.

Abstract

Non-orthogonal multiple access (NOMA)-inspired integrated sensing and communication (ISAC) facilitates spectrum sharing for radar sensing and NOMA communications, whereas facing privacy and security challenges due to open wireless propagation. In this paper, active reconfigurable intelligent surface (RIS) is employed to aid covert communications in NOMA-inspired ISAC wireless system with the aim of maximizing the covert rate. Specifically, a dual-function base-station (BS) transmits the superposition signal to sense multiple targets, while achieving covert and reliable communications for a pair of NOMA covert and public users, respectively, in the presence of a warden. Two superposition transmission schemes, namely, the transmissions with dedicated sensing signal (w-DSS) and without dedicated sensing signal (w/o-DSS), are respectively considered in the formulations of the joint transmission and reflection beamforming optimization problems. Numerical results demonstrate that active-RIS-aided NOMA-ISAC system outperforms the passive-RIS-aided and without-RIS counterparts in terms of covert rate and trade-off between covert communication and sensing performance metrics. Finally, the w/o-DSS scheme, which omits the dedicated sensing signal, achieves a higher covert rate than the w-DSS scheme by allocating more transmit power for the covert transmissions, while preserving a comparable multi-target sensing performance.

Figures (10)

• Figure 1: The active-RIS-aided NOMA-ISAC system model.
• Figure 2: Impacts of CRB threshold on beampattern
• Figure 3: Convergence performance of AO algorithm
• Figure 4: Covert rate versus CRB threshold with $Q=3$.
• Figure 5: Covert rate versus CRB threshold with $Q=1, 2, 3$.