Chance-Constrained Secrecy Optimization in Hybrid RIS-Empowered and UAV-Assisted Networks

TL;DR

This paper develops a robust optimization framework for secure communication in hybrid RIS-UAV networks, accounting for mobility, hardware impairments, and eavesdroppers, and demonstrates significant secrecy improvements through integrated RIS and UAV deployment.

Contribution

It introduces a novel chance-constrained secrecy optimization model for hybrid RIS-UAV systems, with a distributionally robust solution approach and an efficient alternating optimization algorithm.

Findings

Significant reduction in secrecy-outage probability with integrated RIS-UAV systems.

Robustness to channel uncertainties, blockages, and hardware impairments.

Convergence of the proposed optimization algorithm to a stationary point.

Abstract

This paper considers a hybrid reconfigurable environment comprising a UAV-mounted reflecting RIS, an outdoor STAR-RIS enabling simultaneous transmission and reflection, and an indoor holographic RIS (H-RIS), jointly enhancing secure downlink communication for indoor and outdoor users. The system operates under user mobility, dynamic blockages, colluding idle and active eavesdroppers, and transceiver and surface hardware impairments. A 3GPP and ITU-compliant stochastic channel model is developed, capturing mobility-induced covariance evolution, outdoor-indoor penetration losses, and distortion-aware noise due to practical EVM-based impairments. We aim to minimize the aggregate secrecy-outage probability subject to secrecy-rate constraints, QoS requirements, power limitations, and statistical CSI uncertainty. The resulting problem contains coupled secrecy and QoS chance constraints and nonlinear interactions among the BS beamforming vectors, multi-surface phase coefficients, and UAV position. To handle these difficulties, we derive rigorous Bernstein-type deterministic approximations for all chance constraints, yielding a distributionally robust reformulation. Building on this, we propose an alternating optimization framework that employs successive convex approximation (SCA) to convexify each block and solve the BS beamforming, RIS, STAR-RIS, H-RIS configuration, and UAV placement subproblems efficiently. The proposed algorithm is shown to monotonically decrease a smooth surrogate of the secrecy-outage cost and converge to a stationary point of the robustified problem. Simulations based on 3GPP TR 38.901, TR 36.873, and ITU-R P.2109 demonstrate that integrating UAV-RIS, STAR-RIS, and H-RIS significantly reduces secrecy-outage probability compared with benchmark schemes and provides strong robustness to channel uncertainty, blockages, colluding eavesdroppers, and hardware impairments.