[Paper Review] Active Intelligent Reflecting Surface Assisted Secure Air-to-Ground Communication with UAV Jittering

Yimeng Ge, Jiancun Fan|arXiv (Cornell University)|Mar 23, 2022

Advanced Wireless Communication Technologies4 citations

TL;DR

This paper proposes a robust secure transmission scheme for air-to-ground UAV communications using an active intelligent reflecting surface (IRS) to mitigate performance degradation from UAV jittering. By jointly optimizing the UAV's beamforming and the IRS's phase shifts and amplification gains under worst-case secrecy rate constraints, the scheme significantly reduces transmission power while enhancing physical layer security, validated through numerical results showing power savings under channel uncertainty.

ABSTRACT

Unmanned Aerial Vehicles (UAV)-enabled communication is a promising solution for secure air-to-ground (A2G) networks due to the additional secure degrees of freedom afforded by mobility. However, the jittering characteristics caused by the random airflow and the body vibration of the UAV itself have a non-negligible impact on the performance of UAV communication. Considering the impact of UAV jittering, this paper propose a robust and secure transmission design assisted by an novel active intelligent reflecting surface (IRS),where the reflecting elements in IRS not only adjust the phase shift but also amplify the amplitude of signals. Specifically, under the worst-case secrecy rate constraints, we aim to minimize the transmission power by the robust joint design of active IRS's reflecting coefficient and beamforming at the UAV-borne base station (UBS). However,it is challenging to solve the joint optimization problem due to its non-convexity. To tackle this problem, the non-convex problem is reformulated with linear approximation for the channel variations and linear matrix inequality transformed by S-procedure and Schur's complement. Then, we decouple this problem into two sub-problems, namely, passive beamforming and active IRS's reflecting coefficient optimization, and solve them through alternate optimization (AO). Finally, the numerical results demonstrate the potential of active IRS on power saving under secure transmission constraints and the impact of UAV jittering.

Motivation & Objective

• To address the performance degradation in UAV-enabled air-to-ground (A2G) communications caused by UAV jittering due to random airflow and mechanical vibrations.
• To enhance physical layer security in A2G networks by exploiting the secure degrees of freedom provided by active IRS, which offers both phase shifting and signal amplification.
• To minimize the transmission power at the UAV-borne base station (UBS) under worst-case secrecy rate constraints, accounting for channel uncertainty due to UAV jittering.
• To develop a robust joint optimization framework for beamforming and active IRS reflection coefficients that ensures secure and energy-efficient communication.

Proposed method

• Formulates a non-convex optimization problem for joint design of UAV beamforming and active IRS reflection coefficients under worst-case secrecy rate constraints.
• Applies linear approximation to handle channel variations caused by UAV jittering, transforming the problem into a tractable form.
• Uses the S-procedure and Schur’s complement to convert non-convex constraints into linear matrix inequalities (LMIs).
• Decomposes the joint problem into two sub-problems: passive beamforming and active IRS coefficient optimization, solved via alternate optimization (AO).
• Employs a first-order Taylor approximation to convexify the secrecy rate lower bound under channel uncertainty.
• Iteratively optimizes beamforming and IRS reflection coefficients, ensuring convergence under the worst-case channel model.

Experimental results

Research questions

• RQ1 How does UAV jittering impact the secrecy rate and power efficiency in A2G communication systems?
• RQ2 Can active IRS with joint phase shift and amplification capabilities improve secrecy rate and reduce transmission power in the presence of UAV jittering?
• RQ3 What is the optimal robust beamforming and IRS reflection coefficient design under worst-case channel uncertainty due to UAV motion?
• RQ4 How does the proposed scheme compare to conventional passive IRS or direct transmission in terms of power efficiency and security?

Key findings

• The proposed robust beamforming and active IRS optimization scheme achieves significant power savings compared to conventional schemes under the same secrecy rate constraints.
• Numerical results confirm that active IRS effectively mitigates the negative impact of UAV jittering on secrecy performance by enhancing the desired signal and suppressing eavesdropping.
• The worst-case secrecy rate is maintained across all channel uncertainty realizations, validating the robustness of the proposed design.
• The alternate optimization (AO) algorithm converges reliably, demonstrating practical feasibility for real-time implementation.
• The integration of active IRS increases the secure degrees of freedom, enabling better secrecy rate performance than passive IRS or direct links.