[Paper Review] Security Enhancement for Coupled Phase-Shift STAR-RIS Networks

Zheng Zhang, Zhaolin Wang+4 more|arXiv (Cornell University)|Aug 22, 2022

Advanced Wireless Communication Technologies4 citations

TL;DR

This paper proposes a penalty-based secrecy beamforming (PSB) algorithm for coupled phase-shift STAR-RIS networks to maximize the minimum secrecy capacity among indoor and outdoor users, ensuring fairness and security. The method jointly optimizes transmit beamforming and phase-shift coefficients under phase coupling constraints, achieving near-continuous phase-shift performance with only 4-bit quantization.

ABSTRACT

The secure transmission of the simultaneously transmitting and reflecting reconfigurable intelligent surface (STAR-RIS) aided communication system is investigated. Considering the coupled phase shifts of STAR-RISs and the fair secrecy requirement of users, a novel secure beamforming design is proposed for addressing the unique full-space mutual eavesdropping of STAR-RIS aided communication. In particular, a penalty based secrecy beamforming algorithm is developed to solve the resulting non-convex optimization problem, where the closed-form solutions of the coupled transmission/reflection coefficients are obtained in each iteration. Numerical results demonstrate that 1) the proposed scheme achieves higher secrecy capacity than conventional RIS; 2) 4-bit discrete phase shifters are sufficient for secrecy guarantee.

Motivation & Objective

• To address the security challenge in STAR-RIS networks where full-space eavesdropping increases due to simultaneous transmission and reflection.
• To design a secure beamforming scheme that ensures fairness in secrecy capacity between indoor and outdoor users under coupled phase-shift constraints.
• To develop an optimization framework that jointly designs transmit beamforming and STAR-RIS reflection/transmission coefficients under practical phase coupling and quantization.
• To evaluate the secrecy performance gain of STAR-RIS over conventional RIS and determine the minimum phase quantization bits required for near-optimal performance.

Proposed method

• A penalty-based secrecy beamforming (PSB) algorithm is proposed to solve the non-convex optimization problem with coupled phase-shift constraints.
• The algorithm uses an augmented Lagrangian (AL) relaxation in the outer loop to handle the coupled phase-shift constraints.
• In the inner loop, semidefinite relaxation (SDR) is applied to obtain rank-one transmit beamforming vectors.
• First-order optimality conditions are used to derive closed-form solutions for the coupled transmission and reflection coefficients.
• The method ensures convergence and maintains fairness by maximizing the minimum secrecy capacity across both users.
• The system model assumes Rician fading channels and employs a joint optimization of beamforming and STAR-RIS phase shifts under power and phase coupling constraints.

Figure 1: A coupled phase-shift STAR-RIS aided secrecy downlink network.

Experimental results

Research questions

• RQ1 How can physical layer security be enhanced in STAR-RIS networks with coupled phase shifts, where transmission and reflection coefficients are inherently linked?
• RQ2 What is the optimal beamforming strategy that maximizes the minimum secrecy capacity between indoor and outdoor users under fairness constraints?
• RQ3 How does phase quantization affect secrecy performance in coupled phase-shift STAR-RIS systems?
• RQ4 What is the performance gap between continuous phase shifts and discrete phase shifts in terms of secrecy capacity?

Key findings

• The proposed PSB algorithm achieves higher secrecy capacity than conventional RIS, demonstrating the superiority of STAR-RIS in secure communication.
• The scheme ensures fairness by maximizing the minimum secrecy capacity between the indoor and outdoor users.
• With only 4-bit phase quantization, the discrete STAR-RIS achieves secrecy performance comparable to continuous phase shifts.
• The coupled phase-shift STAR-RIS outperforms conventional RIS and random phase-shift schemes, though it slightly lags behind independent phase-shift STAR-RIS due to phase coupling.
• Time-switching (TS) schemes outperform energy-splitting (ES) schemes in low transmit power regions due to interference-free decoding, while ES performs better at high power due to better time utilization.
• Deploying STAR-RIS at the user end yields higher secrecy performance due to reduced path loss in the cascaded links.

Figure 2: Simulation setup for the considered network.