[Paper Review] Secure Wireless Communication via Intelligent Reflecting Surface

Miao Cui, Guangchi Zhang+1 more|arXiv (Cornell University)|May 26, 2019

Advanced Wireless Communication Technologies12 references327 citations

TL;DR

The paper investigates an IRS-aided secure wireless system where a multi-antenna AP and an IRS collaboratively maximize secrecy rate against a single-antenna eavesdropper by jointly designing AP transmit beamforming and IRS reflect beamforming using alternating optimization and SDR techniques.

ABSTRACT

An intelligent reflecting surface (IRS) can adaptively adjust the phase shifts of its reflecting units to strengthen the desired signal and/or suppress the undesired signal. In this letter, we investigate an IRS-aided secure wireless communication system where a multi-antenna access point (AP) sends confidential messages to a single-antenna user in the presence of a single-antenna eavesdropper. In particular, we consider the challenging scenario where the eavesdropping channel is stronger than the legitimate communication channel and they are also highly correlated in space. We maximize the secrecy rate of the legitimate communication link by jointly designing the AP's transmit beamforming and the IRS's reflect beamforming. While the resultant optimization problem is difficult to solve, we propose an efficient algorithm to obtain high-quality suboptimal solution for it by applying the alternating optimization and semidefinite relaxation methods. Simulation results show that the proposed design significantly improves the secrecy communication rate for the considered setup over the case without using the IRS, and outperforms a heuristic scheme.

Motivation & Objective

• Motivate and enable secure wireless communications by leveraging intelligent reflecting surfaces (IRS).
• Formulate and maximize secrecy rate of an AP-to-user link in the presence of an eavesdropper via joint active (AP) and passive (IRS) beamforming.
• Develop an efficient suboptimal algorithm based on alternating optimization and SDR to solve the non-convex problem.
• Demonstrate performance gains of IRS-aided design over no-IRS and heuristic schemes through simulations.

Proposed method

• Model a MISO AP communicating with a single-antenna user in the presence of a single-antenna eavesdropper aided by an IRS with N elements.
• Jointly optimize AP transmit beamforming w and IRS reflect beamforming q to maximize secrecy rate R_sec = [R_U − R_E]^+ under power and unit-modulus constraints.
• Convert sub-problem for w into a generalized eigenvector problem with closed-form solution using the largest eigenvalue of (B+I/P_AP)^{-1}(A+I/P_AP).
• Express the IRS-dependent terms via s = [q^T, 1]^T and apply SDR to relax rank constraints in the resulting fractional objective; use Charnes-Cooper transformation to solve the SDP.
• Apply Gaussian randomization to recover a feasible q from the SDR solution when solving the QoS-like subproblem.
• Iterate between optimizing w and q until convergence.

Experimental results

Research questions

• RQ1 Can an IRS improve secrecy rate when the eavesdropper’s channel is stronger and spatially correlated with the legitimate channel?
• RQ2 What gains can joint active (AP) and passive (IRS) beamforming provide over no-IRS or heuristic AP-MRT with IRS designs?
• RQ3 How can alternating optimization and SDR be applied to jointly design transmit and reflect beamforming for physical-layer security in IRS-assisted networks?
• RQ4 What is the convergence behavior and computational complexity of the proposed alternating-optimization algorithm?

Key findings

• The IRS-aided design significantly increases secrecy rate compared to the case without IRS.
• Joint optimization of AP transmit beamforming and IRS reflect beamforming approaches an upper bound on secrecy performance and outperforms a heuristic AP-MRT-with-IRS scheme.
• Secrecy rate improves with higher AP transmit power and with larger IRS size N.
• The proposed algorithm converges and its complexity scales with O(N_ite(M^3 + (N+1)^{3.5})).
• Simulations show constructive IRS phase tuning boosts user signal at the legitimate receiver while suppressing the eavesdropper’s reception.