Robust and Secure Wireless Communications via Intelligent Reflecting Surfaces

TL;DR

This paper proposes a robust optimization framework using intelligent reflecting surfaces (IRSs) to enhance physical layer security in wireless systems with imperfect channel knowledge, demonstrating significant secrecy improvements.

Contribution

It introduces a joint design of beamformers, artificial noise, and IRS phase shifts under imperfect CSI, with an efficient algorithm for maximizing system sum-rate while ensuring security.

Findings

IRSs significantly improve secrecy performance.

Distributed IRS deployment outperforms single IRS configurations.

The proposed algorithm effectively handles non-convex optimization problems.

Abstract

In this paper, intelligent reflecting surfaces (IRSs) are employed to enhance the physical layer security in a challenging radio environment. In particular, a multi-antenna access point (AP) has to serve multiple single-antenna legitimate users, which do not have line-of-sight communication links, in the presence of multiple multi-antenna potential eavesdroppers whose channel state information (CSI) is not perfectly known. Artificial noise (AN) is transmitted from the AP to deliberately impair the eavesdropping channels for security provisioning. We investigate the joint design of the beamformers and AN covariance matrix at the AP and the phase shifters at the IRSs for maximization of the system sum-rate while limiting the maximum information leakage to the potential eavesdroppers. To this end, we formulate a robust nonconvex optimization problem taking into account the impact of the imperfect CSI of the eavesdropping channels. To address the non-convexity of the optimization problem, an efficient algorithm is developed by capitalizing on alternating optimization, a penalty-based approach, successive convex approximation, and semidefinite relaxation. Simulation results show that IRSs can significantly improve the system secrecy performance compared to conventional architectures without IRS. Furthermore, our results unveil that, for physical layer security, uniformly distributing the reflecting elements among multiple IRSs is preferable over deploying them at a single IRS.