Secure Transmission for Intelligent Reflecting Surface-Assisted mmWave and Terahertz Systems

TL;DR

This paper proposes a joint beamforming and reflecting matrix optimization for IRS-assisted mmWave and THz systems to enhance secrecy rates against eavesdroppers, with solutions derived and validated through simulations.

Contribution

It introduces a novel joint optimization framework for IRS-assisted secure transmission, including closed-form beamforming and SDP-based reflecting matrix design.

Findings

Significant secrecy rate improvement demonstrated in simulations.

Beamforming design is independent of phase shifts under rank-one channels.

Proposed methods are computationally efficient and effective across eavesdropper locations.

Abstract

This letter focuses on the secure transmission for an intelligent reflecting surface (IRS)-assisted millimeter-wave (mmWave) and terahertz (THz) systems, in which a base station (BS) communicates with its destination via an IRS, in the presence of a passive eavesdropper. To maximize the system secrecy rate, the transmit beamforming at the BS and the reflecting matrix at the IRS are jointly optimized with transmit power and discrete phase-shift constraints. It is first proved that the beamforming design is independent of the phase shift design under the rank-one channel assumption. The formulated non-convex problem is then converted into two subproblems, which are solved alternatively. Specifically, the closed-form solution of transmit beamforming at the BS is derived, and the semidefinite programming (SDP)-based method and element-wise block coordinate descent (BCD)-based method are proposed to design the reflecting matrix. The complexity of our proposed methods is analyzed theoretically. Simulation results reveal that the proposed IRS-assisted secure strategy can significantly boost the secrecy rate performance, regardless of eavesdropper's locations (near or blocking the confidential beam).