Secrecy Capacity Maximization for a Hybrid Relay-RIS Scheme in mmWave MIMO Networks

TL;DR

This paper investigates how a hybrid relay-RIS can enhance the secrecy capacity of mmWave MIMO systems, proposing joint optimization techniques and demonstrating significant improvements over traditional passive surfaces.

Contribution

It introduces a novel joint optimization framework for HR-RIS in mmWave MIMO systems, including a closed-form beamformer and a metaheuristic for RIS coefficients.

Findings

HR-RIS significantly improves secrecy capacity

Proposed optimization outperforms conventional passive surfaces

Simulation confirms effectiveness across scenarios

Abstract

The hybrid relay-reflecting intelligent surface (HR-RIS) has been recently introduced as an efficient solution to overcome the double path loss and limited beamforming diversity of the conventional fully passive reflecting surface. This motivates us to investigate the application of the HR-RIS in improving the secrecy capacity of millimeter wave multiple-input-multiple-output (MIMO) systems with the presence of multi-antenna eavesdropper. The joint optimization of the transmit beamformer and the relay-reflecting coefficients at RIS is tackled via alternating optimization. In the proposed solution, a closed-form expression for the optimal transmit beamformer at the transmitter is derived, and a metaheuristic solution based on particle swarm optimization is proposed to optimize the active and passive elements at the HR-RIS. The simulation results verify that under various scenarios, the HR-RIS provides significant improvement in the secrecy capacity with respect to the conventional passive reflecting surface.