Hybrid Relay-Reflecting Intelligent Surface-Aided Covert Communications

TL;DR

This paper introduces a hybrid relay-reflecting intelligent surface (HR-RIS) to improve covert communication by jointly optimizing transmit power and HR-RIS coefficients, demonstrating superior covert rates over traditional RIS with minimal active relays.

Contribution

The paper presents a novel HR-RIS design and formulates an optimization framework for covert communications, including a closed-form upper bound and an efficient solution algorithm.

Findings

HR-RIS outperforms traditional RIS in covert rate.

A small number of active relays (around 5) suffices for near-optimal performance.

Increasing active relays beyond a certain point yields no significant gains.

Abstract

In this paper, we propose to use a hybrid relay-reflecting intelligent surface (HR-RIS) to enhance the performance of a covert communication system. Specifically, the HR-RIS consists of passive reflecting elements and active relay elements to manipulate the wireless signals from a transmitter to a desired receiver while ensuring the covertness of the transmission via avoiding such signals being detected by a warden. To fully explore the benefits offered by the HR-RIS, we first formulate the joint design of the transmit power and relay/reflection coefficients of the HR-RIS as an optimization problem to maximize the covert rate subject to a covertness constraint. To tackle the solution to this optimization problem, we then derive a closed-form expression for an upper bound on covert rate, based on which we develop an alternate algorithm to solve the formulated optimization problem. Our examination shows that the HR-RIS outperforms the traditional RIS in term of achieving a higher covert rate. Interestingly, we also observe the major part of the performance gain brought by the HR-RIS can be obtained by a small number of active relay elements (e.g., 5) and further increasing this number does not improve the covert communication performance.