Citation: Frimpong, E.O.; Oh, B.-H.;
Kim, T.; Bang, I. Physical-Layer
Security with Irregular
Reconfigurable Intelligent Surfaces
for 6G Networks. Sensors 2023, 23,
1881. https://doi.org/10.3390/
s23041881
Academic Editors: Alexandros-
Apostolos Boulogeorgos, Panagiotis
Sarigiannidis, Thomas Lagkas,
Vasileios Argyriou and Pantelis
Angelidis
Received: 7 January 2023
Revised: 2 February 2023
Accepted: 3 February 2023
Published: 7 February 2023
Copyright: © 2023 by the authors.
Licensee MDPI, Basel, Switzerland.
This article is an open access article
distributed under the terms and
conditions of the Creative Commons
Attribution (CC BY) license (https://
creativecommons.org/licenses/by/
4.0/).
Article
Physical-Layer Security with Irregular Reconfigurable
Intelligent Surfaces for 6G Networks
†
Emmanuel Obeng Frimpong
1,‡
, Bong-Hwan Oh
2
, Taehoon Kim
3,
* and Inkyu Bang
1,
*
1
Department of Intelligence Media Engineering, Hanbat National University,
Daejeon 34158, Republic of Korea
2
School of Internet of Things, Xi’an Jiaotong-Liverpool University, No. 111, Taicang Avenue, Taicang,
Suzhou 215488, China
3
Department of Computer Engineering, Hanbat National University, Daejeon 34158, Republic of Korea
* Correspondence: thkim@hanbat.ac.kr (T.K.); ikbang@hanbat.ac.kr (I.B.);
Tel.: +82-42-821-1151 (T.K.); +82-42-821-1209 (I.B.)
†
This paper is an extended version of our paper presented in 2022 KICS (Korean Institute of Communications
and Information Sciences) Summer Conference, Jeju, Republic of Korea, 22–24 June 2022.
‡ Current address: N4-614, Hanbat National University, Daejeon 34158, Republic of Korea.
Abstract:
The goal of 6G is to make far-reaching changes in communication systems with stricter
demands, such as high throughput, extremely low latency, stronger security, and ubiquitous connec-
tivity. Several promising techniques, such as reconfigurable intelligent surfaces (RISs), have been
introduced to achieve these goals. An RIS is a 2D low-cost array of reflecting elements that can
adjust the electromagnetic properties of an incident signal. In this paper, we guarantee secrecy by
using an irregular RIS (IRIS). The main idea of an IRIS is to irregularly activate reflecting elements
for a given number of RIS elements. In this work, we consider a communication scenario in which,
with the aid of an IRIS, a multi-antenna base station establishes a secure link with a legitimate
single-antenna user in the presence of a single-antenna eavesdropper. To this end, we formulate a
topology-and-precoding optimization problem to maximize the secrecy rate. We then propose a Tabu
search-based algorithm to jointly optimize the RIS topology and the precoding design. Finally, we
present simulation results to validate the proposed algorithm, which highlights the performance gain
of the IRIS in improving secure transmissions compared to an RIS. Our results show that exploiting
an IRIS can allow additional spatial diversity to be achieved, resulting in secrecy performance im-
provement and overcoming the limitations of conventional RIS-assisted systems (e.g., a large number
of active elements).
Keywords: physical-layer security; 6G; reconfigurable intelligent surfaces; optimization; secrecy rate
1. Introduction
The goal of 6G is to revolutionize wireless systems with more enhanced requirements,
such as very high data rates, very low latency, always-on broadband global network cover-
age, and intelligence [
1
]. To achieve these goals, several advanced wireless technologies,
such as massive multiple-input and multiple-output (MIMO) transmission, cooperative
communications, and cognitive radio, have been proposed. However, these techniques
focus on processing signals to adapt to the wireless environment and are, in some ways,
affected by the random and negative effects caused by the propagation environment. Re-
cently, a reconfigurable intelligent surface (RIS) was introduced, and it has gained wide
attention due to its ability to control the wireless environment. An RIS is a low-cost uni-
form array of passive reflecting elements where each element can adjust the amplitude
and/or phase of any incident signal to make the propagation medium controllable. This
characteristic can be exploited to constructively (or destructively) add different signals to
enhance (or weaken) the overall signal received [2,3].
Sensors 2023, 23, 1881. https://doi.org/10.3390/s23041881 https://www.mdpi.com/journal/sensors
