Smart Radio Environments Research Articles

Intelligent Reflecting Surface in 6G Vehicular Communications: A Survey

The applications of Intelligent Transportation System (ITS) and autonomous driving in the 6G era heavily rely on the massive information exchange of ultra-wide bandwidth, high reliability, and low latency to guarantee the safety and experience. On the other hand, the signals in high frequency bands including millimeter wave and Terahertz (THz) can be easily blocked by the obstacles. To address this problem, Intelligent Reflecting Surface (IRS) has attracted a lot of attention since it allows for reflecting the signals and changing the propagation directions in a high intelligence and low power consumption way, which creates smart radio environments. Due to its easy deployment and low expense, it has been widely recognized as a promising technology for ground and aerial vehicular networks to enhance signal strength, physical layer security, and positioning accuracy. In this paper, we give a comprehensive survey of current research works on different IRS applications in ground-based vehicular communications and aerial vehicular communications after discussing its basic knowledge and main characteristics. To improve current works and spark more ideas, we summarize the challenges and discuss the future perspectives of IRS-aided 6G vehicular communications.

Reconfigurable Intelligent Edges: Illuminating the Shadow Region in Wireless Networks

This paper proposes a new wireless enabling technology for future smart radio environments. The approach aims to enhance signal coverage within the shadow region(s) of wireless networks with the aid of so-called ’intelligent edges (IEs)’. IEs may be installed at the fringes of shadowing objects such as buildings, walls and other obstacles which obscure the optical signal path from a transmitter to receiver. We investigate two different approaches to illuminating the shadow region in wireless networks using IEs which exploit refraction or diffraction, operating in passive or active mode. The operation of IE-assisted communications are investigated, in particular the ways they can redirect electromagnetic energy towards regions with little or no wireless network coverage. Following from this, a number of variations of IEs are tested in real-world scenarios which consider illuminating the shadow region behind high-rise buildings, first in a city center, and then along a shoreline. Refractive IEs in particular, are shown to provide significant gains compared to the case when no IEs are involved, enhancing signal reception in the shadow region at street level behind a high rise building by as much as 12 dB. Critically, it is shown that intelligent edges offer a low complexity and cost-effective solution for improving connectivity in shadowing-limited environments.

Reconfigurable Intelligent Surfaces: Potentials, Applications, and Challenges for 6G Wireless Networks

Reconfigurable intelligent surfaces (RISs), with the potential to realize smart radio environments, have emerged as an energy-efficient and a cost-effective technology to support the services and demands foreseen for coming decades. By leveraging a large number of low-cost passive reflecting elements, RISs introduce a phase-shift in the impinging signal to create a favorable propagation channel between the transmitter and the receiver. In this article, we provide a tutorial overview of RISs for sixth-generation (6G) wireless networks. Specifically, we present a comprehensive discussion on performance gains that can be achieved by integrating RISs with emerging communication technologies. We address the practical implementation of RIS-assisted networks and expose the crucial challenges, including the RIS reconfiguration, deployment and size optimization, and channel estimation. Furthermore, we explore the integration of RIS and non-orthogonal multiple access (NOMA) under imperfect channel state information (CSI). Our numerical results illustrate the importance of better channel estimation in RIS-assisted networks and indicate the various factors that impact the size of RIS. Finally, we present promising future research directions for realizing RIS-assisted networks in 6G communication.

Performance analysis of RIS-assisted SM with I/Q imbalance

The idea of utilizing reconfigurable intelligent surfaces (RISs) that employ an array of severally-controllable meta-material elements to scatter incoming data in a desirable way, in wireless communication systems has created new opportunities for transmission in various scenarios. For instance, RISs are envisioned as potential candidates to achieve smart radio environments in fifth generation (5G) network technology, which is usually described as incapable of controlling the environment. Nevertheless, existing studies analyzing RIS-empowered wireless networks generally discuss the cases with ideal hardware despite the fact that both physical transceiver and RISs suffer from unignorable hardware imperfections, which may exceedingly reduce the system performance in practical communication scenarios. This paper investigates the performance of an RIS-assisted spatial modulation (SM) scheme in the presence of in-phase/quadrature-phase imbalance (IQI) at the base station (BS) constituting a substantial impairment in direct-conversion transceivers. The concept of the RIS-assisted SM scheme is discussed by comparing three different implementation scenarios depending on the awareness of both the channel phases at the RIS and the severity of the IQI at the destination (D). The average bit error rate (ABER) is derived analytically by use of maximum likelihood (ML) detection method to tackle the distorting impact of the IQI taking the effects of path loss into account. Extensive computer simulation results, which verify theoretical findings, show that the performance of the traditional SM schemes with IQI can remarkably be improved by utilizing RISs, while having the knowledge of the adjusted phases at the receiver (Rx) provides substantial gain in terms of energy.

Macroscopic Modeling of Anomalously Reflecting Metasurfaces: Angular Response and Far-Field Scattering

In view of extremely challenging requirements on the design and optimization of future mobile communication systems, researchers are considering the possibilities of creating intelligent radio environments by using reconfigurable and smart metasurfaces integrated into walls, ceilings, or facades. In this novel communication paradigm, tunable metasurfaces redirect incident waves into the desired directions. To design and characterize such smart radio environments in any realistic scenario, it is necessary to know how these metasurfaces behave when illuminated from other directions and how scattering from finite-sized anomalous reflectors can be estimated. In this work, we analyze the far-field scattering of reflective metasurfaces and study the angular response of anomalous reflectors for arbitrary illumination angles. Using the surface-impedance model, we explain the dependence of the reflection coefficients of phase-gradient metasurfaces on the illumination angle and present numerical examples for typical structures. We also consider scattering from finite-sized metasurfaces and define a route toward including the full-angle response of anomalous reflections into the ray-tracing models of the propagation channel. The developed models apply to other diffraction gratings of finite size.

Reconfigurable Intelligent Surface Enhanced Secure Aerial-Ground Communication

Reconfigurable intelligent surface (RIS), as a revolutionary technique, appears to ameliorate undesirable propagation environment in a controllable manner, which has the potential to substantially boost security of private information in the future smart radio environment. Much recent attention has been directed to the RIS-enhanced secure communication, among which the terrestrial network scenarios are generally concerned, while the exploration of aerial-ground communication integrated with RIS remains an open issue. Furthermore, in the available research, the ideal assumption of line-of-sight channel for aerial-ground link cannot be exploited in complex urban environment. Inspired by this, we provide in this paper the secrecy rate maximization problem under a complex urban scenario, where the drone's high mobility and RIS's tunable capability are utilized to against the potential eavesdropper. Although the established problem is intractable to tackle, we devise an iterative algorithm combining alternating optimization and successive convex approximation method as the solution, which jointly optimizes the phase shifts, the drone's trajectory along with transmit power for the single-user scenario. The proposed designs are also extended to the multi-user multi-eavesdropper system. Eventually, extensive simulation experiments indicate the remarkable benefits brought by our proposed scheme on secrecy performance and the necessity of optimizing phase shifts.

Performance analysis for IRS‐aided communication systems with composite fading/shadowing direct link and discrete phase shifts

AbstractIntelligent reflecting surface (IRS) is an emerging technology and serves as a key component of any smart radio environment. In this article, we consider an IRS that assists communication of a direct link between a single‐antenna transmitter and a receiver. It is assumed that a direct link experiences composite fading/shadowing and is modeled by Generalized‐Kdistribution. Moreover, IRS has line‐of‐sight (LoS) paths, therefore, Rician distribution is used to characterize the fading for these paths. We also consider phase errors that exist due to discrete number of phase shifts. We derive an approximation for the end‐to‐end signal‐to‐noise ratio (SNR) which shows that an amplitude of the direct link is increased by a positive offset because of the presence of IRS. Based on this SNR approximation, a new statistical framework that includes cumulative distribution function (CDF), probability density function (PDF), and moment generating function (MGF) is developed. Leveraging this statistical framework, we derive new and accurate closed‐form approximations for the outage probability, average error probability, ergodic capacity and generalized moments. It is shown that the considered IRS‐aided system (IAS) achieves much better performance even with a few numbers of phase shifts as compared to other baseline systems. Simulations are also provided which verify the tightness of our derived approximations.

Reconfigurable Intelligent Surfaces for Rich Scattering Wireless Communications: Recent Experiments, Challenges, and Opportunities

Recent advances in the fabrication and experimentation of Reconfigurable Intelligent Surfaces (RISs) have motivated the concept of the smart radio environment, according to which the propagation of information-bearing waveforms in the wireless medium is amenable to programmability. Although the vast majority of recent experimental research on RIS-empowered wireless communications gravitates around narrowband beamforming in quasi-free space, RISs are foreseen to revolutionize wideband wireless connectivity in dense urban as well as indoor scenarios, which are usually characterized as strongly reverberant environments exhibiting severe multipath conditions. In this article, capitalizing on recent physics-driven experimental explorations of RIS-empowered wave propagation control in complex scattering cavities, we identify the potential of the spatiotemporal control offered by RISs to boost wireless communications in rich scattering channels via two case studies. First, an RIS is deployed to shape the multipath channel impulse response, which is shown to enable higher achievable communication rates. Second, the RIS-tunable propagation environment is leveraged as an analog multiplexer to localize non-cooperative objects using wave fingerprints, even when they are outside the line of sight. Future research challenges and opportunities in the algorithmic design and experimentation of smart rich scattering wireless environments enabled by RISs for sixth Generation (6G) wireless communications are discussed.

Towards intelligent reflecting surface empowered 6G terahertz communications: A survey

Terahertz (THz) communications have been widely envisioned as a promising enabler to provide adequate bandwidth and achieve ultra-high data rates for sixth generation (6G) wireless networks. In order to mitigate blockage vulnerability caused by serious propagation attenuation and poor diffraction of THz waves, an intelligent reflecting surface (IRS), which manipulates the propagation of incident electromagnetic waves in a programmable manner by adjusting the phase shifts of passive reflecting elements, is proposed to create smart radio environments, improve spectrum efficiency and enhance coverage capability. Firstly, some prospective application scenarios driven by the IRS empowered THz communications are introduced, including wireless mobile communications, secure communications, unmanned aerial vehicle (UAV) scenario, mobile edge computing (MEC) scenario and THz localization scenario. Then, we discuss the enabling technologies employed by the IRS empowered THz system, involving hardware design, channel estimation, capacity optimization, beam control, resource allocation and robustness design. Moreover, the arising challenges and open problems encountered in the future IRS empowered THz communications are also highlighted. Concretely, these emerging problems possibly originate from channel modeling, new material exploration, experimental IRS testbeds and intensive deployment. Ultimately, the combination of THz communications and IRS is capable of accelerating the development of 6G wireless networks.

Reconfigurable Intelligent Surfaces for Localization: Position and Orientation Error Bounds

Next-generation cellular networks could witness the creation of smart radio environments (SREs), where walls and objects will be coated with reconfigurable intelligent surfaces (RISs) to strengthen the communication and localization performance. In fact, RISs have been recently introduced not only to overcome communication blockages due to obstacles but also for high-precision localization of mobile users in GPS denied environments, e.g., indoors. Towards such a vision, this paper presents the localization performance limits for communication scenarios where a single next generation NodeB base station (gNB), equipped with multiple antennas, infers the position and the orientation of a user equipment (UE) in a reconfigurable intelligent surface (RIS)-assisted smart radio environment (SRE). We consider a signal model that is valid also for near-field propagation conditions, as the usually adopted far-field assumption does not always hold, especially for large RISs. For the considered scenario, we derive the Cramer-Rao lower bound (CRLB) for assessing the ultimate localization and orientation performance of synchronous and asynchronous signalling schemes. In addition, we propose a closed-form RIS phase profile that well suits joint communication and localization, and we perform extensive numerical results to assess the performance of our scheme for various localization scenarios and for various RIS phase design. Numerical results show that the proposed scheme can achieve remarkable performance even in asynchronous signalling, and that the proposed phase design, based on signal-to-noise ratio (SNR), approaches the numerical optimal phase design that minimizes the CRLB.

Reconfigurable Intelligent Surfaces Supported Wireless Communications

Reconfigurable Intelligent Surfaces (RISs) have been recently considered in communication environments for focusing signal reflections to create smart radio environments. As it can create favorable propagation conditions by controlling the phase shifts of the reflected signals at the surface to enhance the communication quality. However, the current proofs of concept require complex operations for the RIS configuration, which are mainly realized via wired control connections. When used in wireless networks, RISs may contribute enhance wireless communications. The present paper, compares the RIS technology with the SISO case and evaluates the performance of RIS assisted communication system by giving an approximation of the achievable data rate and the energy efficiency, and investigates the effect of the achievable data rate and distances on the energy efficiency. In this work, based on a work by Emil Björnson [18], we have studied the impact of RIS on the wireless sensor networks by modifying some parameters and giving more interpretations. In particular, we illustrate numerical results that highlight the spectral efficiency gains of RISs when their size is sufficiently large as compared with the wavelength of the radio waves. In addition, we discuss key open issues that need to be addressed for unlocking the potential benefits of RISs for application to wireless communications and networks.

Performance Analysis of Intelligent Reflecting Surface-Assisted Wireless System With Non-Ideal Transceiver

Intelligent Reflecting Surfaces (IRSs) have recently emerged as a promising solution for realizing the smart radio environment concept by leveraging the characteristics of metamaterials and large antenna arrays. However, the existing works on IRS do not account for the transceiver impairments while analyzing the performance of IRS assisted wireless systems. This paper investigates the impact of transceiver hardware imperfections on the performance of IRS-assisted wireless systems. Specifically, we have analyzed the spectral efficiency (SE), the energy efficiency (EE) and the outage probability of IRS-assisted wireless systems by deriving their closed-form expressions and verifying them through simulation results. The results show the importance of modeling and compensating for hardware impairment as they significantly restrict the performance of such systems.

Impact of IQ Imbalance on RIS‐Assisted SISO Communication Systems

Reconfigurable intelligent surface (RIS) for wireless networks has emerged as a promising future transmission technique to create smart radio environments that improve the system performance by turning the wireless channel into an adjustable system block. However, transceivers come with various hardware impairments, such as phase noise and in‐phase/quadrature‐phase imbalance (IQI). Hence, for robust configuration of RIS‐based communication under practical conditions, assuming the identical performance analysis when subject to IQI, will lead to inaccurate analysis. In this paper, the implementation of this novel transmission technique is thoroughly investigated under intensive realistic circumstances. For this purpose, based on the maximum likelihood (ML) detector, a novel analytical expression of average pairwise error probability under IQI is proposed and compared to the standard ML detector. Further, the proposed analytical approaches are confirmed by numerical simulations.

Smart Radio Environments Empowered by Reconfigurable Intelligent Surfaces: How It Works, State of Research, and The Road Ahead

Reconfigurable intelligent surfaces (RISs) are an emerging transmission technology for application to wireless communications. RISs can be realized in different ways, which include (i) large arrays of inexpensive antennas that are usually spaced half of the wavelength apart; and (ii) metamaterial-based planar or conformal large surfaces whose scattering elements have sizes and inter-distances much smaller than the wavelength. Compared with other transmission technologies, e.g., phased arrays, multi-antenna transmitters, and relays, RISs require the largest number of scattering elements, but each of them needs to be backed by the fewest and least costly components. Also, no power amplifiers are usually needed. For these reasons, RISs constitute a promising software-defined architecture that can be realized at reduced cost, size, weight, and power (C-SWaP design), and are regarded as an enabling technology for realizing the emerging concept of smart radio environments (SREs). In this paper, we (i) introduce the emerging research field of RIS-empowered SREs; (ii) overview the most suitable applications of RISs in wireless networks; (iii) present an electromagnetic-based communication-theoretic framework for analyzing and optimizing metamaterial-based RISs; (iv) provide a comprehensive overview of the current state of research; and (v) discuss the most important research issues to tackle. Owing to the interdisciplinary essence of RIS-empowered SREs, finally, we put forth the need of reconciling and reuniting C. E. Shannon's mathematical theory of communication with G. Green's and J. C. Maxwell's mathematical theories of electromagnetism for appropriately modeling, analyzing, optimizing, and deploying future wireless networks empowered by RISs.

A Vision to Smart Radio Environment: Surface Wave Communication Superhighways

Complementary to traditional approaches that focus on transceiver design for bringing the best out of unstable, lossy fading channels, one radical development in wireless communications that has recently emerged is to pursue a smart radio environment by using software-defined materials or programmable metasurfaces for establishing favorable propagation conditions. This article portraits a vision of communication superhighways enabled by surface wave (SW) propagation on intelligent surfaces for future smart radio environments. The concept differs from the mainstream efforts of using passive elements on a large surface for bouncing off radio waves intelligently toward intended user terminals. In the proposed SW vision, energy efficiency can be much higher, due to more desirable propagation characteristics compared to open-space radio propagation. The fact that SW is inherently confined to the smart surface not only greatly simplifies the task of interference management, but also makes possible exceptionally localized high-speed interference-less data access. We shall outline the opportunities and challenges arising from the SW paradigm. We shall also attempt to shed light on several key enabling technologies that make this realizable. One important technology which will be discussed is a software-controlled fluidic waveguiding architecture that permits dynamic creation of high-throughput data highways.

Reconfigurable Intelligent Surfaces for Wireless Communications: Principles, Challenges, and Opportunities

Recently there has been a flurry of research on the use of reconfigurable intelligent surfaces (RIS) in wireless networks to create smart radio environments. In a smart radio environment, surfaces are capable of manipulating the propagation of incident electromagnetic waves in a programmable manner to actively alter the channel realization, which turns the wireless channel into a controllable system block that can be optimized to improve overall system performance. In this article, we provide a tutorial overview of reconfigurable intelligent surfaces (RIS) for wireless communications. We describe the working principles of reconfigurable intelligent surfaces (RIS) and elaborate on different candidate implementations using metasurfaces and reflectarrays. We discuss the channel models suitable for both implementations and examine the feasibility of obtaining accurate channel estimates. Furthermore, we discuss the aspects that differentiate RIS optimization from precoding for traditional MIMO arrays highlighting both the arising challenges and the potential opportunities associated with this emerging technology. Finally, we present numerical results to illustrate the power of an RIS in shaping the key properties of a MIMO channel.

Sparse array of sub-surface aided blockage-free multi-user mmWave communication systems

Recently, Reconfigurable Intelligent Surfaces (RISs) have drawn intensive attention in the realization of the smart radio environment. However, existing works mainly consider the RIS as a whole uniform plane, which may be unrealistic to be installed on the facade of buildings when the RIS is extremely large. In contrast, this paper investigates a practical Sparse Array of Sub-surface (SAoS) deployment of the RIS for uplink multi-user millimeter Wave (mmWave) communication systems, in which the Mobile Stations (MSs) are distributed in the blind coverage area due to the blockage. In order to exploit the benefits of the sparse deployment, the correlation of the effective channel is firstly investigated. Then the approximation and lower bounds of the ergodic spectral efficiency are derived under frequency and spatial multiplexing scenarios, respectively. Based on the autocorrelation of the effective channel, we obtain an optimal reflect coefficient design as well as the deployment guidelines of RIS tiles. Moreover, the RIS tile scheduling algorithms are also proposed. Numerical results show that the ergodic spectral efficiency approximation matches well with the Monte Carlo result under frequency multiplexing scenarios, and the lower bound is tight under spatial multiplexing scenarios only when the effective channel is strongly correlated. On the basis of the RIS tile scheduling algorithm and the reflect coefficient design, the system performance can be significantly improved under frequency multiplexing scenarios. On the other hand, by deploying more sparse RIS tiles, we can increase the multiplexing gain under spatial multiplexing scenarios.

Reconfigurable Intelligent Surface Assisted Multiuser MISO Systems Exploiting Deep Reinforcement Learning

Recently, the reconfigurable intelligent surface (RIS), benefited from the breakthrough on the fabrication of programmable meta-material, has been speculated as one of the key enabling technologies for the future six generation (6G) wireless communication systems scaled up beyond massive multiple input multiple output (Massive-MIMO) technology to achieve smart radio environments. Employed as reflecting arrays, RIS is able to assist MIMO transmissions without the need of radio frequency chains resulting in considerable reduction in power consumption. In this paper, we investigate the joint design of transmit beamforming matrix at the base station and the phase shift matrix at the RIS, by leveraging recent advances in deep reinforcement learning (DRL). We first develop a DRL based algorithm, in which the joint design is obtained through trial-and-error interactions with the environment by observing predefined rewards, in the context of continuous state and action. Unlike the most reported works utilizing the alternating optimization techniques to alternatively obtain the transmit beamforming and phase shifts, the proposed DRL based algorithm obtains the joint design simultaneously as the output of the DRL neural network. Simulation results show that the proposed algorithm is not only able to learn from the environment and gradually improve its behavior, but also obtains the comparable performance compared with two state-of-the-art benchmarks. It is also observed that, appropriate neural network parameter settings will improve significantly the performance and convergence rate of the proposed algorithm.

Reconfigurable Intelligent Surfaces vs. Relaying: Differences, Similarities, and Performance Comparison

Reconfigurable intelligent surfaces (RISs) have the potential of realizing the emerging concept of smart radio environments by leveraging the unique properties of metamaterials and large arrays of inexpensive antennas. In this article, we discuss the potential applications of RISs in wireless networks that operate at high-frequency bands, e.g., millimeter wave (30-100 GHz) and sub-millimeter wave (greater than 100 GHz) frequencies. When used in wireless networks, RISs may operate in a manner similar to relays. The present paper, therefore, elaborates on the key differences and similarities between RISs that are configured to operate as anomalous reflectors and relays. In particular, we illustrate numerical results that highlight the spectral efficiency gains of RISs when their size is sufficiently large as compared with the wavelength of the radio waves. In addition, we discuss key open issues that need to be addressed for unlocking the potential benefits of RISs for application to wireless communications and networks.

Toward Smart Wireless Communications via Intelligent Reflecting Surfaces: A Contemporary Survey

This paper presents a literature review on recent applications and design aspects of the intelligent reflecting surface (IRS) in the future wireless networks. Conventionally, the network optimization has been limited to transmission control at two endpoints, i.e., end users and network controller. The fading wireless channel is uncontrollable and becomes one of the main limiting factors for performance improvement. The IRS is composed of a large array of scattering elements, which can be individually configured to generate additional phase shifts to the signal reflections. Hence, it can actively control the signal propagation properties in favor of signal reception, and thus realize the notion of a smart radio environment. As such, the IRS's phase control, combined with the conventional transmission control, can potentially bring performance gain compared to wireless networks without IRS. In this survey, we first introduce basic concepts of the IRS and the realizations of its reconfigurability. Then, we focus on applications of the IRS in wireless communications. We overview different performance metrics and analytical approaches to characterize the performance improvement of IRS-assisted wireless networks. To exploit the performance gain, we discuss the joint optimization of the IRS's phase control and the transceivers' transmission control in different network design problems, e.g.,~rate maximization and power minimization problems. Furthermore, we extend the discussion of IRS-assisted wireless networks to some emerging use cases. Finally, we highlight important practical challenges and future research directions for realizing IRS-assisted wireless networks in beyond 5G communications.