Best Readings in Reconfigurable Intelligent Surfaces

Future wireless networks will be as pervasive as the air we breathe, not only connecting us but embracing us through a web of systems that support personal and societal wellbeing. That is, the ubiquity, speed and low latency of such networks will allow currently disparate devices and services to become a distributed intelligent communications, sensing, and computing platform. Small cells, massive MIMO, millimeter-wave communications are three fundamental technologies that will spearhead the emergence of 5G wireless networks – Their advantages are undeniable. The question is, however, whether these technologies will be sufﬁcient to meet the requirements of future wireless networks that integrate communications, sensing, and computing in a single platform. Wireless networks, in addition, are rapidly evolving towards a software-deﬁned design paradigm, where every part of the network can be conﬁgured and controlled via software. In this optimization process, however, the wireless environment– the medium or channel – is generally assumed uncontrollable and often an impediment to be reckoned with. For example, signal attenuation limits the network connectivity, multi-path propagation results in fading phenomena, reﬂections and refractions from objects are a source of uncontrollable interference.

Recently, a new concept called Reconﬁgurable Intelligent Surfaces (RISs) has emerged wherein every environmental object is coated with man-made intelligent surfaces of conﬁgurable electromagnetic materials. These materials contain integrated electronic circuits and software that enable the control of the wireless medium. 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), which could potentially enable telecommunication operators to sculpt the communication medium that comprises the network. As such, RISs have the potential to change how wireless networks are designed, usher in that hoped-for wireless future, and are regarded as an enabling technology for realizing the emerging concept of smart radio environments (SREs). But, RISs are not currently well-understood.

With the above vision, this Best Readings of the IEEE Communications Society is aimed at gathering the latest and most promising research advances on the modeling, analysis, design, and implementation of RIS-empowered wireless networks. More precisely, this collection of Best Readings titled “Reconfigurable Intelligent Surfaces” offers a selection of highly selected papers that are conveniently organized in main thematic areas of research, as well at reports the list of past and upcoming special issues related to the topic.

Issued September 2020

Contributors

Marco Di Renzo, CNRS & Paris-Saclay University, France|Yuanwei Liu, Queen Mary University of London, UK
Chau Yuen, Singapore University of Technology and Design, Singapore
Alexios Aravanis, CentraleSupelec, France
Alessio Zappone, University of Cassino and Southern Lazio, Italy
Linglong Dai, Tsinghua University, China
Qingqing Wu, University of Macau, China

Vincenzo Sciancalepore, NEC Europe Labs, Germany
Ertugrul Basar, Koc University, Turkey
Merouane Debbah, Huawei France R&D, France
Mohamed-Slim Alouini, King Abdullah University of Science and Technology, Saudi Arabia
Naofal Aldhahir, The University of Texas at Dallas, USA
Shi Jin, Southeast University, China

Editorial Staff

Matthew C. Valenti
Editor-in-Chief, ComSoc Best Readings
West Virginia University
Morgantown, WV, USA

Xianbin Wang
Associate Editor-in-Chief, ComSoc Best Readings
Western University
London, ON, Canada

Special Interest Groups and Emerging Technology Initiative

IEEE Wireless Communications Technical Committee, Special Interest Group on “Reconfigurable Intelligent Surfaces for Smart Radio Environments (RISE)”.

Signal Processing and Computing for Communications Technical Committee, Special Interest Groups on “REconFigurabLE Intelligent Surfaces for Signal Processing and CommunicatIONS (REFLECTIONS)”.

IEEE Communications Society Emerging Technology Initiative (ETI), “Reconfigurable Intelligent Surfaces”.

Overviews and Tutorials

M. Di Renzo, A. Zappone, M. Debbah, M.-S. Alouini, C. Yuen, J. de Rosny, and S. Tretyakov, “Smart Radio Environments Empowered by Reconfigurable Intelligent Surfaces: How it Works, State of Research, and Road Ahead”, IEEE Journal on Selected Areas in Communications.
This paper provides a comprehensive and critical overview of RIS, the key technology enabler to realize the smart radio environment. It introduces the concept of RIS, the main operating principles and envisioned potential applications, the current state of research, and the major open research challenges to realize smart radio environments.

M. Di Renzo et al., “Smart Radio Environments Empowered by AI Reconfigurable Meta-Surfaces: An Idea Whose Time Has Come,” EURASIP Journal on Wireless Communications and Networking, article 129, May 2019.
This article reviews the recent research efforts on smart radio environments and the corresponding key technologies, wherein two types of intelligent reconfigurable metasurfaces are discussed. It further highlights the need of new communication-theoretic models for analysis and design, and reveals the long-term and open research issues in the field.

E. Basar, M. Di Renzo, J. de Rosny, M. Debbah, M.-S. Alouini, and R. Zhang, “Wireless Communications through Reconfigurable Intelligent Surfaces,” IEEE Access, vol. 7, pp. 116 753–116 773, August 2019.
This article summarizes the current research activities on RIS-empowered wireless networks and illustrates the main differences between RISs and other technologies. Moreover, the error performance of RIS-assisted systems with M-ary phase shift keying/quadrature amplitude modulation (PSK/QAM) is studied. The potential use cases and fundamental research issues of RISs are also discussed.

C. Huang, S. Hu, G. C. Alexandropoulos, A. Zappone, C. Yuen, R. Zhang, M. Di Renzo, and M. Debbah, “Holographic MIMO Surfaces for 6G Wireless Networks: Opportunities, Challenges, and Trends,” IEEE Wireless Communications.
This paper provides an overview of holographic multiple input multiple output surfaces. It first presents the available hardware architectures, functionalities and characteristics, as well as the applications. The design challenges and research opportunities are highlighted.

Y. Liu, X. Liu, X. Mu, T. Hou, J. Xu, Z. Qin, M. Di Renzo, and N. Al-Dhahir, “Reconﬁgurable Intelligent Surfaces: Principles and Opportunities”, arXiv, July 2020. [Online].
This paper provides a comprehensive overview of the state-of-the-art on RISs. It includes the operating principles of RISs, performance evaluation, beamforming design and resource management, applications of machine learning to RIS-enhanced wireless networks, and the integration of RISs with other emerging technologies in next-generation networks.

C. Liaskos, S. Nie, A. Tsioliaridou, A. Pitsillides, S. Ioannidis, and I. Akyildiz, “A New Wireless Communication Paradigm through Software-Controlled Metasurfaces,” IEEE Communications Magazine, vol. 56, no. 9, pp. 162–169, September 2018.
This article introduces the concept of programmable wireless environments, and proposes the HyperSurface tile, which enables deterministic, programmable control over the behavior of wireless environments. Moreover, the performance gain is evaluated via simulations.

C. Liaskos, A. Tsioliaridou, A. Pitsillides, S. Ioannidis, and I. F. Akyildiz, “Using any Surface to Realize a New Paradigm for Wireless Communications,” Communications of the ACM., vol. 61, no. 11, pp. 30–33, November 2018.
This paper proposes the idea of HyperSurface to produce programmable wireless environments, thus achieving substantial gains in communication quality, coverage, and power saving.

Y.-C. Liang, R. Long, Q. Zhang, J. Chen, H. V. Cheng, and H. Guo, “Large Intelligent Surface/Antennas (LISA): Making Reflective Radios Smart,” Journal of Communications and Information Networks, vol. 4, no. 2, pp. 40-50, June 2019.
This paper introduces the reflective radio basics as well as the fundamentals and implementations of LISA technology, and then provides an overview of the state-of-the-art research on the applications of LISA. It also discusses the limitations, challenges, and open issues associated with LISA.

Q. Wu and R. Zhang, “Towards Smart and Reconfigurable Environment: Intelligent Reflecting Surface Aided Wireless Network,” IEEE Communications, vol. 58, no. 1, pp. 106–112, January 2020.
This article introduces the IRS technology in terms of the main applications, advantages over other existing technologies, hardware architecture as well as the signal model. In addition, it presents the main design challenges and provides some numerical results to demonstrate the performance enhancement by deploying the IRS.

X. Yuan, Y.-J. Zhang, Y. Shi, W. Yan, and H. Liu, “Reconfigurable-Intelligent-Surface Empowered 6G Wireless Communications: Challenges and Opportunities,” arXiv, January 2020. [Online].
This paper focuses on three main challenges for implementing RISs in wireless networks, including channel state information acquisition, passive information transfer, and resource allocation. It summarizes the state-of-the-art solutions and discusses potential research directions.

S. Gong, X. Lu, D. T. Hoang, D. Niyato, L. Shu, D. I. Kim, and Y.-C. Liang, “Towards Smart Radio Environment for Wireless Communications via Intelligent Reflecting Surfaces: A Contemporary Survey,” IEEE Communications Surveys & Tutorials.
This survey reviews recent applications and design aspects of intelligent reflecting surfaces (IRSs) in the future wireless networks. It first introduces the basic theory and implementation of IRSs, and then presents performance analysis and optimization. Some practical challenges and future research directions are also discussed.

Q. Wu, S. Zhang, B. Zheng, C. You, and R. Zhang, “Intelligent Reflecting Surface Aided Wireless Communications: A Tutorial,” Invited paper, arXiv, Jun. 2020. [Online].
This tutorial paper provides in-depth technical discussion to facilitate and inspire future research in modeling, analysis, design, optimization, and implementation of IRS-aided wireless networks. In particular, it offers a systematic treatment on how to address three key design issues, namely IRS passive reflection optimization, IRS channel estimation, and IRS deployment from various communication perspectives.

Early Research Works (2003-2014)

E. F. Kuester, M. A. Mohamed, M. Piket-May, and C. L. Holloway, “Averaged Transition Conditions for Electromagnetic Fields at a Metaﬁlm,” IEEE Transactions on Antennas and Propagation, vol. 51, no. 10, pp. 2641-2651, October 2003.

C. L. Holloway, M. A. Mohamed, E. F. Kuester, and A. Dienstfrey, “Reﬂection and Transmission Properties of a Metaﬁlm: With an Application to a Controllable Surface Composed of Resonant Particles,” IEEE Transactions on Electromagnetic Compatibility, vol. 47, no. 4, pp. 853-865, November 2005.

In these two papers, the authors write:

“The GSTCs are expected to have wide application to the design and analysis of antennas, reﬂectors, and other devices where controllable scatterers are used to form a smart surface” - “This could in principle enable us to realize controllable surfaces, able for instance to switch electronically between reﬂecting and transmitting states”
“We see that if a scatterer can be chosen such that the electric and/or magnetic polarizability is changeable on demand, then a smart or controllable surface would be realized”

L. Subrt, D. Grace, and P. Pechac, “Controlling the Short-range Propagation Environment Using Active Frequency Selective Surfaces,” Radioengineering, vol. 19, no. 4, pp. 610–615, Dec. 2010.

L. Subrt and P. Pechac, “Controlling Propagation Environments Using Intelligent Walls,” 6th European Conference on Antennas and Propagation (EUCAP), Prague, March 2012, pp. 1-5.

L. Subrt and P. Pechac, “Intelligent Walls as Autonomous Parts of Smart Indoor Environments,” IET Communications, vol. 6, no. 8, pp. 1004–1010, May. 2012.

In these three papers, the authors write: “This paper deals with a new approach to the control of the propagation environment in indoor scenarios using intelligent walls. The intelligent wall is a conventional wall situated inside a building, but equipped with an active frequency selective surface and sensors. The intelligent wall can be designed as a self-conﬁguring and self-optimizing autonomous part of a collaborative infrastructure working within a high-capacity mobile radio system. The paper shows how such surfaces can be used to adjust the electromagnetic characteristics of the wall in response to changes in trafﬁc demand, monitored using a network of sensors, thereby controlling the propagation environment inside the building”.

N. Kaina, M. Dupre, G. Lerosey, and M. Fink, “Shaping Complex Microwave Fields in Reverberating Media with Binary Tunable Metasurfaces,” Scientific Reports, vol. 4, no. 6693, pp. 1–8, October 2014.

In this paper, the authors write: “We propose to use electronically tunable metasurfaces as spatial microwave modulators. We demonstrate that like spatial light modulators, which have been recently proved to be ideal tools for controlling light propagation through multiple scattering media, spatial microwave modulators can efﬁciently shape in a passive way complex existing microwave ﬁelds in reverberating environments with a non-coherent energy feedback. We expect that spatial microwave modulators will be interesting tools for fundamental physics and will have applications in the ﬁeld of wireless communications”.

Special Issues

Wireless Networks Empowered by Reconfigurable Intelligent Surfaces, IEEE Journal on Selected Areas in Communications, Third Quarter 2020.

Reconfigurable Intelligent Surface-Based Communications for 6G Wireless Networks, IEEE Open Journal of the Communications Society, First Quarter 2021.

Intelligent Surfaces for Smart Wireless Communications, IEEE Transactions on Cognitive Communications and Networking, Second Quarter 2021.

Reconfigurable Intelligent Surfaces: Design and Implementation, IEEE Communications, June 2021.

Intelligent Surfaces for 5G and Beyond, IEEE Wireless Communications, December 2021.

Wireless Communications with Reconfigurable Intelligent Surfaces, China Communications, Feature Topic, Vol.18, No.2, 2021.

Multimedia Content

M. Di Renzo, “Smart Radio Environments Empowered by Reconﬁgurable Intelligent Surfaces: How it Works, State of Research, and the Road ahead”, VTC & ICASSP 2020.

M. Di Renzo, “Smart radio Environments Empowered by Reconﬁgurable Intelligent Surfaces: How it works, State of Research, and the Road ahead”, WTC July 2020.

M. Di Renzo, “Smart Radio Environments Empowered by Reconﬁgurable Intelligent Surfaces: How it Works, State of Research, and the Road ahead”, ISEP & PIMRC July 2020.

Topic: Channel Modeling and Measurements

A major research issue for analyzing the ultimate performance limits, optimizing the operation, and assessing the advantages and limitations of RIS-empowered SREs is the development of simple but sufficiently accurate models for the power received at a given location in space when a transmitter emits radio waves that illuminate an RIS. This is an open and challenging research issue to tackle. Recently, however, a few initial research works have attempted to shed light on modeling the path-loss of RISs.

W. Tang, M. Z. Chen, X. Chen, J. Y. Dai, Y. Han, M. Di Renzo, Y. Zeng, S. Jin, Q. Cheng, and T. J. Cui, “Wireless Communications with Reconﬁgurable Intelligent Surface: Path-loss Modeling and Experimental Measurement,” arXiv, November 2019. [Online].
This paper develops free-space path loss models of RIS-assisted wireless communications in different scenarios, while taking the physics and electromagnetic nature of the RISs into account. Experimental measurements are further carried out to validate the proposed path loss models. The paper shows that the measurement results match quite well with the modeling results.

M. Di Renzo, F. H. Danufane, X. Xi, J. de Rosny, and S. A. Tretyakov, “Analytical Modeling of the Path-loss for Reconﬁgurable Intelligent Surfaces - Anomalous Mirror or Scatterer ?” IEEE 21st International Workshop on Signal Processing Advances in Wireless Communications (SPAWC), Atlanta, GA, USA, 2020, pp. 1-5.
This paper proposes closed-form expressions for computing the power received from the RIS by leveraging the general scalar theory of diffraction and the Huygens-Fresnel principle. The conditions under which the RISs can act as an anomalous mirror are identified.

F. H. Danufane, M. Di Renzo, X. Xi, J. de Rosny, and S. A. Tretyakov, “On the Path-Loss of Reconfigurable Intelligent Surfaces: An Approach Based on Green's Theorem Applied to Vector Fields” arXiv, July 2020. [Online].
This paper introduces a new and general channel model for RISs, by using the Green theorem, which can be applied to general metasurfaces either for transmission or reflection. Asymptotic analysis shows that the path-loss has a different scaling law as a function of the transmission distance and the size of the surface in the near and far field regimes.

J. B. Garcia, A. Sibille, and M. Kamoun, “Reconﬁgurable Intelligent Surfaces: Bridging the Gap between Scattering and Reﬂection,” IEEE Journal on Selected Areas in Communications. [Online].
This paper unifies the opposite behavior of RIS as a scatterer and as a mirror, which helps to identify different application scenarios. In addition, it introduces a model for the signal-level characterization of a transmitter-receiver link in the presence of RIS.

W. Khawaja, O. Ozdemir, Y. Yapici, F. Erden, M. Ezuma, and I. Guvenc, “Coverage Enhancement for NLOS mmwave Links Using Passive Reﬂectors,” IEEE Open Journal of the Communications Society, vol. 1, pp. 263-281, 2020.
To overcome the high path loss at mmWave frequencies, this paper uses passive metallic reflectors to improve 28 GHz mmWave signal coverage for both indoor and outdoor NLOS scenarios. The achieved gains are quantified via experimental measurements, analytical expressions, and ray tracing simulations.

E. Basar, I. Yildirim, and I. F. Akyildiz, “Indoor and Outdoor Physical Channel Modeling and Efficient Positioning for Reconfigurable Intelligent Surfaces in mmWave Bands“, arXiv, May 2020. [Online].
This paper introduces SimRIS Channel Simulator, which is an open-source and MATLAB-based simulator. This simulator can be used in channel modeling of RIS-assisted systems with tunable operating frequency, terminal locations, number of RIS elements, and environments.

G. Gradoni and M. Di Renzo, “End-to-End Mutual-Coupling-Aware Communication Model for Reconfigurable Intelligent Surfaces: An Electromagnetic-Compliant Approach Based on Mutual Impedances”, arXiv, September 2020. [Online].
This paper introduces a physics and electromagnetic (EM) compliant communication model for RIS-aided wireless systems. The proposed model has four main characteristics: (i) it is end-to-end, i.e., it is formulated in terms of an equivalent channel that yields a one-to-one mapping between the voltages fed into the ports of a transmitter and the voltages measured at the ports of a receiver; (ii) it is EM-compliant, i.e., it accounts for the generation and propagation of the EM fields; (iii) it is mutual-coupling-aware, i.e., it accounts for the mutual coupling among the sub-wavelength unit cells of the RIS; and (iv) it is unit-cell-aware, i.e., it accounts for the intertwinement between the amplitude and phase response of the unit cells of the RIS.

Topic: Surface-Based Modulation and Encoding

One of the emerging and promising applications of RISs is to employ them for modulating and encoding data into their individual reconfigurable elements. This application of RISs can be viewed as an instance of spatial modulation and index modulation.

A. Li, M. Wen, and M. Di Renzo, “Single-RF MIMO: From Spatial Modulation to Metasurface-Based Modulation,” arXiv, September 2020. [Online].
In multiple-input multiple-output (MIMO), multiple radio frequency (RF) chains are usually required to simultaneously transmit multiple data streams. As a special MIMO technology, spatial modulation (SM) activates one transmit antenna with one RF chain and exploits the index of the active antenna for information transfer at each time slot. Recently, reconfigurable metasurfaces have emerged as a promising technology that is able to reconfigure the wireless propagation environment by altering the amplitude and/or phase of the incident signal. In this article, the authors discuss implementations of single-RF MIMO based on metasurface-based modulation.

E. Basar, M. Di Renzo, J. de Rosny, M. Debbah, M.-S. Alouini, and R. Zhang, “Wireless Communications through Reconﬁgurable Intelligent Surfaces,” IEEE Access, vol. 7, pp. 116753-116773, 2019.
This article summarizes the current research activities on RIS-empowered wireless networks and illustrates the main differences between RISs and other technologies. Moreover, the error performance of RIS-assisted systems with M-ary phase shift keying/quadrature amplitude modulation (PSK/QAM) is studied. The potential use cases and fundamental research issues of RISs are also discussed.

R. Karasik, O. Simeone, M. Di Renzo, and S. Shamai, “Beyond Max-SNR: Joint Encoding for Reconﬁgurable Intelligent Surfaces,” IEEE International Symposium on Information Theory (ISIT), Los Angeles, CA, USA, 2020, pp. 2965-2970.
This paper investigates the capacity of a RIS-aided channel from an information-theoretic perspective. It proves that the capacity-achieving scheme is to jointly encode information in both the RIS configuration and the transmitted signal instead of maximizing the received SNR. In addition, a suboptimal but practical transmission strategy is proposed based on layered encoding and successive cancellation decoding techniques.

E. Basar, "Reconfigurable Intelligent Surface-Based Index Modulation: A New Beyond MIMO Paradigm for 6G," IEEE Transactions on Communications, vol. 68, no. 5, pp. 3187-3196, May 2020.
This paper introduces the concept of RIS-based index modulation (IM). Two schemes, namely, RIS-space shift keying (RIS-SSK) and RIS-spatial modulation (RIS-SM) are proposed. Based on this, greedy and maximum likelihood (ML) detectors of both schemes are designed and the corresponding theoretical error performance is analyzed.

S. Guo, S. Lv, H. Zhang, J. Ye and P. Zhang, "Reflecting Modulation," IEEE Journal on Selected Areas in Communications.
This paper proposes a reflecting modulation (RM) scheme for RIS-based communications, where both the reflecting patterns and transmit signals can carry information. RM is further divided into two categories: jointly mapped RM (JRM) and separately mapped RM (SRM). To improve the performance, the paper describes designs for discrete and continuous optimization-based joint signal mapping, shaping, and reflecting approaches. The proposed designs are validate in various systems.

Topic: Channel Estimation

Channel estimation is a fundamental issue to be tackled in SREs wherein nearly-passive RISs are employed. In contrast to other communication systems, e.g., relays, which are equipped with sufficiently powerful signal processing capabilities, nearly-passive RISs are equipped with minimal on-board signal processing units that are not intended to be used during their normal operation phase. Therefore, new algorithms and protocols are necessary in order to perform channel estimation, while keeping the complexity of RISs as low as possible and avoiding on-board signal processing operations as much as possible.

T. L. Jensen and E. De Carvalho, "An Optimal Channel Estimation Scheme for Intelligent Reflecting Surfaces Based on a Minimum Variance Unbiased Estimator," IEEE International Conference on Acoustics, Speech and Signal Processing, pp. 5000-5004, Barcelona, Spain, 2020.
This paper proposes an optimal channel estimation scheme for IRS-aided communication, where the IRS activation patterns follow the rows of a discrete Fourier transform (DFT). It shows that the proposed scheme achieves one order lower estimation variance compared to existing on/off IRS activation patterns methods.

D. Mishra and H. Johansson, “Channel Estimation and Low-complexity Beamforming Design for Passive Intelligent Surface Assisted MISO Wireless Energy Transfer,” IEEE International Conference on Acoustics, Speech and Signal Processing, pp. 4659–4663, Brighton, United Kingdom, May. 2019.
This paper presents a novel channel estimation protocol for passive intelligent surface (PIS)-assisted energy transfer systems, which requires no active participation from the PIS. Moreover, near-optimal energy beamforming at both the power beacon and PIS are derived in closed-form expressions.

C. You, B. Zheng and R. Zhang, "Channel Estimation and Passive Beamforming for Intelligent Reflecting Surface: Discrete Phase Shift and Progressive Refinement," IEEE Journal on Selected Areas in Communications, early access, 2020.
This paper investigates the channel estimation and passive beamforming design under the constraint of discrete phase shifts. It first proposes a novel hierarchical training reflection design, which progressively estimate the channels by grouping and partitioning the reflection elements. Based on this, the progressive passive beamforming is further designed with the aim of improving the achievable rate for data transmission.

Z.-Q. He and X. Yuan, “Cascaded Channel Estimation for Large Intelligent Metasurface Assisted Massive MIMO,” IEEE Wireless Communications Letters, vol. 9, no. 2, pp. 210–214, February 2020.
This paper investigates the cascaded channel estimation problem for large intelligent metasurface (LIM) assisted massive MIMO systems. A two-stage algorithm is proposed, including a sparse matrix factorization stage and a matrix completion stage. Simulation results demonstrate the accuracy of the proposed design.

S. Liu, Z. Gao, J. Zhang, M. Di Renzo, and M.-S. Alouini, “Deep Denoising Neural Network Assisted Compressive Channel Estimation for mmWave Intelligent Reflecting Surfaces”, IEEE Transactions on Vehicular Technology, vol. 69, no. 8, pp. 9223-9228, August 2020.
To reduce the training overhead for mmWave intelligent reflecting surfaces (IRS) systems, this paper proposes a deep denoising neural network assisted compressive channel estimation method. Based on a hybrid passive/active IRS architecture, the complete channel matrix is reconstructed with only a few measurements. It further proposes a complex-valued denoising convolution neural network for improving performance.

Q. U. Nadeem, A. Kammoun, A. Chaaban, M. Debbah, and M. -S. Alouini, "Intelligent Reflecting Surface Assisted Multi-user MISO Communication: Channel Estimation and Beamforming Design", IEEE Open Journal of the Communications Society, vol. 1, pp. 661-680, 2020.
In this paper, the authors propose an MMSE-DFT based channel estimation protocol to estimate the direct and IRS-assisted links, and compare it with existing LS based channel estimation protocols.

Topic: Performance Evaluation

Since their inception, RISs have been under study for unveiling their fundamental performance limits and the impact of the imperfect knowledge of various system parameters on their achievable performance. In this context, several exact, approximate, and asymptotic analytical frameworks have been developed in order to quantify the advantages and limitations of RISs in different network scenarios.

Q. Nadeem, A. Kammoun, A. Chaaban, M. Debbah and M. Alouini, "Asymptotic Max-Min SINR Analysis of Reconfigurable Intelligent Surface Assisted MISO Systems," IEEE Transactions on Wireless Communications, early access, 2020.
This article investigates the max-min SINR performance of an RIS-assisted multi-user MISO communication system, where the BS-RIS link is assumed to be in LoS. For the rank-one LoS channel scenario, it shows that the minimum SINR goes to zero with the number of users. This reveals the importance of high-rank LoS channel for serving multiple users. For the full-rank BS-RIS channel, deterministic approximations for the parameters of optimal linear precoder are developed. Based on this, the reflection phases can be designed with the channel’s large-scale statistics.

M.-A. Badiu and J. P. Coon, “Communication through a Large Reﬂecting Surface with Phase Errors,” IEEE Wireless Communications Letters, vol. 9, no. 2, pp. 184–188, February 2020.
This paper investigates the communication through a large reflecting surface (LRS) with phase errors. It shows that the LRS-based composite channel is equivalent to a direct channel with Nakagami scalar fading. Based on this, theoretical performance studies are conducted.

T. Hou, Y. Liu, Z. Song, X. Sun, Y. Chen and L. Hanzo, "Reconfigurable Intelligent Surface Aided NOMA Networks," IEEE Journal on Selected Areas in Communications, early access, 2020.
This paper evaluates the performance of an RIS-aided NOMA network. It derives new channel statistics, the outage probability, the ergodic rate, energy efficiency, and spectral efficiency in both best-case and worst-case scenarios. In addition, the diversity orders are investigated.

X. Qian, M. Di Renzo, J. Liu, A. Kammoun, and M.-S. Alouini, “Beamforming through Reconﬁgurable Intelligent Surfaces in Single User MIMO Systems: SNR Distribution and Scaling Laws in the Presence of Channel Fading and Phase Noise,” IEEE Wireless Communications Letters, 2020.
This paper studies RIS-assisted MIMO communication over fading channels. In the presence of phase noise at the RIS, the authors presents an analytical approach for characterizing the distribution and scaling laws of the SNR.

H. Zhang, B. Di, L. Song and Z. Han, "Reconfigurable Intelligent Surfaces Assisted Communications With Limited Phase Shifts: How Many Phase Shifts Are Enough?," IEEE Transactions on Vehicular Technology, vol. 69, no. 4, pp. 4498-4502, April 2020.
This paper first derives the achievable data rate performance of an RIS assisted uplink transmission under continuous phase shifts, and then discusses the impact of finite resolution phase shifts on the derived achievable data rate.

Y. Han, W. Tang, S. Jin, C. K. Wen, and X. Ma, “Large Intelligent Surface-assisted Wireless Communication Exploiting Statistical CSI”, IEEE Transactions on Vehicular Technology, vol. 68, no. 8, pp. 8238-8242, August 2019.
In this paper, the authors evaluate the performance of an RIS-assisted large-scale antenna system by formulating a tight upper bound of the ergodic spectral efficiency and investigate the effect of the phase shifts on the ergodic spectral efficiency in different propagation scenarios.

Topic: Stochastic Geometry Based Analysis

Random spatial processes are considered to be the most suitable analytical tool to shed light on the ultimate performance limits of innovative technologies when applied in wireless networks and to guide the design of optimal algorithms and protocols for attaining such ultimate limits. In particular, random spatial processes have been successfully applied to the analysis of cellular networks, multi-tier cellular networks, millimeter-wave cellular networks, multi-antenna cellular networks, wireless information and power transfer, energy efficiency optimization, etc. Due to the importance of quantifying the performance gains offered by RISs in large-scale wireless networks, some researchers have recently attempted to fill this fundamental research gap.

M. Di Renzo and J. Song, “Reﬂection Probability in Wireless Networks with Metasurface-coated Environmental Objects: An Approach based on Random Spatial Processes,” EURASIP Journal on Wireless Communications and Networking, no. 99, Apr. 2019.
By utilizing tools from point processes, stochastic geometry, and random spatial processes, this paper presents the analytical framework to compute the probability that a randomly distributed object coated with a reconfigurable metasurface can act as a reflector for a given pair of transmitter and receiver.

T. Hou, Y. Liu, Z. Song, X. Sun, Y. Chen, and L. Hanzo, “MIMO Assisted Networks Relying on Large Intelligent Surfaces: A Stochastic Geometry Model,” arXiv, October 2019. [Online].
This paper focuses on a LIS-assisted MIMO network. By utilizing stochastic geometry tools, channel statistics for characterizing the effective channel gains, the outage probability, the ergodic rate of users, as well as the spectrum efficiency and energy efficiency of the network are derived.

M. A. Kishk and M.-S. Alouini, “Exploiting Randomly-located Blockages for Large-scale Deployment of Intelligent Surfaces,” IEEE Journal on Selected Areas in Communications, early access, 2020.
This article investigates the effect of large-scale deployment of RISs on the performance of cellular networks with the existence of blockages. The authors derive the probability that a typical mobile user can be associated with a BS via an RIS and the probability distribution of the corresponding path-loss. The analysis provides useful design insights for system design.

J. Lyu and R. Zhang, “Hybrid Active/Passive Wireless Network Aided by Intelligent Reflecting Surface: System Modeling and Performance Analysis”, arXiv, Apr. 2020. [Online].
This article focuses on a hybrid wireless network comprising both active BSs and passive IRSs. Based on stochastic geometry, the downlink achievable spatial throughput and other key performance indicators are derived under random channel fading and BSs/IRSs distributions.

Topic: Optimization and Resource Allocation: Model-Based and Data-Driven

Optimization of the system radio resources is one of the most researched topics in the context of RIS-aided wireless networks. A significant share of research articles has considered the problem of passive and active beamforming in MISO systems, i.e., the optimization of the transmit (active) beamforming and the (passive) RIS phase shifts, by resorting to the popular tool of alternating maximization in order to handle the non-convexity of the optimization problem. In addition, data-driven optimization methods based on machine learning and artificial intelligence have recently been applied to RISs.

C. Huang, A. Zappone, G. C. Alexandropoulos, M. Debbah, and C. Yuen, “Reconﬁgurable Intelligent Surfaces for Energy Efﬁciency in Wireless Communication,” IEEE Transactions on Wireless Communications, vol. 18, no. 8, pp. 4157–4170, August 2019.
This paper develops a realistic RIS power consumption model, which depends on the number of reflector units and the phase resolution. Based on this, the energy efficiency is maximized by jointly optimizing the RIS phase shifts and transmit power. Two low complexity algorithms are proposed to solve the formulated problem by invoking sequential fractional programming and conjugate gradient search methods.

S. Zhou, W. Xu, K. Wang, M. Di Renzo, and M.-S. Alouini, “Spectral and Energy Efficiency of IRS-Assisted MISO Communication with Hardware Impairments”, IEEE Wireless Communications Letters, vol. 9, no. 9, pp. 1366-1369, Sept. 2020.
This paper analyzes the spectral and energy efficiency of an IRS-assisted MISO system while considering hardware impairments, which include RF impairments at the AP and phase noise at the IRS. The paper shows that the achievable performance is limited due to the non-ideal AP and IRS.

S. Abeywickrama, R. Zhang, Q. Wu, and C. Yuen, “Intelligent Reﬂecting Surface: Practical Phase Shift Model and Beamforming Optimization,” IEEE International Conference on Communications (ICC), Dublin, Ireland, 2020, pp. 1-6.
This paper proposes a practical IRS phase shift model which captures the phase-dependent amplitude of the reflection elements. Based on this model, the authors maximize the achievable performance by using alternating optimization algorithm.

H. Han, J. Zhao, D. Niyato, M. Di Renzo, and Q.-V. Pham, “Intelligent Reﬂecting Surface Aided Network: Power Control for Physical-layer Broadcasting,” IEEE International Conference on Communications (ICC), Dublin, Ireland, 2020, pp. 1-7.
This paper investigates an IRS-assisted physical-layer broadcasting system. The transmit power is minimized by jointly designing the transmit beamforming at the BS and the phase shifts at the IRS, subject to QoS requirements to the mobile users. In addition, a lower bound of the minimum transmit power is derived.

Q. Wu and R. Zhang, “Intelligent Reﬂecting Surface Enhanced Wireless Network via Joint Active and Passive Beamforming,” IEEE Transactions on Wireless Communications, vol. 18, no. 11, pp. 5394-5409, November 2019.
This paper investigates the joint active and passive beamforming problem in both single-user and multi-user scenarios, with the target of minimizing the total transmit power. It also reveals that the receive SNR increases quadratically with the number of reflection elements, which is more cost efficient compared to the conventional massive MIMO and multi-antenna AF relay.

G. Zhou, C. Pan, H. Ren, K. Wang, M. Di Renzo and A. Nallanathan, "Robust Beamforming Design for Intelligent Reflecting Surface Aided MISO Communication Systems," IEEE Wireless Communications Letters, early access, 2020.
This paper investigates the robust joint beamforming under the assumption of imperfect CSI. By utilizing approximation and transformation techniques, the considered problem is converted into a sequence of semidefinite program subproblems.

Q. Wu and R. Zhang, “Beamforming Optimization for Wireless Network Aided by Intelligent Reﬂecting Surface with Discrete Phase Shifts,” IEEE Transactions on Communications, vol. 68, no. 3, pp. 1838-1851, March 2020.
This paper considers practical discrete IRS phase shifts, where the total transmit power minimization problem is formulated by jointly optimizing transmit and passive beamforming. In both single-user and multi-user scenarios, globally optimal solutions are firstly proposed by invoking the branch-and-bound method. Then, low complexity successive refinement based suboptimal solutions are designed, which are capable of achieving near-optimal performance.

X. Mu, Y. Liu, L. Guo, J. Lin, and N. Al-Dhahir, “Capacity and Optimal Resource Allocation for IRS-assisted Multi-user Communication Systems,” arXiv, January 2020. [Online].
This paper reveals the fundamental capacity limits of IRS-assisted multi-user wireless communication systems. The capacity and rate regions for both capacity-achieving NOMA and practical OMA transmission schemes are characterized. Numerical results show that the achieved IRS capacity gain can be further improved by dynamically configuring the reflection matrix.

X. Mu, Y. Liu, L. Guo, J. Lin, and R. Schober, “Joint Deployment and Multiple Access Design for Intelligent Reflecting Surface Assisted Networks,” arXiv, May. 2020. [Online].
This paper focuses on the optimal IRS deployment problem under different multiple access schemes. It proposes monotonic optimization based and alternating optimization methods in order to obtain performance upper bound and suboptimal solutions. The paper shows that asymmetric and symmetric IRS deployment strategies are preferable for NOMA and OMA, respectively.

A. Zappone, M. Di Renzo, F. Shams, X. Qian, and M. Debbah, “Overhead-aware Design of Reconfigurable Intelligent Surfaces in Smart Radio Environments,” IEEE Transactions on Wireless Communications, early access, 2020.
This article proposes an overhead model which captures the time needed for channel estimation and information exchange of the RIS. Closed-form solutions for the RIS phase shifts are provided, and numerical solutions for transmit and receive filters are given for maximizing the system rate and energy efficiency. Numerical results reveal a trade-off between the radio resource optimization and the overhead for carrying out the optimized solution.

J. Ye, S. Guo, and M. -S. Alouini, "Joint Reflecting and Precoding Designs for SER Minimization in Reconfigurable Intelligent Surfaces Assisted MIMO Systems", IEEE Transactions on Wireless Communications, vol. 19, no. 8, pp. 5561-5574, August 2020.
The authors investigate the use of an RIS to aid point-to-point multi-data-stream multiple-input multiple-output wireless communications. With practical finite alphabet input, the reflecting elements at the RIS and the precoder at the transmitter are alternatively optimized to minimize the symbol error rate.

A. Zappone, M. Di Renzo, and M. Debbah, “Wireless Networks Design in the Era of Deep Learning: Model-based, AI-based, or both?” IEEE Transactions on Communications, vol. 67, no. 10, pp. 7331–7376, October 2019.
This paper provides an extensive overview of wireless applications of deep learning. It includes the elaboration on the connection between model-based and data-driven methodologies, promising RIS-based network architectures, several case studies for deep learning, and the theoretical foundations of deep learning. The connections between deep learning and other machine learning frameworks are also discussed.

H. Gacanin and M. Di Renzo, “Wireless 2.0: Towards an Intelligent Radio Environment Empowered by Reconfigurable Meta-surfaces and Artificial Intelligence,” arXiv, February 2020. [Online].
In this paper, the concept of “Wireless 2.0” is introduced, where the radio environment becomes controllable, programmable, and intelligent with the emerging technologies of reconfigurable metasurfaces and artificial intelligence.

C. Huang, G. C. Alexandropoulos, C. Yuen, and M. Debbah, “Indoor Signal Focusing with Deep Learning Designed Reconﬁgurable Intelligent Surfaces,” IEEE International Workshop on Signal Processing Advances in Wireless Communications (SPAWC), pp. 1–5, Cannes, France, July 2019.
In this paper, an indoor RIS-assisted communication environment is investigated. A deep learning method is proposed for online configuring the RIS phase shifts, with the target of improving the transmitted signal focusing. To facilitate this, a Deep Neural Network (DNN) is designed to learn the mapping between the measured position information of the user and the optimal configuration of the RIS.

A. Taha, Y. Zhang, F. B. Mismar, and A. Alkhateeb, “Deep Reinforcement Learning for Intelligent Reﬂecting Surfaces: Towards Standalone Operation,” IEEE 21st International Workshop on Signal Processing Advances in Wireless Communications (SPAWC), Atlanta, GA, USA, 2020, pp. 1-5.
This paper proposes a novel deep reinforcement learning (DRL) framework for predicting the IRS reflection coefficients, which requires minimal training overhead. Compared with supervised learning based solutions, the proposed solution does not require an initial dataset collection phase.

A. Taha, M. Alrabeiah, and A. Alkhateeb, “Enabling Large Intelligent Surfaces with Compressive Sensing and Deep Learning,” arXiv, Apr. 2019. [Online].
This paper first proposes a novel LIS architecture, where a few active reflection elements are deployed. Based on this architecture, compressive sensing based and deep learning based solutions are proposed for design the LIS reflection matrices with negligible training overhead. Numerical results show that the proposed solutions approach the performance achieved with perfect channel knowledge.

N. S Perović, L. N Tran, M. Di Renzo, and M. F. Flanagan, “Achievable Rate Optimization for MIMO Systems with Reconfigurable Intelligent Surfaces,” arXiv, August 2020. [Online].
In this paper, the authors study the achievable rate optimization for a multi-stream multiple-input multiple-output (MIMO) system equipped with an RIS, and formulate a joint optimization problem of the covariance matrix of the transmitted signal and the RIS elements. To solve the considered optimization problem, the authors propose an iterative optimization algorithm that is based on the projected gradient method (PGM). Simulation results show that the proposed PGM achieves the same achievable rate as a state-of-the-art benchmark scheme based on alternating optimization (AO), but with a significantly lower computational complexity.

Topic: Comparison with other Transmission Technologies

When new technologies come into the limelight, it is dutiful to critically investigate the potential benefits and limitations that they may provide as compared with similar and well established technologies. Therefore, it is sensible to compare RISs with transmission technologies that may be considered to be closely related to them. In the research literature, two technologies are often considered to be the most similar to RISs: Relays and massive MIMO. In general terms, RISs are different with respect to relays and massive MIMO because the former emerging technology is intended to be realized in a passive way with minimal power amplification and signal processing capabilities, while the latter two technologies rely upon the utilization of individual RF chains and powerful signal processing algorithms. The long-term vision of RIS-based SREs consists of realizing a multi-function platform in which the transmission, processing, and computation are, as much as possible, moved to the EM level. Relays and massive MIMO, have been envisioned, on the other hand, as platforms in which the EM signal are first converted into the digital domain and, after processing, are converted back to the EM domain. Despite these general considerations, it is important to identify a fair basis for the comparison of RISs against relays and massive MIMO. A few research papers have recently attempted to shed light on this issue.

M. Di Renzo, K. Ntontin, J. Song, F. Danufane, X. Qian, F. Lazarakis, J. de Rosny, D.-T. Phan-Huy, O. Simeone, R. Zhang, M. Debbah, G. Lerosey, M. Fink, S. Tretyakov, and S. Shamai, “Reconfigurable Intelligent Surfaces vs. Relaying: Differences, Similarities, and Performance Comparison,” IEEE Open Journal of the Communications Society, vol. 1, pp. 798-807, 2020.
This paper discusses the key differences and similarities between RISs and conventional relays. Extensive numerical results are provided to compare the performance of RISs and relays with different key system parameters. The paper shows that sufficiently large RISs can achieve higher data rate than relay-aided systems.

E. Bjornson, O. Ozdogan, and E. G. Larsson, “Intelligent Reﬂecting Surface vs. Decode-and-forward: How Large Surfaces are Needed to Beat Relaying?” IEEE Wireless Communications Letters, vol. 9, no. 2, pp. 244–248, February 2020.
This paper provides a performance comparison between IRSs and classic decode-and-forward (DF) relays. It reveals that a large number of IRS elements is required to achieve IRS performance gain over DF relays.

S. Hu, F. Rusek, and O. Edfors, “Beyond Massive-MIMO: The Potential of Data-transmission with Large Intelligent Surfaces,” IEEE Transactions on Signal Processing, vol. 66, no. 10, pp. 2746-2758, May 2018.
This paper investigates the potentials of using LIS for data transmission. It provides an information-theoretical analysis on the normalized capacity measured per unit surface while considering different terminal deployment cases. Numerical results show that LIS is a promising research direction for future data transmission.

D. Dardari, “Communicating with Large Intelligent Surfaces: Fundamental Limits and Models,” IEEE Journal on Selected Areas in Communications, early access 2020.
This paper derives simple but accurate analytical expressions for the link gain and communication DoF, which provides useful insights and design guidelines for employing LISs. It reveals that LIS is a promising technology to achieve significant enhancement in spatial capacity density, compared with classical MIMO.

Topic: Localization, Positioning, and Sensing

Future wireless networks are expected to offer more than allowing people, mobile devices, and objects to communicate with each other. Future wireless networks need to be turned into a distributed intelligent communication, sensing, and computing platform. Besides connectivity, more specifically, this envisioned 6G platform is expected to be capable of sensing the environment, as well as locally storing and processing information, in order to provide network applications and services with context-awareness capabilities. Such processing could accommodate the time critical, ultra-reliable, and energy-efficient delivery of data, and the accurate localization of people and devices. Besides enabling enhanced connectivity, therefore, RISs are envisioned to play an important role to accomplish several other tasks that complement and support communications. Thanks to the possibility of equipping smart surfaces with energy-harvesting sensors, RISs may be able to offer a dense and capillary network for sensing the environment, and for creating environmental maps supporting a variety of emerging applications. Thanks to the possibility of realizing large-size smart surfaces, RISs may offer a platform that provides localization and positioning services with high accuracy in outdoor and outdoor scenarios, as well as they may enable near-field highly focusing capabilities for supporting the communication of massive deployments of devices. Thanks to the possibility of performing algebraic operations and functions directly on the impinging radio waves, RISs may offer the opportunity to realize a fully electromagnetic-based computing platform unlocking the potential of reconfigurable backscatter communications. For all these reasons, several researchers have started investigating the potential opportunities offered by RISs for applications beyond enhanced connectivity.

S. Hu, F. Rusek, and O. Edfors, “Beyond Massive MIMO: The Potential of Positioning with Large Intelligent Surfaces,” IEEE Transactions on Signal Processing, vol. 66, no. 7, pp. 1761–1774, Apr. 2018.
This paper investigates the potential applications of LIS for positioning. It first derives the Fisher-information matrix (FIM) and Cramer-Rao lower bound (CRLB) in closed-form expressions. The CRLB is analyzed under the condition that an unknown phase exists in the analog circuits of the LIS. In addition, the performance of centralized and distributed LIS deployment strategies are compared.

J. He, H. Wymeersch, L. Kong, O. Silvén and M. Juntti, "Large Intelligent Surface for Positioning in Millimeter Wave MIMO Systems," IEEE 91st Vehicular Technology Conference (VTC2020-Spring), Antwerp, Belgium, 2020, pp. 1-5.
This paper investigates LIS-aided mmWave MIMO systems for positioning. The paper analyzes the impact of the number of LIS elements and phase shifts on the position estimation accuracy. Numerical results show that the performance of positioning is improved with the aid of LIS.

H. Wymeersch, J. He, B. Denis, A. Clemente, and M. Juntti, “Radio Localization and Mapping with Reconﬁgurable Intelligent Surfaces,” arXiv, Dec. 2019. [Online].
This paper provides an overview of the benefits and challenges of invoking RISs to localization and mapping.

Topic: Experimental Assessment and Testbeds

From a technological point of view, RISs are a new and emerging paradigm in the context of wireless communications. Even though reflect-arrays have been used for long time in several communication-related applications, the realization of metasurface-based RISs bring new implementation challenges, and necessitate the realization of proof-of-concept platforms and hardware testbeds in order to substantiate theoretical findings and to develop realistic channel and signal models for system design. Even though several research works on the design of metasurfaces with advanced functionalities can be found, research on the realization of hardware platforms for application to wireless communications is still at its infancy. However, a few relevant research works can be found already.

A. Welkie, L. Shangguan, J. Gummeson, W. Hu, and K. Jamieson, “Programmable Radio Environments for Smart Spaces,” ACM Workshop on Hot Topics in Networks, pp. 36–42, November 2017.
This paper proposes to deploy low-cost devices embedded in the walls to realize programmable radio environments. It introduces the design and implementation of the proposed Programmable Radio Environment for Smart Spaces (PRESS). Some preliminary experiments are provided to demonstrate the effectiveness of PRESS.

Z. Li, Y. Xie, L. Shangguan, R. I. Zelaya, J. Gummeson, W. Hu, and K. Jamieson, “Towards Programming the Radio Environment with Large Arrays of Inexpensive Antennas,” USENIX Symposium on Networked Systems Design and Implementation, pp. 285–299, February 2019.
This paper proposes to augment the indoor environment with large arrays of inexpensive antennas (LAIA). It also designs algorithms to configure LAIA elements in real time. Experiments are carried out with a 32-element LAIA in a real indoor home environment. It shows that significant performance enhancement can be achieved by reconfiguring the wireless environment.

V. Arun and H. Balakrishnan, “RFocus: Beamforming Using Thousands of Passive Antennas,” USENIX Symposium on Networked Systems Design and Implementation, February 2020, pp. 1047–1061.
This paper introduces the RFocus prototype, which compromises 3200 inexpensive antennas on a 6 square-meter surface. It also proposes an optimization algorithm for controlling the state of the elements. Experimental results in an office building are provided to show the communication enhancement achieved by the prototype.

M. Dunna, C. Zhang, D. Sievenpiper, and D. Bharadia, “ScatterMIMO: Enabling Virtual MIMO with Smart Surfaces,” ACM Annual International Conference on Mobile Computing and Networking, no. 10, p. 1-14, September 2020.
This article proposes the concept of ScatterMIMO, which uses smart surfaces to increase the scattering in the environment, thus providing MIMO spatial multiplexing gain. It first introduces the mathematical framework to design smart reflectors, and then provides practical smart surface design, implementation, and evaluation. Experimental results validate the performance gain.

W. Tang, J. Y. Dai, M. Chen, X. Li, Q. Cheng, S. Jin, K.-K. Wong, and T. J. Cui, “Programmable Metasurface-based RF Chain-free 8PSK Wireless Transmitter,” IET Electronics Letters, vol. 55, no. 7, pp. 417–420, Apr. 2019.
This letter presents a cost-efficient architecture, where the transmitter uses programmable metasurfaces for 8PSK transmission. Experimental results show that the proposed programmable metasurface-based transmitter achieves comparable bit error rate performance as conventional transmitters.

W. Tang, X. Li, J. Y. Dai, S. Jin, Y. Zeng, Q. Cheng, and T. J. Cui, “Wireless Communications with Programmable Metasurface: Transceiver Design and Experimental Results,” IEEE China Communications, vol. 16, no. 5, pp. 46–61, May 2019.
This paper first introduces the fundamental principle of applying programmable metasurfaces as transmitters for wireless communications, and then presents a prototype system of metasurface-based transmitter. Based on this prototype, experiments are carried out, and the obtained results show that the metasurface-based architecture can achieve comparable performance as the conventional architecture.

W. Tang, M. Z. Chen, J. Y. Dai, Y. Zeng, X. Zhao, S. Jin, Q. Cheng, and T. J. Cui, "Wireless Communications with Programmable Metasurface: New Paradigms, Opportunities, and Challenges on Transceiver Design," IEEE Wireless Communications, vol. 27, no. 2, pp. 180-187, April 2020.
This paper introduces two paradigms to utilize the programmable metasurfaces, namely, RF chain-free transmitter and space-down-conversion receiver. The paper presents prototype design architectures and preliminary experimental results. Challenges and future research directions are also discussed.

W. Tang, J. Y. Dai, M. Z. Chen, K.-K. Wong, X. Li, X. Zhao, S. Jin, Q. Cheng, and T. J. Cui, “MIMO Transmission through Reconﬁgurable Intelligent Surface: System Design, Analysis, and Implementation,” IEEE Journal on Selected Areas in Communications, early access, 2020.
This paper proposes an RIS architecture achieving high-order modulation. It also analyzes the impact of the hardware constraints of the RIS on the system design. Moreover, the prototype that implements real-time RIS-based MIMO-QAM wireless communication is presented. Experimental results validate the proposed designs.

L. Dai, B. Wang, M. Wang, X. Yang, J. Tan, S. Bi, S. Xu, F. Yang, Z. Chen, M. Di Renzo, C. Chae, and L. Hanzo, “Reconﬁgurable Intelligent Surface-based Wireless Communication: Antenna Design, Prototyping and Experimental Results,” IEEE Access, vol. 8, pp. 45913– 45923, March 2020.
This paper develops an RIS compromising 256 2-bit phase resolution elements, and introduces the corresponding RIS-based wireless communication prototype. Experimental results show that significant antenna gain can be achieved at both 2.3 GHz and 28.5 GHz.

NTT DOCOMO, “DOCOMO and Metawave Announce Successful Demonstration of 28 GHz Band,” Dec. 2018. [Online].
It provides a successful demonstration of a 5G mobile communication system using 28GHz-band and meta-structure reflect-array technology. Results show that 560 Mbps communication speed can be achieved with metasurface-based reflect-arrays, compared to 60 Mbps with no reflector.

NTT DOCOMO, “DOCOMO Conducts World’s First Successful Trial of Transparent Dynamic Metasurface,” January 2020. [Online].
It presents a prototype of transparent dynamic metasurface using 28 GHz 5G radio signals. The transparent dynamic metasurface can work in two modes, namely, full penetration and full reflection.

Topic: Integration with Other Transmission Technologies

As an emerging and promising transmission technology, RISs can be employed for several applications and can be amalgamated with other technologies. Notable examples reported in this section includes: physical layer security, non-orthogonal multiple access, Internet of Things and backscattering communications, aerial communications, wireless power transfer, multiple-access edge computing, millimeter-wave, terahertz, and optical wireless communications, as well as their integration with software defined networking protocols for their control and programmability.

Physical Layer Security

L. Yang, J. Yang, W. Xie, M. O. Hasna, T. Tsiftsis, and M. Di Renzo, “Secrecy Performance Analysis of RIS-Aided Wireless Communication Systems”, IEEE Transactions on Vehicular Technology, early access 2020.
In this paper, the secrecy performance of an RIS-aided communication system is analyzed. The authors derive the secrecy outage probability and provide an asymptotic analysis to evaluate the effect of the main parameters on the secrecy performance of the considered system.

X. Yu, D. Xu and R. Schober, "Enabling Secure Wireless Communications via Intelligent Reflecting Surfaces," 2019 IEEE Global Communications Conference (GLOBECOM), Waikoloa, HI, USA, 2019, pp. 1-6.
In this paper, the secrecy rate is maximized by jointly optimizing the beamformer at the transmitter and the IRS phase shifts. Two efficient algorithms are proposed for small-scale and large scale IRSs, by leveraging the block coordinate descent and minorization-maximization methods.

S. Hong, C. Pan, H. Ren, K. Wang, and A. Nallanathan, “Artiﬁcial-noise-aided Secure MIMO Wireless Communications via Intelligent Reﬂecting Surface,” February 2020. [Online].
This article exploits IRSs to maximize the secrecy rate of artificial noise (AN)-aided MIMO communication systems, by joinytly optimizing the transmit precoding matrix and the AN covariance matrix at the BS, and phase shifts at the IRS.

X. Guan, Q. Wu and R. Zhang, "Intelligent Reflecting Surface Assisted Secrecy Communication: Is Artificial Noise Helpful or Not?," IEEE Wireless Communications Letters, vol. 9, no. 6, pp. 778-782, June 2020.
This paper investigates the application of artificial noise (AN) to improve the secrecy rate of an IRS-assisted communication system. An alternating optimization based algorithm is proposed for jointly transmit beamforming with AN and the IRS reflect beamforming.

J. Qiao and M. -S. Alouini, "Secure Transmission for Intelligent Reflecting Surface-assisted mmWave and Terahertz Systems", IEEE Wireless Communications Letters, early access, 2020.
The authors study the secure transmission for an IRS-assisted millimeter-wave and terahertz systems, and maximize the system secrecy rate, the transmit beamforming at the base station and the reflecting matrix at the IRS are jointly optimized with transmit power and discrete phase-shift constraints.

Non-Orthogonal Multiple Access

X. Mu, Y. Liu, L. Guo, J. Lin, and N. Al-Dhahir, “Exploiting Intelligent Reflecting Surfaces in NOMA Networks: Joint Beamforming Optimization,” IEEE Transactions on Wireless Communications, early access, 2020.
This paper focuses on the joint beamforming optimization problem for a downlink MISO NOMA network, subject to SIC decoding rate conditions and various IRS reflection element constraints. In terms of the characteristics of reflection amplitudes and phase shifts, three efficient algorithms are proposed to maximize the system sum rate.

Z. Ding and H. Vincent Poor, "A Simple Design of IRS-NOMA Transmission," IEEE Communications Letters, vol. 24, no. 5, pp. 1119-1123, May 2020.
This letter proposes an IRS-assisted NOMA downlink transmission architecture, where an IRS is deployed to serve the cell-edge users who share the same beam for the near users. It provides outage performance analysis while considering finite-resolution phase shifters.

G. Yang, X. Xu, and Y.-C. Liang, “Intelligent Reﬂecting Surface Assisted Non-orthogonal Multiple Access,” IEEE Wireless Communications and Networking Conference (WCNC), Seoul, Korea (South), 2020, pp. 1-6.
This paper maximizes the minimum decoding SINR in an IRS-assisted downlink NOMA system by jointly optimizing the active and passive beamforming. A combined-channel-strength based user ordering scheme is proposed, which achieves near-optimal performance.

B. Zheng, Q. Wu, and R. Zhang, “Intelligent Reﬂecting Surface-assisted Multiple Access with User Pairing: NOMA or OMA?” IEEE Communications Letters, vol. 24, no. 4, pp. 753–757, January 2020.
This paper provides a theoretical performance comparison between NOMA and OMA in the IRS-assisted downlink communication. It reveals that NOMA does not always outperform TDMA, especially when applying symmetric user paring strategy. Asymmetric user paring is suggested to exploit the NOMA gain over OMA.

J. Zuo, Y. Liu, Z. Qin, and N. Al-Dhahir, “Resource Allocation in Intelligent Reflecting Surface Assisted NOMA Systems,” IEEE Transactions on Communications, early access, 2020.
This paper focuses on a multi-channel IRS-aided downlink NOMA network. The authors maximize the system throughput by jointly optimizing the channel assignment, decoding order, power allocation, and reflection coefficients. A three-step algorithm is proposed to solve the formulated NP-hard problem, by invoking matching theory and alternating optimization methods.

X. Liu, Y. Liu, Y. Chen, and H. V. Poor, “RIS Enhanced Massive Non-orthogonal Multiple Access Networks: Deployment and Passive Beamforming Design,” IEEE Journal on Selected Areas in Communications, early access, 2020.
This paper investigates the deployment and passive beamforming design in the RIS-enhanced NOMA network. With the aim of maximizing the energy efficiency, a long short-term memory (LSTM) based echo state network (ESN) algorithm is firstly designed to predict the users’ tele-traffic demand. Then, a decaying double deep Q-network based algorithm is proposed for designing the RIS deployment and phase shifts.

Internet of Things and Backscattering

S. Y. Park and D. I. Kim, “Intelligent Reﬂecting Surface-aided Phase Shift Backscatter Communication,” IEEE International Conference on Ubiquitous Information Management and Communication, pp. 1–5, Taichung, Taiwan, January 2020.
This paper proposes an IRS-aided backscatter communication, where the IRS is utilized to simultaneously enhance the transmission of the primary and secondary systems. The spectrum efficiency of the secondary system is maximized via power splitting, while satisfying the minimum spectrum efficiency of the primary system.

Q. Zhang, Y.-C. Liang, and H. V. Poor, “Large Intelligent Surface/antennas (LISA) Assisted Symbiotic Radio for IoT Communications,” arXiv, February 2020. [Online].
This paper proposes a LISA-assisted symbiotic radio system, where the LISA simultaneously assists the primary transmission between the BS and the PR, and transmits messages to an IoT receiver (IR) using reflecting radio techniques. The total transmit power for satisfying the SNR requirements of PR and IR is minimized, by jointly designing the active and passive beamforming.

W. Zhao, G. Wang, S. Atapattu, T. A. Tsiftsis and X. Ma, "Performance Analysis of Large Intelligent Surface Aided Backscatter Communication Systems," IEEE Wireless Communications Letters, vol. 9, no. 7, pp. 962-966, July 2020.
This paper evaluates the performance of an LIS-aided backscatter system. The symbol error probability (SEP) is analyzed under both intelligent and random LIS phase adjustments. In addition, an upper bound for the average SEP in each scenario is derived.

Aerial Communications

S. Li, B. Duo, X. Yuan, Y. Liang and M. Di Renzo, "Reconfigurable Intelligent Surface Assisted UAV Communication: Joint Trajectory Design and Passive Beamforming," IEEE Wireless Communications Letters, vol. 9, no. 5, pp. 716-720, May 2020.
This paper studies RISs to enhance UAV-aided communications. By jointly optimizing the UAV trajectory and RIS phase shifts, the average achievable rate is maximized. Closed-form solutions and the successive convex approximation (SCA) method are proposed for designing the RIS phase shifts and UAV trajectory, respectively.

L. Yang, F. Meng, J. Zhang, M. O. Hasna, and M. Di Renzo, “On the Performance of RIS-Assisted Dual-Hop UAV Communication Systems” IEEE Transactions on Vehicular Technology, early access, 2020.
This paper evaluates the performance of an RIS-assisted UAV communication system. An analytical framework is proposed for estimating the outage probability, average bit-error rate, and average capacity. It shows that RISs can significantly improve the UAV communication.

H. Lu, Y. Zeng, S. Jin, and R. Zhang, “Enabling Panoramic Full-angle Reﬂection via Aerial Intelligent Reﬂecting Surface,” IEEE International Conference on Communications Workshops (ICC Workshops), Dublin, Ireland, 2020, pp. 1-6.
This paper proposes an aerial IRS architecture to achieve panoramic full-angle reflection. Under this model, the authors maximize the worst-case SNR among a given coverage area, by jointly optimizing the transmit beamforming, aerial IRS placement, and phase shifts.

Wireless Power Transfer

Q. Wu and R. Zhang, “Weighted Sum Power Maximization for Intelligent Reflecting Surface Aided SWIPT,’’ IEEE Wireless Communications Letters, vol. 9, no. 5, pp. 586–590, May 2020.
This paper exploits IRSs to improve the rate-energy trade-off in SWIPT systems. The key contribution is to theoretically prove that dedicated energy beamforming is not required for sum harvested power maximization problem.

C. Pan, H. Ren, K. Wang, M. Elkashlan, A. Nallanathan, J. Wang, and L. Hanzo, "Intelligent Reflecting Surface Aided MIMO Broadcasting for Simultaneous Wireless Information and Power Transfer," IEEE Journal on Selected Areas in Communications, vol. 38, no. 8, pp. 1719-1734, August 2020.
This paper investigates the advantages of using an IRS to assist SWIPT systems. The authors maximize the weighted sum rate of information receivers, subject to energy harvesting requirements of the energy receivers. An algorithm is proposed for joint beamforming optimization, which is guaranteed to converge to the Karush-Kuhn-Tucker point.

Q. Wu and R. Zhang, “Joint Active and Passive Beamforming Optimization for Intelligent Reﬂecting Surface Assisted SWIPT under QoS Constraints,” IEEE Journal on Selected Areas in Communications, vol. 38, no. 8, pp. 1735-1748, August 2020.
In this paper, multiple IRSs are deployed to assist SWIPT systems. With the aim of minimizing the total required transmit power for satisfying the QoS constraints of all users, a general penalty-based optimization framework is proposed for joint beamforming optimization. In addition, an alternating low-complexity algorithm is proposed.

Multiple-Access Edge Computing

T. Bai, C. Pan, Y. Deng, M. Elkashlan, A. Nallanathan and L. Hanzo, "Latency Minimization for Intelligent Reflecting Surface Aided Mobile Edge Computing," IEEE Journal on Selected Areas in Communications, early access, 2020.
This paper investigates the potential benefits of IRSs in mobile edge computing (MEC) systems. The computational latency minimization problem is formulated, where an efficient algorithm is proposed to alternatively optimize computing and communications related variables. The paper shows that the device-average computational latency is significantly reduced with the aid of IRSs.

Y. Liu, J. Zhao, Z. Xiong, D. Niyato, Y. Chau, C. Pan, and B. Huang, “Intelligent Reﬂecting Surface Meets Mobile Edge Computing: Enhancing Wireless Communications for Computation Ofﬂoading,” arXiv, January 2020. [Online].
In this paper, an IRS is deployed to enhance the computation tasks offloading from mobile devices to an edge server. The authors maximize the earning of the edge server by optimizing the IRS phase shifts. Two efficient algorithms are proposed to check the feasibility and solve the formulated problem.

Millimeter-Wave, Terahertz, and Optical Wireless Communications

K. Ying, Z. Gao, S. Lyu, Y. Wu, H. Wang and M. Alouini, "GMD-Based Hybrid Beamforming for Large Reconfigurable Intelligent Surface Assisted Millimeter-Wave Massive MIMO," IEEE Access, vol. 8, pp. 19530-19539, 2020.
This paper investigates an RIS-assisted broadband mmWave MIMO system. A geometric mean decomposition (GMD) based approach and simultaneous orthogonal matching pursuit algorithm are proposed for designing the digital and analog beamforming, respectively. Moreover, the IRS phase shifts are designed with LoS information of the reflection channels. The paper shows that the proposed design can achieve a better BER performance by deploying an IRS.

J. Zuo, Y. Liu, E. Basar, and O. Dobre, “Intelligent Reflecting Surface Enhanced Millimeter-Wave NOMA Systems”, IEEE Communication Letters, early access, 2020.
This paper investigates a downlink IRS-enhanced mmWave NOMA system. The sum rate is maximized by jointly optimizing the active and passive beamforming, as well as the power allocation. An iterative algorithm is proposed by invoking alternating optimization and successive convex approximation methods.

N. S. Perovic, M. Di Renzo, and M. F. Flanagan, “Channel Capacity Optimization Using Reconﬁgurable Intelligent Surfaces in Indoor mmwave Environments,” IEEE International Conference on Communications (ICC), Dublin, Ireland, 2020, pp. 1-7.
This paper focuses on RIS-assisted indoor mmWave communications, where an RIS is deployed to address the blockage problem. Two optimization schemes are proposed in order to maximize the channel capacity. In addition, an approximate expression of the channel capacity is derived, which reveals how the system parameters affect the achievable channel capacity.

M. Najafi and R. Schober, "Intelligent Reflecting Surfaces for Free Space Optical Communications," IEEE Global Communications Conference (GLOBECOM), Waikoloa, HI, USA, 2019, pp. 1-7.
This paper proposes an IRS-aided free space optical communication system. It provides a statistical channel model that captures the impact of the physical parameters of the IRS, and on this basis, performance analysis can be carried out.

W. Chen, X. Ma, Z. Li, and N. Kuang, “Sum-rate Maximization for Intelligent Reﬂecting Surface Based Terahertz Communication Systems,” IEEE/CIC International Conference on Communications Workshops in China (ICCC Workshops), Changchun, China, 2019, pp. 153-157.
In this paper, the joint beamforming design is investigated in an IRS-assisted THz communication system. By invoking local search and cross-entropy methods, two algorithms are proposed to maximize the sum rate.

Software Defined Networking Based Design and Nano-Communication Networks

S. Abadal, C. Liaskos, A. Tsioliaridou, S. Ioannidis, A. Pitsillides, J. Sole-Pareta, E. Alarcan, and A. Cabellos-Aparicio, “Computing and Communications for the Software-deﬁned Metamaterial Paradigm: A Context Analysis,” IEEE Access, vol. 5, pp. 6225–6235, Apr. 2017.
This paper provides a context analysis of software-defined metamaterials (SDMs) from both the computing and communication perspectives. It first introduces the concept of software-defined metasurfaces, and then gives an overview of current computing and networking approaches that could be amenable to SDMs. Open issues and research challenges are also discussed.

C. Liaskos, S. Nie, A. Tsioliaridou, A. Pitsillides, S. Ioannidis, and I. F. Akyildiz, “A Novel Communication Paradigm for High Capacity and Security via Programmable Indoor Wireless Environments in Next Generation Wireless Systems,” Elsevier Ad Hoc Networks, vol. 87, pp. 1–16, May 2019.
This paper proposes the HyperSurface-based programmable wireless environment model. It first provides an overview of recent research on programmable wireless environments, and introduces prerequisites on metasurfaces. Then, the theoretical foundations of HyperSurfaces and performance evaluations are presented.

C. Liaskos, A. Tsioliaridou, S. Nie, A. Pitsillides, S. Ioannidis, and I. F. Akyildiz, “On the Network-layer Modeling and Conﬁguration of Programmable Wireless Environments,” IEEE/ACM Transactions on Networking, vol. 27, no. 4, pp. 1696–1713, Apr. 2019.
This paper proposes a graph-based model of programmable environments, which transforms different performance objectives to path search problems and captures core physical restrictions. Moreover, a configuration scheme called KPCONFIG is designed, and extensive performance evaluations are carried out.

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