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A Survey of Intelligent Reflecting Surfaces (IRSs): Towards 6G Wireless Communication Networks

Jun Zhao

arXiv:1907.04789v3eess.SPcs.ITcs.NI

TL;DR

IRS research addresses how programmable reflecting surfaces can improve wireless efficiency for prospective 6G networks, while the literature remains broad and heterogeneous. This paper surveys and categorizes studies across communications, implementations, and applications, then identifies future research directions. It concludes that IRSs are a promising technology for facilitating 6G communications and may play a fundamental role in 6G networks.

  • Problem

    The paper addresses the need to organize rapidly growing research on IRSs as a promising technology for improving spectrum and energy efficiency in 6G communications.

    Source: §I · p. 1, §II · p. 2

  • Method

    The paper surveys and categorizes IRS studies across communication analyses and optimizations, implementations, secure communications, terminal positioning, and other applications.

    Source: §II · p. 2, §I · p. 2

  • Results

    The survey identifies IRSs as a promising technology for facilitating 6G wireless communications and envisions a fundamental role for them in 6G networks.

    Source: §V · p. 5, §IV · p. 5

  • Takeaways & Limitations

    The paper provides a timely synthesis of IRS research and identifies future directions concerning real-world validation, physical-material-aware modeling, and performance scaling laws.

    Source: §III · p. 5, §IV · p. 5

Abstract

from arXiv · show

Intelligent reflecting surfaces (IRSs) tune wireless environments to increase spectrum and energy efficiencies. In view of much recent attention to the IRS concept as a promising technology for 6G wireless communications, we present a survey of IRSs in this paper. Specifically, we categorize recent research studies of IRSs as follows. For IRS-aided communications, the summary includes capacity/data rate analyses, power/spectral optimizations, channel estimation, deep learning-based design, and reliability analysis. Then we review IRSs implementations as well as the use of IRSs in secure communications, terminal-positioning, and other novel applications. We further identify future research directions for IRSs, with an envision of the IRS technology playing a critical role in 6G communication networks similar to that of massive MIMO in 5G networks. As a timely summary of IRSs, our work will be of interest to both researchers and practitioners working on IRSs for 6G networks.

I. INTRODUCTION

IRSs are presented as a 6G technology that tunes wireless propagation to support more efficient and reliable communications. The survey distinguishes IRSs from related surface concepts and organizes the paper around recent studies and future directions.

  • An intelligent reflecting surface comprises independently controllable units that modify incident signals, including their phase, amplitude, or frequency.Source: §I · p. 1
  • IRS-aided links can connect a base station with mobile users, UAVs, smart vehicles, or other terminals, including when a tree blocks the line of sight.Source: §I · p. 1, Figure 1 · p. 1
  • Unlike massive MIMO, which uses large antenna arrays, IRSs tune the wireless propagation environment for communication.Source: §I · p. 1
  • The survey uses IRS as its consistent name, distinguishing it from actively transmitting large intelligent surfaces and frequency selective surfaces used for different purposes.Source: §I · p. 2
  • The paper classifies recent IRS studies, compares its coverage with earlier reviews, identifies future research directions, and concludes with the survey’s findings.Source: §I · pp. 1–2

II. CATEGORIZING RECENT STUDIES OF INTELLIGENT REFLECTING SURFACES

The survey categorizes recent IRS research across IRS-aided communications, implementations, and applications beyond conventional data transmission.

  • The communication categories cover capacity and data rates, power and spectral optimization, channel estimation, deep-learning design, and reliability analysis.Source: §II · p. 2
  • The survey also reviews IRS implementations and applications in secure communications, terminal positioning, and other novel areas.Source: §II · p. 2

A. Capacity/data rate analyses of IRS-aided communications

Capacity and data-rate research examines IRS-enabled information transfer under idealized, impaired, estimated, correlated, and multi-user channel conditions. Optimization studies jointly design active base-station transmission and passive IRS configurations.

B. Power/spectral optimizations in IRS-aided communications

Power and spectral-efficiency studies formulate IRS-aided downlink designs around energy efficiency, transmit-power reduction, and spectral-efficiency maximization, jointly tuning base-station transmission and IRS phase shifts.

C. Channel estimation for IRS-aided communications

Channel estimation studies address passive IRS constraints by estimating channels at the base station and configuring IRS phase shifts, using staged protocols, active elements, or structured recovery methods.

D. Deep learning-based design for IRS-aided communications

Deep learning is used both to configure IRSs for wireless communications and to infer channel qualities at units whose channels are not directly observed.

  • Wireless propagation is modeled as a deep neural network whose neurons are IRS units and whose links are their cross-interactions.Training on data enables the network to learn propagation basics and configure the IRS to an optimal setting.Source: D. Deep learning-based design for… · p. 3
  • A deep neural network estimates channel qualities at all IRS units using observations from active units connected to the IRS controller baseband.Deep learning also guides the IRS in learning its optimal interaction with incident signals.Source: D. Deep learning-based design for… · p. 3

E. Reliability analysis of IRS-aided communications

Reliability analysis characterizes IRS-aided communications through rate distributions and outage probabilities, while related implementation studies examine surface geometry and active/passive architectures.

E. Reliability analysis of IRS-aided communications

  • Uplink reliability is analyzed through the data sum-rate distribution and the probability that the sum-rate falls below a desired value.The distribution is obtained using the Lyapunov central limit theorem, and the resulting outage probability supports outage computation.Source: E. Reliability analysis of IRS-aided… · p. 3

F. IRS implementations

  • The hexagonal lattice minimizes IRS surface area for a desired number of independent signal dimensions when each antenna provides one signal-space dimension.Source: F. Implementations of IRSs · p. 3
  • The lattice analysis draws on classical lattice theory, which also has applications in information theory, cryptography, machine learning, and knowledge representation.Source: F. Implementations of IRSs · p. 4
  • IRS architectures can combine active units connected to the controller baseband with passive units that are not connected.Channel information from active units captures environmental conditions and sender/receiver locations for system optimization.Source: F. Implementations of IRSs · p. 4

G. IRSs for secure communications

IRSs are studied for physical-layer security by shaping rates toward legitimate receivers and eavesdroppers, with optimization extending from one receiver and eavesdropper to multiple users.

  • IRS-assisted secure communications build on wiretap-channel models including broadcast, compound, Gaussian, and MIMO variants.Source: G. IRSs for secure communications · p. 4
  • An IRS can increase a legitimate receiver’s data rate while decreasing an eavesdropper’s, improving the secrecy data rate.The secrecy data rate is the legitimate receiver’s rate minus the eavesdropper’s rate.Source: G. IRSs for secure communications · p. 4
  • For one legitimate receiver and one eavesdropper, studies jointly design base-station transmit beamforming and IRS phase shifts.One approach alternates between exact transmit-beamforming updates and approximate IRS phase-shift optimization.Source: G. IRSs for secure communications · p. 4
  • For multiple legitimate receivers and eavesdroppers, optimization maximizes the minimum secrecy data rate among all legitimate receivers.The design jointly searches for transmit beamforming and IRS phase shifts in an IRS-aided downlink broadcast system.Source: G. IRSs for secure communications · p. 4

H. IRSs for terminal-positioning and other novel applications

The survey describes IRS applications beyond conventional communications, including terminal positioning, index modulation, over-the-air computation, access-point functionality, and wireless energy transfer.

Terminal-positioning

  • IRS-aided terminal positioning yields CRLBs that generally decrease quadratically with IRS surface area.For a terminal perpendicular to the IRS center, the distance CRLB instead decreases linearly with surface area.Source: H. IRSs for terminal-positioning and… · p. 4

Index modulation

  • IRS-space shift keying and IRS-spatial modulation convey information by selecting receive antenna indices according to information bits.These schemes realize index modulation through IRS-assisted spatial signaling.Source: H. IRSs for terminal-positioning and… · p. 4

Over-the-air computation

  • IRS-assisted AirComp jointly optimizes IRS phase shifts and base-station decoding vectors to minimize mean squared decoding distortion.An alternating difference-of-convex algorithm is proposed to address the non-convex optimization problem.Source: H. IRSs for terminal-positioning and… · p. 4

IRS as an access point

Wireless energy transfer

  • IRS-assisted wireless energy transfer combines channel estimation with energy beamforming at a multi-antenna power beacon and the IRS.The application targets power delivery from the beacon to a single-antenna user.Source: H. IRSs for terminal-positioning and… · p. 4

III. RECENT REVIEWS OF INTELLIGENT REFLECTING SURFACES AND RELATED TECHNOLOGIES

The paper distinguishes its survey from earlier reviews by combining broad categorization with coverage of implementations, applications, and future research directions. It identifies real-world validation, physically grounded modeling, and scaling laws as major research needs.

Recent reviews

  • Earlier reviews emphasize mathematical IRS communication analyses, artificial intelligence, or design and implementation challenges, whereas this paper provides a broader categorization.Source: §III · p. 5

Related technologies

  • Other reviews focus on physical IRS implementations, massive MIMO 2.0, or technologies related to massive MIMO.Source: §III · p. 5

Future research directions

  • The paper envisions IRSs playing a fundamental role in 6G networks similar to massive MIMO's role in 5G.Source: §IV · p. 5
  • Most existing IRS communication studies rely on theoretical analyses validated with simulations, motivating confirmation through real-world implementations and experiments.Source: §IV · p. 5
  • Existing models of IRS signal transformation are simple despite dependence on physical materials and manufacturing processes.More physically informed models can guide IRS optimization more accurately.Source: §IV · p. 5
  • Scaling laws are needed to establish fundamental performance limits for IRS-aided communications.The paper links this need to understanding how IRSs affect traditional information-theoretic models.Source: §IV · p. 5

V. CONCLUSION

The survey categorizes IRS research and identifies future directions while presenting IRSs as a promising technology for improving spectrum and energy efficiencies in 6G communications.

  • The survey categorizes IRS studies and identifies future research directions for IRS technology.Source: §V · p. 5
  • IRSs are presented as a promising technology that induces smart radio environments to increase spectrum and energy efficiencies.Source: §V · p. 5

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