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A Wideband Reconfigurable Intelligent Surface for 5G Millimeter-Wave Applications

Ruiqi Wang, Yiming Yang, Behrooz Makki, Atif Shamim

arXiv:2304.11572v2eess.SPcs.SYeess.SY

TL;DR

Published RIS literature contains little experimental evidence, with most works focused on idealized theoretical analyses. The authors design, fabricate, and test a PIN-switch RIS using a circular-cutout unit cell that excites adjacent high-order resonances. The measured RIS has a 3 dB peak gain bandwidth of 26.9% from 22.5 to 29.5 GHz and scans reflected beams up to 50°.

  • Problem

    Published RIS literature contains little experimental evidence, with most works focused on idealized theoretical analyses.

    Source: §I · p. 1

  • Method

    The authors design, fabricate, and test a PIN-switch RIS using a circular-cutout unit cell that excites adjacent high-order resonances.

    Source: §I · p. 2, §II · p. 2

  • Results

    The measured RIS has a 3 dB peak gain bandwidth of 26.9% from 22.5 to 29.5 GHz and scans reflected beams up to 50°.

    Source: §IV · pp. 6–7, §V · p. 8

  • Takeaways & Limitations

    The authors conclude that the measured wideband reflection characteristics make the design suitable for 5G mm-wave communication systems.

    Source: §V · pp. 8–9

Abstract

from arXiv · show

Despite the growing interest in reconfigurable intelligent surfaces (RISs) for millimeter-wave (mm-wave) bands, and the considerable theoretical work reported by the communication community, there is a limited number of published works demonstrating practical implementations and experimental results. To the authors' knowledge, no published literature has reported experimental results for RISs covering the n257 and n258 mm-wave bands. In this work, we propose a novel wideband RIS design that covers the entire mm-wave 5G n257 and n258 bands. In simulations, the unit cell can maintain a phase difference of 180{\deg} +- 20{\deg} and a reflection magnitude greater than -2.8 dB within 22.7 to 30.5 GHz (29.3% bandwidth) using one-bit PIN switches. The proposed unit cell design with four circular cutouts and long vias could realize wideband performance by exciting two adjacent high-order resonances (2.5f and 3.5f). The periodic unit cells can maintain an angular stability of 30{\deg}. Based on the proposed unit cell, a 20 by 20 RIS array is designed and fabricated with a size of 7.1{\lambda} by 7.1{\lambda}. The measurement results demonstrate that the proposed RIS could maintain a 3 dB peak gain variation bandwidth among various array configurations within 22.5 to 29.5 GHz (26.9%) and with a beam scanning capability of 50{\deg}, making this design a good candidate for 5G mm-wave applications.

I. INTRODUCTION

Practical mm-wave RIS designs remain sparsely experimentally characterized, and the stated n257/n258 coverage target is challenging. The work presents a fabricated RIS designed for these bands, with measured wideband performance and beam scanning.

I. INTRODUCTION

  • In mm-wave systems, atmospheric attenuation can leave links vulnerable to blockage; an RIS can provide a secondary line of sight.The introduction frames this as a way to address signal blind spots.Source: §I · p. 1
  • Practical RIS experiments at mm-wave frequencies are scarce; prior experiments mostly target sub-6 GHz or around 10 GHz, with few mm-wave designs.The cited mm-wave examples include limited bandwidth or simulations without experimental results.Source: §I · pp. 1–2
  • Covering n257 and n258 requires 24.25–29.5 GHz, and no previously reported PIN-switch-based RIS covered that entire range.The proposed design is reported to cover both bands.Source: §I · p. 2
  • A fabricated 20×20 RIS measures a 3 dB gain bandwidth of 26.9% and a 50° beam-scanning range.The array measures 7.1λ×7.1λ.Source: §I · p. 2

II. UNIT CELL DESIGN AND SIMULATION

The proposed unit cell uses circular cutouts and PIN-switch reconfiguration to provide wideband phase control. Its design combines paired resonant modes with angular stability and a compact layout.

II. UNIT CELL DESIGN AND SIMULATION

  • Four circular cutouts shape a 3 mm square patch, while vias connect its sides through the middle ground to the bottom feeding network.The design also uses a 1.575 mm Rogers 5880 top substrate and a 0.2 mm FR4 bottom substrate.Source: §II · p. 2
  • Backside PIN switches toggle the unit cell between ON and OFF states, while RF chokes keep DC feeding lines from affecting RF performance.The switches connect the middle ground and top patch through short and long vias, respectively.Source: §II · p. 2
  • The ON/OFF phase difference stays within 180°±20° from 22.7 to 30.5 GHz (29.3%), with reflection magnitude greater than -2.8 dB.The paired 2.5f and 3.5f modes are attributed to the circular cutouts; without them, the patch has only one ON-state 3.5f resonance.Source: §II · p. 2
  • The PIN switches and RF chokes sit beneath the patch, leaving boundary space for bias lines routed on the same layer without extra layers.The cutouts and backside PIN placement are also described as supporting the compact cell structure.Source: §II · p. 3
  • The 0.34λ×0.34λ unit cell can accommodate more elements in a fixed physical area than a standard half-wavelength cell.Its physical dimensions are 3.85×3.85 mm.Source: §II · p. 3
  • For incident angles from 0° to 30°, reflection magnitude exceeds -2.5 dB and phase difference stays between 140° and 200° across 24.25–29.5 GHz.This is the stated angular-stability performance in the desired 5G mm-wave band.Source: §II · p. 3

III. RIS DESIGN AND SIMULATION

The section presents a 20×20 RIS array and a full-wave simulation using discretized unit-cell phase states. At 27.5 GHz, the simulated pattern has a 20.9 dBi peak gain at 0° reflection.

III. RIS DESIGN AND SIMULATION

  • The proposed array has 400 unit cells in a 20×20 configuration and measures 77×77 mm, or 7.1λ×7.1λ at 27.5 GHz.The front and back array views are shown in Fig. 8.Source: §III · p. 3, Figure 8 · p. 4
  • Each array element uses a discretized phase distribution, with ON/OFF unit-cell phases differing by 180°±20° across the band.The ON and OFF phase values are obtained from HFSS simulations of the proposed unit cell.Source: §III · pp. 3–4
  • 20.9 dBi peak gain at 0° reflection validates the proposed RIS's reconfigurability in full-wave simulation.The 3D radiation pattern is shown at 27.5 GHz.Source: §III · p. 4, Figure 10 · p. 5

IV. FABRICATION AND MEASUREMENT

Measurements of the fabricated RIS demonstrate wideband reflection and beam steering across the 5G mm-wave bands. Tests also compare ON/OFF operation and near- and far-field performance.

IV. FABRICATION AND MEASUREMENT

  • The measured simulation and experiment results were consistent, with discrepancies attributed to the printed support, control circuit, and fabrication and assembly errors.The near-field setup used a Tx horn 20 cm from the RIS and moved the Rx horn from –80° to 0° in 10° steps.Source: §IV · pp. 5–6, Figure 14 · p. 6
  • At 27.5 GHz, switching the RIS pattern ON improved received signal strength by 24.7 dB at 0° reflection and 30° incidence.Average enhancement across the n257 and n258 bands was 21.2 dB; each frequency point used its own RIS pattern.Source: §IV · p. 6, Figure 15 · p. 7
  • 26.9% 3 dB peak-gain bandwidth was measured from 22.5 to 29.5 GHz, covering the n257 and n258 bands.The measured S21 changed from –12.5 to –14.7 dB across the band.Source: §IV · p. 6, Figure 17 · p. 7
  • The measured 0° reflection S21 was –12.5 to –14.7 dB across 22.5–29.5 GHz, while the RIS and PEC reflections were compared at eight frequencies.The measurement used a distinct array pattern at each frequency to redirect a 30° incident wave to 0° reflection.Source: §IV · p. 6, Figure 18 · p. 7
  • The RIS steered the reflected beam from 0° to 50° as the array pattern changed, with measured S21 values from –11.3 to –14.5 dB.The measured S21 values for directions 0°, 10°, 20°, 30°, 40°, and 50° were –12.5, –12.8, –11.7, –11.3, –11.9, and –14.5 dB.Source: §IV · pp. 7–8, Figure 19 · p. 8
  • In the far-field test, the RIS produced an average 19.6 dB signal enhancement across the n257 and n258 bands, 5.1 dB less than in the near-field test.The Tx horn was placed 70 cm from the RIS for the far-field test.Source: §IV · p. 8, Figure 20 · p. 8

V. CONCLUSION

The conclusion summarizes a wideband RIS design for 5G mm-wave operation. Its fabricated array achieved measured wideband gain performance and beam scanning.

V. CONCLUSION

  • The fabricated 20×20 RIS array measured 7.1λ×7.1λ and achieved 26.9% 3 dB gain bandwidth covering the n257 and n258 bands.The conclusion reports unit-cell bandwidth of 29.3% from 22.7 to 30.5 GHz, enabled by resonances at 2.5f and 3.5f with angular stability of ±30°.Source: §V · p. 8
  • The RIS design scanned its reflected beam up to 50°.The conclusion identifies the design as suitable for 5G mm-wave communication systems.Source: §V · pp. 8–9
  • The proposed unit cell uses two adjacent high-order resonances to realize wideband performance.The reported resonant operation frequencies are 2.5f and 3.5f.Source: §V · p. 8

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