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A Wideband STAR-RIS Employing Shared-Aperture Polarization Conversion and Diagonal Switching Topology for 6G Applications

Ruiqi Wang, Atif Shamim

arXiv:2610.19040v1eess.SP

TL;DR

Practical STAR-RIS demonstrations have had limited bandwidth, and simultaneous wideband reflection and transmission through a common aperture remains challenging. This work proposes a shared-aperture, diagonal-switching design using polarization conversion and current reversal. A fabricated 10 × 10 prototype demonstrates wide measured bandwidths and near-field signal enhancement in both modes.

  • Problem

    Practical STAR-RIS demonstrations have limited bandwidth, while achieving wideband reflection and transmission simultaneously through a common aperture remains challenging.

    Source: §I · p. 1

  • Method

    A shared-aperture, four-state polarization-converting unit cell uses diagonal switching and mirror-symmetric current reversal for 1-bit phase control in both modes.

    Source: §I · p. 2, §II · p. 3

  • Results

    The 10 × 10 prototype measured 3-dB bandwidths of 19.8% (reflection) and 21.9% (transmission), with average near-field signal enhancements of 19.8 and 22.6 dB, respectively, over 6.8–8.4 GHz.

    Source: §I · p. 2, C. Near-Field Measurement · pp. 8–9

  • Takeaways & Limitations

    The measured prototype demonstrates wideband wavefront manipulation on both sides of a shared aperture across a common frequency range.

    Source: C. Near-Field Measurement · p. 9, §V · p. 12

Abstract

from arXiv · show

Simultaneously transmitting and reflecting reconfigurable intelligent surfaces (STAR-RIS) enable programmable wave control on both sides of a surface, extending wireless coverage beyond single-sided operation. However, practical STAR-RIS demonstrations in literature have narrow bandwidths, because it is challenging to achieve wideband reflection and transmission simultaneously from the same aperture. To overcome this limitation, a wideband STAR-RIS architecture is proposed based on shared-aperture polarization conversion and diagonal switching topology. Here, multiple resonances enable wideband magnitude responses and mirror-symmetric current reversal maintains $180^{\circ}$ phase difference in both reflection and transmission modes. Accordingly, a diagonal four-branch unit cell configuration is developed, where the tailored top patch directs the received wave either through a pair of blind vias connected by a buried strip to the shared top aperture for reflection, or to the bottom ring patch using a through via for transmission. Based on the proposed design, a 10 by 10 STAR-RIS prototype is fabricated and experimentally characterized. The measured 3-dB bandwidths reach 19.8% in reflection mode and 21.9% in transmission mode. Near-field measurements show average signal enhancements of 19.8 and 22.6 dB for reflection and transmission, respectively, over the wide frequency range of 6.8-8.4 GHz. Far-field measurements further demonstrate average received-power enhancements of 11.2 dB in reflection and 11.9 dB in transmission. To the best of the authors' knowledge, this work presents the first experimentally demonstrated wideband STAR-RIS covering the 7.125-8.4 GHz band within the emerging 6G FR3 spectrum.

I. INTRODUCTION

STAR-RIS can extend programmable coverage to both sides of a surface, but practical wideband reflection and transmission through a common aperture remain challenging. The proposed diagonal-switching unit cell combines shared-aperture routing and current reversal; simulations show wideband, overlapping reflection and transmission responses.

I. INTRODUCTION

  • STAR-RIS extends programmable wave manipulation from the incident-wave side to both sides, while practical FR3 hardware requires broadband reflection and transmission from a common aperture.Prior hardware demonstrations had relatively limited operating bandwidths, motivating a wideband design for the 7.125–8.4 GHz band.Source: §I · pp. 1–2

II. UNIT CELL DESIGN AND SIMULATION

  • The unit cell uses a tailored shared receiving patch and four diagonal switching branches to route waves into reflection or transmission paths, with two 1-bit phase states per mode.Reflection uses blind vias and a buried strip to return the wave to the top aperture; transmission uses a through via to feed the bottom ring patch.Source: §II · pp. 2–3
  • Mirror-symmetric switching reverses current orientation within each mode, producing approximately 180° phase separation for wideband 1-bit operation.The reflection states concentrate current mainly on the top radiating and routing structures; transmission states excite the bottom ring patch.Source: §II · p. 3
  • Reflection and transmission bands both cover 7.125–8.4 GHz, with simulated magnitude above −3 dB from 6.85–8.41 GHz and 6.83–8.59 GHz, respectively.Reflection-state phase separation is 181°–190°; transmission-state separation is 176°–181°.Source: §II · pp. 3–4
  • Simulated reflection and transmission responses retain wideband characteristics for TE- and TM-polarized incidence from 0° to 30°, despite slight frequency shifts and magnitude variations.This supports unit-cell operation under non-normal illumination within the tested angular range.Source: §II · p. 4

A. STAR-RIS Array Design

The 10 × 10 array preserves the unit cell’s multilayer architecture and embeds RF and DC routing between ground planes. Its periodic routing and peripheral control interfaces are designed to support extension to larger apertures.

A. STAR-RIS Array Design

  • The 10 × 10 array uses 12.7 mm unit-cell spacing in both directions, forming a 127 × 127 mm aperture.The multilayer stack includes top and bottom patch layers, two substrates, ground planes, prepregs, and a central FR4 routing layer.Source: A. STAR-RIS Array Design · p. 4
  • Embedding the RF routing and DC biasing network between two ground planes shields it from the radiating apertures and reduces undesired interaction with their responses.The routing network can be extended periodically with the array.Source: A. STAR-RIS Array Design · p. 4
  • Peripheral FFC connectors link the internal biasing network to digital control boards, and additional cells and control channels can be added by extending the routing and connector interfaces.The authors describe this architecture as a practical, scalable platform for larger programmable apertures.Source: A. STAR-RIS Array Design · p. 4

B. Full-Wave Beamforming Simulation

The array-level simulations apply a shared phase-synthesis framework, compensating for the illuminating horn’s spatial phase while imposing a gradient for the desired outgoing direction. They produce directive beams in both reflection and transmission modes.

B. Full-Wave Beamforming Simulation

  • The 1-bit phase patterns are derived by compensating for incident-field phase variation and adding the spatial phase gradient for the desired reflected or transmitted beam.Continuous phase values are quantized to the two available states in the selected mode.Source: B. Full-Wave Beamforming Simulation · pp. 4–5
  • With normal incidence and both target angles set to 30°, the 10 × 10 full-wave model uses mode-specific 1-bit states to redirect waves into the reflection and transmission hemispheres.The illumination horn’s phase center is approximately 80 mm from the array center.Source: B. Full-Wave Beamforming Simulation · p. 5
  • 8.0 dBi reflection and 9.8 dBi transmission realized gains accompany directive simulated beams toward the desired directions.The simulated patterns demonstrate that the unit-cell characteristics extend to array-level beamforming.Source: B. Full-Wave Beamforming Simulation · p. 5, Figure 11 · p. 5

C. Control Circuit Design

A programmable digital system converts calculated reflection or transmission phase patterns into control signals for the STAR-RIS. Cascaded shift registers and logic buffers provide the array’s bias-control outputs.

C. Control Circuit Design

  • The control architecture’s pattern-generation flow is organized around the input parameters, phase calculation, 1-bit mapping, and digital delivery to the STAR-RIS.The circuit layout includes shift-register and buffer/LED boards.Source: Figure 13 · p. 6, C. Control Circuit Design · p. 6, Figure 12 · p. 6
  • 74HC595 shift registers convert serial data from the ESP-WROOM-32 into parallel control outputs, while SN74ACT244 buffers provide drive capability to the biasing network.Cascading shift registers allows many channels to be addressed through a limited number of digital interfaces.Source: C. Control Circuit Design · p. 6, Figure 12 · p. 6
  • The control flow calculates a mode-specific phase distribution from the operating frequency, illumination geometry, and desired beam direction, then quantizes and maps it to available states.The ESP-WROOM-32 sends the resulting digital sequence through shift registers and buffers to the surface.Source: C. Control Circuit Design · p. 6

IV. FABRICATION AND MEASUREMENT

The authors fabricated a complete 10 × 10 STAR-RIS with integrated RF and bias networks and external digital-control boards. Measurements show broad 3-dB bandwidths in both modes, with measured phase differences that remain usable for 1-bit beamforming.

A. Fabricated Prototypes

  • The complete 10 × 10 array integrates the RF components and DC-bias networks in a multilayer PCB and connects to external control circuitry through peripheral FFC connectors.The programmable platform includes separate shift-register and buffer/LED boards interconnected with the array.Source: A. Fabricated Prototypes · p. 6, Figure 14 · p. 7

B. RIS Characterization

  • Uniformly programming each array state and switching between the two states per mode lets the two-horn measurements extract magnitude responses and phase differences over frequency.The horns are arranged symmetrically about the surface normal for reflection and on opposite sides for transmission.Source: B. RIS Characterization · p. 6
  • The fabricated STAR-RIS has a measured reflection 3-dB bandwidth of 6.86–8.37 GHz and a measured transmission 3-dB bandwidth of 6.69–8.34 GHz, corresponding to 21.9%.Source: B. RIS Characterization · pp. 6–7
  • The measured phase difference is approximately 150°–195° for reflection states and 134°–176° for transmission states over 6.4–8.8 GHz.The transmission-state separation remains relatively stable and enables practical 1-bit transmission beamforming.Source: B. RIS Characterization · p. 7

C. Near-Field Measurement

Near-field measurements test programmable beamforming across 6.8–8.4 GHz and at multiple steering angles. The measured RIS ON/OFF comparisons show average signal enhancements in both reflection and transmission modes.

C. Near-Field Measurement

  • For reflection, the transmitting horn illuminates the array at normal incidence from about 20 cm, while the receiving horn is about 25 cm away and rotates across observation angles.The short Tx–RIS and RIS–Rx distances are selected to characterize near-field beamforming.Source: C. Near-Field Measurement · p. 8
  • 19.8 dB reflection and 22.6 dB transmission are the average measured signal enhancements from 6.8 to 8.4 GHz.The ON/OFF comparisons are shown in Figs. 18 and 21, respectively.Source: C. Near-Field Measurement · pp. 8–9, Figure 18 · p. 8, Figure 21 · p. 9
  • At six frequencies from 6.8 to 8.4 GHz, the measured 30° reflection beam varies by 2.5 dB and remains above the RIS OFF response.The highest sidelobes are approximately 7.1–9.2 dB below the desired beams.Source: C. Near-Field Measurement · p. 8, Figure 19 · p. 8
  • At 7.6 GHz, changing only the 1-bit phase pattern steers reflection to −30°, −15°, 15°, and 30° with at most 0.8 dB variation in desired-direction S21.The highest sidelobes are approximately 8.5–9.9 dB below the main beams.Source: C. Near-Field Measurement · p. 9, Figure 20 · p. 9
  • At the same six frequencies, the measured 30° transmission beam varies by 2.4 dB, while the RIS OFF responses are substantially weaker.The measured desired-direction S21 levels range from −29.8 to −32.2 dB.Source: C. Near-Field Measurement · p. 9, Figure 22 · p. 9
  • At 7.6 GHz, transmission steering to −30°, −15°, 15°, and 30° yields only 1.0 dB variation in desired-direction S21.The measured sidelobes are approximately 7.5–12.4 dB below the desired beams.Source: C. Near-Field Measurement · p. 10, Figure 23 · p. 10

D. Far-Field Measurement

Far-field measurements show average received-power enhancements of 11.2 dB in reflection and 11.9 dB in transmission across 6.8–8.4 GHz. Measurements at six frequencies confirm performance at the desired 30° direction in both modes.

D. Far-Field Measurement

  • The reflection-mode Tx and Rx antennas were approximately 2 m and 2.3 m from the surface, respectively, placing them in its far-field region over the investigated band.The Rx horn was positioned at the desired 30° reflection direction; for transmission it was moved to the opposite hemisphere while illumination and measurement conditions were kept the same.Source: D. Far-Field Measurement · pp. 10–11, Figure 24 · p. 10, Figure 26 · p. 11
  • 9.9–13.7 dB of received-power enhancement was measured for 30° reflection across six frequencies from 6.8 to 8.4 GHz.The average enhancement was approximately 11.2 dB; Fig. 25 presents the RIS ON and OFF measurements.Source: D. Far-Field Measurement · p. 10, Figure 25 · p. 10
  • 9.7–14.9 dB of received-power enhancement was measured for 30° transmission across six frequencies from 6.8 to 8.4 GHz.The average enhancement was approximately 11.9 dB, with relatively small variation in RIS ON received power; Fig. 27 presents the measurements.Source: D. Far-Field Measurement · p. 11, Figure 27 · p. 11
  • 11.2 dB in reflection and 11.9 dB in transmission were the average far-field received-power enhancements across 6.8–8.4 GHz.The measurements verify programmable wavefront manipulation toward both hemispheres under different propagation conditions.Source: D. Far-Field Measurement · p. 11

E. Comparison and Discussion

Prior RIS designs offered wide bandwidth in only reflection or transmission, while practical STAR-RIS prototypes offered both modes but had limited bandwidth or implementation constraints. The proposed design is presented as a wider-band, embedded-network platform for FR3 applications.

E. Comparison and Discussion

  • Earlier wideband RIS designs supported either reflection or transmission, limiting programmable coverage to one hemisphere rather than both sides.The cited reflection designs, and transmissive design each had this single-mode limitation.Source: E. Comparison and Discussion · p. 11
  • Earlier practical STAR-RIS prototypes had bandwidths of approximately 10% and 8.4%, 1.9% in both modes, or approximately 8.3% in both modes.The discussion also notes exposed biasing, interconnected RF modules, or a roughly 0.7λ profile as implementation constraints; the reported prototypes operated in sub-6-GHz bands.Source: E. Comparison and Discussion · p. 11

TABLE Ⅱ. PERFORMANCE COMPARISON WITH PREVIOUS REPORTED WORKS

The proposed planar STAR-RIS targets future FR3 systems, where wideband operation, low profile, and scalable hardware are desired together. Its measured bandwidths exceed those of the compared STAR-RIS hardware, with RF feeding and DC biasing embedded between ground planes.

TABLE Ⅱ. PERFORMANCE COMPARISON WITH PREVIOUS REPORTED WORKS

  • Earlier compared STAR-RIS prototypes operated in the sub-6-GHz range, while this design targets 7.6 GHz for emerging FR3 applications.The work frames wideband operation, low profile, and scalable hardware as desired properties for practical future-6G STAR-RIS.Source: TABLE Ⅱ. PERFORMANCE COMPARISON WITH… · p. 12
  • The proposed design achieves measured bandwidths of 19.8% in reflection and 21.9% in transmission, providing substantially wider bandwidth than the compared STAR-RIS hardware.Its RF feeding and DC biasing network is embedded between two ground planes to reduce undesired interaction with the external apertures.Source: TABLE Ⅱ. PERFORMANCE COMPARISON WITH… · p. 12

V. CONCLUSION

The paper concludes that its planar STAR-RIS provides 1-bit phase control in reflection and transmission using electrically controlled current reversal. A fabricated 10 × 10 prototype was experimentally validated, supporting its proposed role as a practical wideband platform for 6G FR3.

V. CONCLUSION

  • The four-state polarization-converting unit cell provides 1-bit phase control in both modes through electrically controlled current reversal.The embedded RF/DC network supports a low-profile and scalable implementation.Source: §V · p. 12
  • The fabricated 10 × 10 prototype achieved measured 3-dB bandwidths of 19.8% in reflection and 21.9% in transmission.Near- and far-field measurements confirmed wideband beamforming and programmable control on both sides of the surface.Source: §V · p. 12
  • The authors conclude that the architecture provides a practical wideband STAR-RIS solution for future 6G FR3 deployment.Source: §V · p. 12
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