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A Reconfigurable Intelligent Surface at mmWave based on a binary phase tunable metasurface
Jean-Baptiste Gros, Vladislav Popov, Mikhail A. Odit, Vladimir Lenets, Geoffroy Lerosey
TL;DR
The paper addresses how to realize a low-complexity RIS that adaptively controls mmWave reflections for future wireless environments. It designs and experimentally validates a PIN-diode binary-phase metasurface, including its unit cell, complete 10 cm × 10 cm array, and near- and far-field configurations. The RIS controls mmWave beams and restores a blocked wireless path, with reported gains obtained through PIN-diode power dissipation.
Problem
Future 6G wireless systems need channel-level adaptation, while RIS offers a route to smart environments without increasing the number of signal sources.
Method
The paper designs, models, fabricates, and experimentally characterizes a PIN-diode-controlled binary-phase mmWave metasurface and its complete RIS in near- and far-field configurations.
Results
The RIS experimentally controls mmWave beams in near- and far-field configurations, including a 25 dB gain in a blocked-path access-point-extender scenario.
Takeaways & Limitations
The binary RIS provides experimentally demonstrated mmWave reflection control for reflectarray and access-point-extender use cases.
Abstract
from arXiv · showhide
Originally introduced in the early 2010's, the idea of smart environments through reconfigurable intelligent surfaces (RIS) controlling the reflections of the electromagnetic waves has attracted much attention in recent years in preparation for the future 6G. Since reconfigurable intelligent surfaces are not based on increasing the number of sources, they could indeed pave the way to greener and potentially limitless wireless communications. In this paper, we design, model and demonstrate experimentally a millimeter wave reconfigurable intelligent surface based on an electronically tunable metasurface with binary phase modulation. We first study numerically the unit cell of the metasurface, based on a PIN diode, and obtain a good phase shift and return loss for both polarizations, over a wide frequency range around 28.5 GHz. We then fabricate and characterize the unit cell and verify its properties, before fabricating the whole 10 cm x10 cm reconfigurable intelligent surface. We propose an analytical description of the use that can be done of the binary phase RIS, both in the near field (reflectarray configuration) and in the far field (access point extender). We finally verify experimentally that the designed RIS works as expected, performing laboratory experiments of millimeter wave beamforming both in the near field and far field configuration. Our experimental results demonstrate the high efficiency of our binary phase RIS to control millimeter waves in any kind of scenario and this at the sole cost of the energy dissipated by the PIN diodes used in our design.
I. INTRODUCTION
The paper frames RIS as a missing channel-level component for adaptive, greener 6G wireless environments and proposes a low-complexity mmWave binary-phase implementation.
- RIS can modify the wireless channel in real time without increasing the number of signal sources.
- Smart environments adapt electromagnetic reflectiveness to improve wireless signal distribution according to changing needs.
- The proposed RIS uses an electronically tunable binary-phase metasurface to control electromagnetic-wave reflections around 30 GHz.
- Its unit cell uses a capacitively coupled parasitic resonator loaded with a PIN diode to control reflection phase.
A. Unit cell design
The unit cell combines a grounded patch with a PIN-diode-controlled parasitic resonator to switch the reflection phase while limiting dissipation.
- The unit cell is a copper square patch on a grounded low-loss dielectric substrate, designed for a patch resonance near 27.5 GHz.
- The simulated unit-cell reflection is compared between OFF and ON states, with dashed lines marking the operating frequency range.
- Binary operation requires maximum reflected amplitude and two reflection states separated by a π-phase difference.
- A split parasitic resonator loaded with a PIN diode changes mutual coupling with the patch, shifting resonance and reflection phase.
- Across the operating range, the phase difference exceeds 160 degrees while reflection amplitude remains above -5 dB.
B. Unit cell measurements
The fabricated pixel was characterized inside a WR-34 waveguide using controlled PIN-diode states and measured reflection coefficients.
- A WR-34 rectangular waveguide was used to characterize the reflection properties of the fabricated single pixel.
- The pixel was sized to the waveguide aperture and surrounded by vias that isolate the electromagnetic field and limit substrate leakage.
- A vector network analyzer measured reflection while a constant-voltage source controlled the diode states.
C. Reconfigurable intelligent surface fabrication
The fabricated RIS is a 10 × 10 cm2, 20 × 20-pixel array with independently controlled polarization states and FPGA-based diode control.
- The RIS contains 400 unit cells arranged on a 20 × 20 lattice with λ/2 = 0.5 cm spacing at 30 GHz.
- Independent control of the two reflected-field polarizations uses 400 vertical-polarization diodes and 400 horizontal-polarization diodes.
- The six-layer PCB combines a low-loss first layer carrying the unit cells with five FR4 layers supporting electronics.
- A 10 × 10 shift-register matrix and FPGA board independently set diode states through LAN or USB-controlled Python or MATLAB software.
III. ANALYTICAL MODEL OF BINARY PHASE RIS
The analytical model assigns binary phase states to RIS pixels from the incident and desired reflected phase distributions, then estimates the resulting radiation pattern.
- The RIS radiation pattern is approximated as that of an array of patch-like antennas weighted by each pixel’s local reflection and incident-field response.The model includes the individual-element factor cos(θ), local reflection coefficient Γ_nm, incident field E_i(x_n, y_m), and pixel viewing angle θ_nm.
- PIN-diode states set each pixel’s local reflection phase to either 0 or π for each polarization.The binary phase state is selected from the phase difference between the desired reflected and incident waves.
- The desired phase profile is determined by the difference between reflected and incident phase distributions at each pixel.For phase differences from −π/2 to π/2 the state is 0; otherwise it is π.
- The model treats near-field illumination as spherical and assumes the receiving horn remains far from the RIS.The two cases are distinguished by the Tx distance relative to the RIS size, while D_Rx ≫ D_m is assumed.
A. Tx antenna is close to RIS: Near-field configuration
In the near-field configuration, a nearby Tx horn produces spherical illumination while the RIS is configured to reflect a plane wave toward a chosen Rx direction.
- The near-field scenario models the Tx antenna as radiating a spherical wave and the desired reflected field as a plane wave toward the Rx antenna.The Rx direction is specified by θ_Rx and ϕ_Rx.
- The Tx antenna coordinates are expressed through its distance and spherical angles when constructing the near-field phase profile.The Cartesian coordinates are z_Tx = D_Tx cos(θ_Tx), x_Tx = D_Tx sin(θ_Tx) cos(ϕ_Tx), and y_Tx = D_Tx sin(θ_Tx) sin(ϕ_Tx).
- 27.9 dBi is the modeled directivity at θ_Rx = 30°, decreasing to 25.2 dBi at θ_Rx = 60°.The corresponding maximum possible directivity for a uniform aperture of the same area is approximately 30.5 dBi.
- The modeled directivity falls with steering angle because the physical aperture decreases, binary phase states create secondary lobes, and spherical illumination is nonuniform.These three factors explain the difference from the uniform-aperture maximum.
B. Tx antenna is far from RIS: Far-field configuration
In the far-field configuration, plane-wave illumination produces a periodic binary phase profile that steers energy but cannot fully control the radiation pattern.
- The far-field case approximates the incident field as a plane wave because the Tx antenna is much farther from the RIS than the RIS size.
- The analytical examples evaluate steering toward θ_Rx = 30°, 45°, and 60° with ϕ_Rx = 0°.For the spherical-illumination examples, the Tx antenna is at D_Tx = 145 mm, θ_Tx = 45°, and ϕ_Tx = 270°.
- A periodic binary phase profile splits a normally incident plane wave into two symmetrical beams and produces a strong undesired lobe.Only two phase states cannot accurately approximate the periodic phase profile.
- Plane-wave illumination achieves aperture efficiency of 1, so modeled directivities remain close to those of the corresponding near-field configurations despite the secondary lobe.
IV. EXPERIMENTAL RESULTS
The experiments test the 10 cm by 10 cm RIS in near-field reflectarray and far-field radiation-pattern configurations.
- The experiments address RIS placement in both the Tx antenna’s near field and far field.
- The near-field experiment uses a 10 cm by 10 cm RIS in a reflectarray setup, with radiation patterns measured on an elevation-azimuth rotation stage.
- The far-field measurement places the reflectarray setup before a fixed horn antenna to measure its radiation pattern.
- Frequency responses and normalized far-field radiation patterns are measured for four steering angles under initial and optimized RIS configurations.The steering angles are θ_Rx = 0°, 30°, 45°, and 60° with ϕ_Rx = 0°.
A. Reflectarray setup: RIS in Near-field configuration
The RIS is experimentally configured as a near-field reflectarray and optimized pixel by pixel to steer millimeter-wave beams across a broad angular range.
- A. Reflectarray setup: RIS in Near-field configuration: A horn antenna illuminates the RIS from 145 mm away, and the assembly is measured on an elevation-azimuth rotation stage in an anechoic chamber.The feed horn is tilted 45° to reduce shading.
- A. Reflectarray setup: RIS in Near-field configuration: The optimization tests each pixel’s four PIN-diode combinations and retains the state maximizing the measured transmission amplitude |S12| at 27.5 GHz.This iterative procedure is repeated for all pixels and for each relative angular position.
- A. Reflectarray setup: RIS in Near-field configuration: 30 dB: optimized transmissions at the optimized frequency exceed the initial configuration for each tested azimuth angle.The comparison uses frequency responses for initial and optimized RIS configurations.
- A. Reflectarray setup: RIS in Near-field configuration: The measured radiation patterns agree with the numerical model and demonstrate beam steering up to 60° optimization angles.The optimized configurations produce patterns for multiple azimuth angles and elevation angle θ0 = 0°.
- A. Reflectarray setup: RIS in Near-field configuration: A 2.5 dB higher main-beam field is obtained experimentally, while first sidelobes remain about −13 dB as expected for a rectangular aperture.The optimization can account for manufacturing imperfections and slight experimental misalignment.
- A. Reflectarray setup: RIS in Near-field configuration: The beam is steered over a 120° range with 0.2° precision, although increasing steering angles broaden the beam and reduce gain.The effective geometric aperture decreases at higher optimization angles.
B. RIS in access-point extender configuration
In an obstructed wireless link, the RIS is evaluated as a far-field access-point extender that redirects millimeter waves around a right-angle barrier.
- B. RIS in access-point extender configuration: The experiment places the RIS between horn antennas separated by a right-angle barrier, with each barrier arm 1 m long and no direct line of sight.The setup is outside the anechoic chamber and places the RIS substantially farther from the antennas than the near-field experiment.
- B. RIS in access-point extender configuration: The RIS can be treated as being in an antenna’s far-field when that antenna’s field is well approximated by a plane wave at the RIS position.For the approximately 17 dBi horn antennas used here, the far-field zone is around 15 cm.
- B. RIS in access-point extender configuration: Replacing the tilted metallic plate with an optimized tilted RIS changes the measured transmission in the blocked-link scenario.Scenario I uses the metallic plate reference, while scenario II uses the optimized RIS over a 2 GHz range around 27.5 GHz.
- B. RIS in access-point extender configuration: 25 dB gain: the RIS restores wireless transmission around the barrier at the cost of PIN-diode power dissipation.The reported average dissipation is 8 W for the whole metasurface, reduced to 4 W in a later version.
- B. RIS in access-point extender configuration: A random RIS configuration can also serve as a benchmark, but its result depends on the chosen configuration and requires averaging over many configurations.The experiment itself uses a metal plate as the reference.
V. CONCLUSION
The paper presents a binary Ka-band RIS for both near-field reflectarray operation and far-field access-point extension, with low reflection loss, near-π phase switching, and wide instantaneous bandwidth.
- V. CONCLUSION: The binary RIS operates in the transmitting spectrum of the Ka-band and is characterized by low reflection-wave dissipation, near-π phase difference, and wide instantaneous bandwidth.The paper identifies these as key performance parameters of the single pixel.
- V. CONCLUSION: The demonstrated RIS supports two configurations: a reflectarray illuminated by a closely positioned horn and an access-point extender for obstructed links.In the extender configuration, the RIS amplifies the signal between transmitter and receiver when no line of sight originally exists.
- V. CONCLUSION: Future work will integrate the RIS into a Software-Defined-Radio communication system and examine link robustness to power noise potentially introduced by the RIS.This is identified as a planned direction rather than a demonstrated result.