Compact Reconfigurable Intelligent Surface with Phase-Gradient Coded Beam Steering and Controlled Substrate Loss

Abstract: This paper presents a 1-bit reconfigurable intelligent surface (RIS) fabricated using a three-layer structure. It employs a manual layer stackup incorporating an optimal air gap to reduce the effective dielectric losses while using a low-cost FR4 substrate. The new design of the unit cells of the proposed RIS is outlined, with each unit cell featuring a PIN-diode-based, compact, simplified biasing network that simplifies the control circuit while maintaining distinct $\boldsymbol{0^\circ/180^\circ \pm 20^\circ}$ phase states between ON/OFF conditions. The designed RIS is in the form of a $\boldsymbol{10\times10}$ array with a compact size of $\boldsymbol{2.9λ_g \times 2.9λ_g}$. Additionally, a phase-gradient coding scheme is presented and utilized that achieves measured beam steering up to $\boldsymbol{\pm30^\circ}$ in both anechoic and noisy environments. Controlled and driven by an Arduino-cum-digital interface, the proposed RIS exhibits measured reflected wave gain enhancement of about 9\,dB over an incident wave angular range of $\boldsymbol{\pm 30^\circ}$. Furthermore, the design is also experimentally validated by transmitting quadrature phase-shift keying-modulated symbols via the RIS-assisted wireless channel. The proposed RIS works for the range 3.38--3.67\,GHz (8.3\%), and is suitable for deployment for the 5G n78 \mbox{band (3.5\,GHz).}

• The paper presents a compact 1-bit RIS architecture that mitigates dielectric loss through an innovative FR4-air gap design and optimized phase control.
• The experimental results show a 320° phase tuning range, beam steering up to ±30° in azimuth, and an approximate 9 dB gain improvement.
• The design features a low-power biasing network with integrated digital control, offering scalable and cost-effective deployment for modern wireless systems.

Compact 1-Bit Reconfigurable Intelligent Surface with Phase-Gradient Coded Beam Steering and Controlled Substrate Loss

Introduction and Motivation

The paper presents a comprehensive design, fabrication, and experimental demonstration of a 1-bit reconfigurable intelligent surface (RIS) focused on practical wireless communication deployments at the sub-6 GHz bands, specifically 3.5 GHz for 5G n78. A key research emphasis is on synthesizing a compact RIS array leveraging low-cost FR4 substrates with tailored material stackup and air gap to alleviate the high dielectric loss, which historically makes FR4 unattractive for frequencies above 3 GHz. Existing RIS implementations either rely on expensive low-loss dielectrics, complex multi-bit control circuitry, or suffer from high hardware complexity, large aperture footprints, and limited focus on substrate/biasing network optimization. This paper proposes a holistic architecture that addresses these limitations.

Architectural and Electromagnetic Design

The RIS is realized as a $10 \times 10$ array of novel multilayer unit cells, each integrating a radiating patch, ground, and biasing network layers. The unit cell architecture incorporates:

• Two FR4 substrate layers separated by a precisely-controlled 0.5 mm air gap, which substantially reduces RF loss via reduction of the effective loss tangent, as characterized by a loss participation ratio (LPR)-based analysis.
• A circular patch with symmetric slots for polarization-independent, stable phase response, and resonance tuning.
• Optimized via placement, determined by Floquet-mode current distribution analysis, enabling maximal phase control with minimal conductivity degradation.

Simulation shows that the phase tuning range reaches 320°, and $|S_{11}|$ is between $-4.9$ dB and $-5.7$ dB for ON/OFF states in the targeted band (3.38–3.67 GHz, fractional bandwidth 8.3%), indicating efficient radiative performance despite FR4 usage.

Biasing and Digital Control Network

Each unit cell incorporates a PIN diode (Skyworks SMP1345-079LF) biased via a compact RF-DC network. The topology employs minimal parasitics, dual-path choke/capacitors for RF/DC isolation, and a radial stub to ensure broadband operation and low DC power (≤0.05 W/cell). Digital phase configuration is controlled by a microcontroller (Arduino ATmega2560) interfaced through cascaded shift registers, achieving fully independent cell switching with minimal hardware fanout.

Phase-Gradient Coding and Beam Steering

Beam steering is achieved through quantized phase-gradient coding. Far-field steering principles are analytically derived, mapping desired linear phase profiles to 1-bit (0/π) RIS states, with quantization error minimized by incorporating a correction term in the mapping function.

The experimental prototype exhibits robust, programmable beam steering up to ±30° in azimuth, verified in both anechoic and noisy environments. Experimental main-lobe direction deviates by ±3.75° on average compared to simulation, demonstrating precise control in practical conditions.

Experimental Characterization

The RIS is assessed using a bi-static setup with patch antennas and a vector network analyzer, with the RIS interposed to force all Rx energy through controlled reflection. Key results include:

• Main-lobe gain improvement of ~9 dB over an FR4 copper plate for the steered direction.
• Normalized radiation patterns reveal clear, code-dependent main lobes consistent between simulation and measurement across the beam steering range.
• The array maintains compactness ($2.9\lambda_g \times 2.9\lambda_g$), a significant reduction in aperture area compared to typical RISs at similar frequencies.

Comparison to State of the Art

Relative to other recent RIS hardware, the proposed design demonstrates:

• Comparable or superior phase range and gain with a substantially reduced aperture size.
• Simpler, scalable biasing via integrated digital control and low-parasitic DC network, whereas most competitors rely on complex FPGA circuits or larger arrays.
• Maintenance of effective beam steering over the band of interest, despite FR4-induced dielectric losses, due to the air gap and optimized unit cell EM characteristics.

RIS-Enabled QPSK Communication: System-level Implications

A USRP-2901 SDR platform is used to realize QPSK transmission through a RIS-assisted link. The channel with RIS exhibits increased effective channel gain, yielding QPSK constellations with improved symbol separation and tighter clusters—empirically corroborating the analytical prediction that larger $|h_{\text{eff}}|$ reduces post-equalization noise and enhances symbol detection reliability. This validates the RIS as a practical enabler for robust higher-order modulation schemes in signal-challenged or multipath-rich environments.

Implications and Prospects

This work demonstrates that the integration of controlled substrate engineering—specifically, the air gap/FR4 hybrid layer, advanced biasing design, and phase-gradient coded steering—enables efficient, low-cost, and compact RIS hardware suitable for immediate 5G/B5G wireless test beds. The tradeoff between fabrication complexity, cost, and radiative performance is judiciously balanced and validated through comprehensive measurement.

On a theoretical front, the emphasis on Floquet-based EM analysis and LPR-driven loss mitigation provides replicable methodologies for future RIS miniaturization below 4 GHz. The practical coding and MCU control schemes open scalable reconfigurability without the escalating control complexity seen in multi-bit/FPGA solutions.

Looking forward, this RIS platform is extendable to higher frequency millimeter-wave (mmW) bands, larger array tiling, and the implementation of more advanced phase quantization/codebooks. It also serves as a foundation for AI-driven RIS-channel adaptation, integrated sensing, and smart radio environments.

Conclusion

The paper provides a complete solution for compact, low-complexity RIS with effective phase-gradient beam steering and controlled loss, validated in both EM and communication system terms. The approach bridges the theoretical–practical gap in RIS deployments at sub-6 GHz, and sets a reference for future scalable, programmable metasurface designs targeting modern wireless infrastructure (2604.04625).