⚡ electrical engineering

Radar Sensing using Dual-Beam Reconfigurable Intelligent Surface

This paper investigates how practical phase quantization impacts Reconfigurable Intelligent Surface (RIS) performance in around-the-corner radar sensing by comparing an idealized analog phase-shift configuration with a low-complexity one-bit quantized dual-beam setup through both simulations and measurements.

Original authors: Kainat Yasmeen, Shobha Sundar Ram, Debidas Kundu

Published 2026-02-13

📖 5 min read🧠 Deep dive

CC BY 4.0

Original authors: Kainat Yasmeen, Shobha Sundar Ram, Debidas Kundu

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine you are trying to spot a friend hiding behind a tall wall in a busy city. You have a flashlight (your radar), but you can't see around the corner. In the old days, you'd have to rely on the light bouncing off the wall like a mirror. But mirrors only reflect light back at the same angle it came in. If your friend is standing in a "shadow" spot where the mirror reflection doesn't reach, they remain invisible.

This paper introduces a clever new tool called a Reconfigurable Intelligent Surface (RIS) to solve this problem. Think of an RIS not as a solid wall, but as a giant, high-tech smart mirror made of thousands of tiny, programmable tiles.

Here is the breakdown of what the researchers did, using simple analogies:

1. The Problem: The "Mirror" Limitation

Standard walls act like static mirrors. If you shine a flashlight at a wall at a 30-degree angle, the light bounces off at a 30-degree angle. If your target is standing at a 60-degree angle, the wall ignores them. This is called specular reflection. It works great if you know exactly where to look, but terrible for finding things hiding in unexpected spots.

2. The Solution: The "Smart Mirror" (RIS)

The researchers built a special surface (the RIS) that can change how it reflects signals. Instead of being a passive wall, it's an active team of tiny tiles.

The Analogy: Imagine a choir. In a normal wall, everyone sings the same note at the same time, creating a single loud sound in one direction. In an RIS, the conductor (the computer) can tell each singer to start slightly earlier or later. By doing this, the sound waves can be steered to sing loudly in a different direction, effectively "bending" the light around the corner.

3. The Two Approaches: The "Perfectionist" vs. The "Budget-Friendly"

The paper compares two ways to program these tiles:

Approach A: The Ideal Single-Beam (The Perfectionist)
- How it works: The computer calculates the exact perfect timing for every single tile to create one perfect beam pointing exactly where you want.
- The Catch: In the real world, it's very hard to build hardware that can do infinite, perfect adjustments. It's like trying to tune a guitar string to a frequency so precise that it requires a microscope to see the difference. It's theoretically perfect but practically difficult.
Approach B: The Dual-Beam (The Budget-Friendly)
- How it works: To make it simple and cheap, the researchers decided to only give the tiles two options: "ON" or "OFF" (or 0 degrees and 180 degrees). This is called one-bit quantization.
- The Magic Trick: Because they can only use two settings, the physics forces the surface to create two beams instead of one. One beam points to the left, and a "twin" beam points to the right.
- The Benefit: Even though it's a "simpler" system, it's actually better for radar! Why? Because it covers two targets at once. If you have two friends hiding behind the wall, one on the left and one on the right, this "dual-beam" mirror catches both of them simultaneously.

4. The Experiment: Testing the Mirror

The team tested this in two ways:

Computer Simulation: They ran complex math models to see how the beams would behave.
Real-World Test: They built a physical prototype (a panel with 160 tiles) and used a radar system to bounce signals off it.

What they found:

The Metal Plate: When they used a plain metal sheet, the signal only bounced back at the "mirror angle." If the target wasn't there, the radar saw nothing.
The RIS: When they used their smart surface, they could steer the signal to hit targets that were completely hidden from the radar's direct view.
The Trade-off: The "Dual-Beam" (simple) version was slightly less powerful than the "Single-Beam" (perfect) version because the energy was split between two directions. However, it was much easier to build and was excellent at spotting multiple targets at once.

5. Why This Matters

This technology is like giving self-driving cars "super-vision."

Current Tech: A car's radar can only see what's directly in front of it or what bounces off a wall at a perfect angle.
Future Tech: With these smart surfaces installed on buildings, a car could "look" around a blind corner, detect a pedestrian stepping out from behind a bus, or spot a cyclist in a side alley, all without needing a direct line of sight.

The Bottom Line

The paper proves that by using a simple, low-cost "on/off" switch for thousands of tiny tiles, we can create a smart surface that acts like a magic mirror. It can bend radar waves around corners to find hidden objects, and it does so well enough to spot two targets at the same time, making it a game-changer for safety and surveillance in cities.

1. Problem Statement

Radar sensing in Non-Line-of-Sight (NLOS) scenarios, such as "around-the-corner" detection, relies on multipath scattering along walls to detect targets blocked from the radar's direct line of sight. However, traditional wall reflections are limited to specular angles (where the angle of incidence equals the angle of reflection). Consequently, radar performance deteriorates significantly when targets are located in non-specular regions.

While Reconfigurable Intelligent Surfaces (RIS) have been proposed to steer beams toward non-specular angles to enhance coverage, practical implementation faces challenges:

Hardware Constraints: Real-world RIS unit cells cannot achieve continuous, ideal analog phase shifts. They are limited to discrete, quantized phase values.
Complexity vs. Performance: High-resolution phase control is complex and costly. There is a need to evaluate low-complexity solutions (e.g., 1-bit quantization) for radar applications.
Reciprocity: Radar involves two-way propagation (transmit to target and back). The reciprocity of RIS scattering in forward and reverse paths under quantized conditions is not well understood.
Multi-Target Detection: Unlike point-to-point communications, radar must detect multiple uncooperative targets simultaneously, requiring the ability to steer multiple beams with low algorithmic and hardware complexity.

2. Methodology

The authors propose and evaluate a Dual-Beam RIS configuration using one-bit quantized phase shifts for radar sensing.

Theoretical Framework:
- The study models a 2D monostatic radar system with a 1D RIS array ( $N$ unit cells).
- Ideal Single-Beam: Uses generalized Snell's law with continuous phase shifts ( $\zeta_n$ ) to steer a main lobe to a desired angle $\phi_d$ .
- Dual-Beam (Proposed): Uses 1-bit quantization where the phase shift $\zeta_n$ is restricted to two values ( $0^\circ$ or $180^\circ$ ) based on the sign of the ideal phase. This creates two symmetric beams at angles $+\phi_d$ and $-\phi_d$ .
- RCS Calculation: The Radar Cross-Section (RCS) is computed for both the forward path (Radar $\to$ RIS $\to$ Target) and reverse path (Target $\to$ RIS $\to$ Radar) to evaluate reciprocity.
Simulation Setup:
- Environment: 2D simulation at 5.5 GHz.
- Configuration: Monostatic radar at $(-2.5, 4.3)$ m; RIS with 16 elements centered at $(-3, 2.7)$ m.
- Targets: Two targets performing circular motion with different angular velocities to generate distinct micro-Doppler signatures.
- Comparisons: The RIS performance is benchmarked against:
  1. A passive metal plate of identical dimensions.
  2. An idealized single-beam RIS (continuous phase).
  3. The proposed 1-bit dual-beam RIS.
Experimental Setup:
- Hardware: A custom-designed RIS ( $16 \times 10$ unit cells, $256 \times 160$ mm) using PIN diodes for 1-bit switching (ON/OFF states with $180^\circ \pm 20^\circ$ phase difference).
- Instrumentation: A Vector Network Analyzer (VNA) acting as a narrowband radar (5.5 GHz, +3 dBm) and a Spectrum Analyzer for receiving signals.
- Procedure: Measurements were taken for forward and reverse scattering at various incident ( $\phi_i$ ) and desired ( $\phi_d$ ) angles.

3. Key Contributions

Dual-Beam RIS Architecture: Introduction of a low-complexity RIS configuration using 1-bit quantization that naturally generates two symmetric beams, enabling simultaneous detection of targets in two different angular sectors without complex beam-steering algorithms.
Reciprocity Analysis: A rigorous evaluation of the reciprocity of RIS scattering. The study demonstrates that while reciprocity holds at specular angles, it deviates at non-specular angles due to practical phase quantization errors and hardware limitations.
Benchmarking: Comprehensive comparison of RIS performance against passive metal plates and idealized RIS, highlighting the trade-offs between beam steering capability, RCS magnitude, and hardware complexity.
Experimental Validation: Provision of real-world measurement data validating the simulation results, including beam squint error analysis and RCS measurements.

4. Key Results

Beam Steering Capability:
- Metal Plate: Only reflects at the specular angle ( $\phi_r = \phi_i$ ), failing to detect targets in non-specular regions.
- Ideal Single-Beam RIS: Successfully steers the main lobe to desired angles but suffers from grating lobes at certain angles and requires complex hardware.
- Dual-Beam RIS: Successfully generates two beams at $\pm \phi_d$ . While it exhibits a slight steering error (beam squint) and higher grating lobes compared to the ideal case, it effectively covers a wider angular sector.
Radar Cross-Section (RCS):
- Specular Angles: The RIS behaves similarly to a metal plate.
- Non-Specular Angles: The RIS provides significantly higher RCS (up to 20+ dB improvement over metal) because it directs energy to the target, whereas the metal plate only radiates weak sidelobes.
- Dual-Beam Trade-off: The dual-beam RIS has an RCS 3–5 dB lower than the single-beam RIS because the incident power is split between two lobes.
- Reciprocity Deviation: At non-specular angles, the forward RCS ( $\sigma_f$ ) and reverse RCS ( $\sigma_r$ ) are not identical, indicating a breakdown of strict reciprocity in practical 1-bit RIS implementations.
Target Detection (Micro-Doppler):
- In a scenario with two rotating targets, the dual-beam RIS successfully detected both targets with clear micro-Doppler signatures.
- The single-beam RIS only clearly detected the target aligned with its beam; the second target appeared significantly weaker.
- The metal plate failed to provide a clear detection of either target in the non-specular configuration.
Experimental vs. Simulation:
- Experimental results showed a 1° beam squint error (measured $46^\circ$ vs. desired $45^\circ$ ).
- Measured RCS values were approximately 2 dB lower than simulations due to ohmic losses in the PIN diodes and system losses (cables/adapters).

5. Significance and Conclusion

This work demonstrates that low-complexity, 1-bit quantized RIS is a viable and effective solution for NLOS radar sensing, particularly for multi-target scenarios.

Practicality: The dual-beam approach eliminates the need for complex, high-resolution phase control hardware, making RIS deployment more feasible for radar applications.
Performance: Despite the power splitting penalty (lower RCS per beam), the dual-beam RIS offers superior coverage and detection capability compared to passive walls or single-beam systems in NLOS environments.
Future Directions: The authors suggest future work should focus on minimizing grating lobes in 1-bit metasurfaces using prephasing techniques and further experimental validation of target detection algorithms.

In summary, the paper establishes that RIS-enhanced radar can overcome the limitations of specular reflection, and specifically, that a simple, low-cost 1-bit dual-beam configuration offers a robust trade-off between hardware complexity and multi-target detection performance.