Smart surfaces could represent a powerless solution to multipath signal interference

The evolution of wireless communications and the miniaturization of electrical circuits have fundamentally reshaped our lives and the digital landscape. However, as we push toward higher-frequency communications in an increasingly connected world, engineers face growing challenges from multipath propagation—a phenomenon where the same radio signal reaches receiving antennas through multiple routes, usually with time delays and altered amplitudes.

Multipath interference leads to many reliability issues, ranging from “ghosting” in television broadcasts to signal fading in wireless communications.

Addressing multipath interference has long presented two fundamental physical challenges. First, multipath signals share the same frequency with the main (leading) signal, rendering conventional frequency-based filtering techniques ineffective. Second, the incident angles of these signals are variable and unpredictable. These limitations have made passive solutions particularly difficult to implement, as traditional materials bound by linear time-invariant (LTI) responses maintain the same scattering profile for a given frequency, regardless of when the signal arrives.

Moreover, without active control systems requiring additional power resources, the angular dependence of conventional filters remains fixed at any given frequency.

Against this backdrop, a research team led by Associate Professor Hiroki Wakatsuchi from Nagoya Institute of Technology, Japan, has developed a groundbreaking approach to overcome these limitations. Their paper, co-authored by Shunsuke Saruwatari of Osaka University and Kiichi Niitsu of Kyoto University, is published in Physical Review Letters .

The team designed a passive metasurface-based filtering system that breaks free from LTI constraints through an innovative time-varying interlocking mechanism. The design incorporates metasurface panels with internally coupled circuit elements, including metal-oxide-semiconductor field-effect transistors (MOSFETs). The proposed system, which acts as a shield, can selectively allow only the first incoming wave to pass through while rejecting time-delayed signals from other angles—all without requiring active biasing or control systems.

The key innovation lies in how the metasurface creates a time-varying response without active components. Each unit cell, positioned on a panel facing a particular direction, contains a MOSFET that acts as a dynamic switch, creating either an open circuit point or a short circuit depending on the transistor’s gate-source voltage. When the first signal arrives, it maintains the metasurface panel’s resonance to strongly transmit the incoming signal.

However, this first signal also triggers changes in the internal circuit configuration of unit cells in other panels, effectively altering the spatial impedance to reject subsequent signals from different angles.

This mechanism was demonstrated via both simulations and experiments using a hexagonal prism structure with two interlinked metasurface unit cells and a receiver positioned within the prism. Adjacent sides of the prism each received signals from different transmitters with a time delay, simulating a realistic multipath scenario.

In their proof-of-concept experiments, the researchers demonstrated that their approach enhanced the magnitude of the first incoming signal by approximately 10 dB while successfully suppressing subsequent waves, regardless of their arrival direction. This breakthrough represents the first passive filtering design capable of overcoming the two physical limitations imposed by signals with the same frequency and variable incident angles.

“Our proposed working mechanism is totally different from previously reported designs,” remarks Wakatsuchi. “This approach has advantages over conventional techniques, since ours does not require many calculations and modulation/demodulation circuits. Thus, it is suitable for low-cost application scenarios such as IoT devices.”

Additionally, unlike existing hardware methods based on adaptive arrays, this innovative strategy does not require additional direct current energy sources. While the current prototype uses simplified antenna designs and commercial diode products, the team believes performance can be further enhanced through advanced semiconductor technologies and optimized configurations.

It is worth noting that beyond addressing multipath issues, this interlocking approach shows promise for autonomously controlling various types of electromagnetic devices, potentially revolutionizing wireless communication systems for IoT applications where computational resources are limited.

“The concept of our passive filter design can potentially create new kinds of next-generation radio-frequency devices and applications, including antennas, sensors, imagers, and reconfigurable intelligent surfaces,” explains Wakatsuchi. “In particular, our passive interlocking solution finds effective applications in versatile, low-cost communication devices that are unable to adopt conventional modulation- or signal-processing-based approaches due to their large computational resources and expensive costs.”

With continued advancements in wireless communication technology, the world may become increasingly interconnected.