Breakthrough metasurface acts as power-free RF filter
Researchers in Japan have developed a passive metasurface that can tackle multipath signal interference without the need for a powered filter.
The time-varying interlocking passive metasurface developed at the Nagoya Institute of Technology transmits the first signal while blocking delayed ones from other angles without power or processing. This enables low-cost, reliable wireless communication in IoT applications and environments that are prone to interference.
A proof-of-concept enhanced the magnitude of the first incoming signal by approximately 10 dB while successfully suppressing subsequent waves, regardless of their arrival direction. This represents the first passive filtering design capable of overcoming the two physical limitations imposed by signals with the same frequency and variable incident angles.
Engineers face growing challenges from multipath propagation 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’ (or 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.
The team at Nagoya 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.
“Our proposed working mechanism is totally different from previously reported designs,” says Associate Professor Hiroki Wakatsuchi who led the team “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.”
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.
Unlike existing hardware methods based on adaptive arrays, this 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.
“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,” says 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.”
10.1103/PhysRevLett.134.157001; www.nitech.ac.jp/eng/index.html
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