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Reconfigurable Metasurfaces for 6G Mobile Communication

Reconfigurable Metasurfaces for 6G Mobile Communication

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Metamaterials are electromagnetic structures that have the property of producing macroscopic electromagnetic behavior different from that of natural materials through the periodic arrangement of microscopic (electrically small) unit cells. Thus, a metamaterial surface (meta-surface for short) can be designed that reflects a wave incident from a certain angle at a predefined angle that contradicts Snell's law of refraction (see above).

Metasurfaces are therefore coming into focus for applications in the mm-wave range because the detail of microscopic unit cells, especially at small wavelengths, gives them great advantages in beamsteering over other passive arrays such as reflectarrays.

The unit cells, whose periodic composition creates a metasurface, are typically fabricated as a planar structure consisting of a conductor on a dielectric substrate. In the mm-wave domain, an approach using resonant elements is usually employed. In conventional methods, the single cells are designed based on an equivalent circuit. Using the example of a split-ring resonator, the coupling of the electric or magnetic field with the conducting structure can be shown in a simple way [1]. Thus, the capacitor-like structure creates a resonance with a fundamental mode that couples strongly with the electric field, and the surrounding ring creates a coupling to the magnetic field.

• Reconfigurable Intelligent Surfaces

Reconfigurable intelligent surfaces (RIS) are an upcoming technology for wireless communication for realizing the emerging concept of smart radio environments [2]. RIS can be implemented in a variety of ways, including large arrays of low-cost antennas, which are typically half a wavelength apart, and metamaterial-based planar or conformal large areas whose scattering elements have sizes and spacings are in the sub-wavelength domain. Compared to other transmission technologies, e.g., phased arrays, arrays, multi-antenna transmitters and relays, RIS require the largest number of scattering elements, but each of these elements are backed by the fewest and least expensive components. In addition, power amplifiers are not required. For these reasons RIS represents a promising architecture that can be implemented at reduced cost, size, weight, and power.

• Modeling of Anomalous Reflecting Metasurfaces Using Characteristic Modes

In order to be able to utilize the Characteristic Modes for the design of metasurfaces a structure on which the MoM can be calculated has to be defined. Therefore a Unit Cell is modeled as piecewise. To fullfil the aproximation of the local reflection coefficient

$Z_S(x)=\frac{Z_0}{\sqrt{\cos\theta_i\cos\theta_r}}\frac{\sqrt{\cos\theta_r}+\sqrt{\cos\theta_i}e^{j\phi_r(x)}}{\sqrt{\cos\theta_i}-\sqrt{\cos\theta_i}e^{j\phi_r(x)}}$

the width of the patches and the gaps respectevly have be of subwave length domain.

Based on this assumption, the Characterictic Modes (CM) can be used to optimize the structure towards the desired reflection, analyzing the modal farfields and their dedicated modal current densities. In this process the width and length of the patches is shaped in a way to support modal current densities belonging to modal farfield patterns with, directivity towards the desired reflection angle while suppressing current densities with high farfield directivity towards other directions. To evaluate which modal current densities are of greater importance, the modal weighting coefficient can be determined from the total current density of the structure which is excited by an incident wave from a predefined direction. Whereby modal distributions with higher weighting coefficient are contributing more to the total field than others. By repeating this process, the scattered field is optimized with a refelected wave towards the desired direction. [3]

• Project ReMoTe

The cooperative project ReMoTe aims at realizing an intelligent reconfigurable surface whose reflection behavior for radio waves in 6G mobile communications can be dynamically adapted to the necessary requirements. In particular, reconfigurable reflecting surfaces are important for the artificial design of communication channels and can be seen as efficient passive repeater. The required passive surface is developed by the Institute of Microwave and Wireless Systems in close coordination with the project partners from industry.

The project ReMoTe is funded by the Federal Ministry of Education and Research. Protection under grant 16KIS1634 within the Innovationspotential klein und mittlere Unternehmen (KMU).

• References

[1] D. Schurig, J. J. Mock, and D. R. Smith, “Electric-field-coupled resonators for negative permittivity metamaterials,” Appl. Phys. Lett.,vol. 88, no. 4, 2006, Art. no. 041109.

[2] M. Di Renzo et al., "Smart Radio Environments Empowered by Reconfigurable Intelligent Surfaces: How It Works, State of Research, and The Road Ahead," in IEEE Journal on Selected Areas in Communications, vol. 38, no. 11, pp. 2450-2525, Nov. 2020, doi: 10.1109/JSAC.2020.3007211.

[3] A. Hoffmann, D. Manteuffel, “Investigation of Modeling of Anomalous Reflecting Metasurfaces Using Characteristic Modes” 2022 International Workshop on Antenna Technology: Small Antennas, Novel EM Structures and Materials, and Applications (iWAT), 2022

Corresponding Lectures

Axel Hoffmann, M. Sc.
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