Mutual Coupling in Dynamic Metasurface Antennas: Foe, but also Friend

TL;DR

This work reframes mutual coupling in dynamic metasurface antennas (DMAs) from a detrimental nuisance to a design asset. By developing a physics-consistent forward model and applying it to chaotic-cavity-backed DMAs, the authors show that stronger inter-element coupling increases the sensitivity of the radiation pattern to DMA configurations, enabling higher-fidelity pattern synthesis via adjoint-based optimization. The findings imply that DMA design should deliberately embrace mutual coupling, accompanied by research into frugal calibration, efficient optimization, and physics-based bounds to guide end-to-end hardware development. The work lays out concrete open questions for practically deploying highly-coupled DMAs, including compact forward models, robust optimization under nonlinearity, and hardware layouts that maximize ergodic coupling. Overall, the paper provides a principled shift in DMA design philosophy with potential for more capable, compact 6G base stations and sensing systems.

Abstract

Dynamic metasurface antennas (DMAs), surfaces patterned with reconfigurable metamaterial elements (meta-atoms) that couple waves from waveguides or cavities to free space, are a promising technology to realize 6G wireless base stations and access points with low cost and power consumption. Mutual coupling between the DMA's meta-atoms results in a non-linear dependence of the radiation pattern on the DMA configuration, significantly complicating modeling and optimization. Therefore, mutual coupling has to date been considered a vexing nuance that is frequently neglected in theoretical studies and deliberately mitigated in experimental prototypes. Here, we demonstrate the overlooked property of mutual coupling to boost the control over the DMA's radiation pattern. Based on a physics-compliant DMA model, we demonstrate that the radiation pattern's sensitivity to the DMA configuration significantly depends on the mutual coupling strength. We further evidence how the enhanced sensitivity under strong mutual coupling translates into a higher fidelity in radiation pattern synthesis, benefiting applications ranging from dynamic beamforming to end-to-end optimized sensing and imaging. Our insights suggest that DMA design should be fundamentally rethought to embrace the benefits of mutual coupling. We also discuss ensuing future research directions related to the frugal characterization of DMAs based on compact physics-compliant models.

Figures (4)

• Figure 1: Architecture and working principle of chaotic-cavity-backed DMA (CCB-DMA). (a) The considered CCB-DMA consists of a quasi-2D cavity made up of two parallel conducting plates and a via fence. It is excited by a single feed from the back; 64 randomly placed reconfigurable meta-atoms at the front leak energy from the cavity to the far field. The meta-atoms are continuously tunable cELC resonators parametrized by varactor diodes (see sleasman2020implementation for details). The via fence has an irregular shape to induce wave chaos. (b) The radiation pattern magnitude at 10 GHz observed 1m in front of the DMA is illustrated for a non-optimized uniform DMA configuration. The corresponding field distribution inside the DMA and the meta-atoms' dipole moments are provided in the insets; magnitude and phase of the complex-valued field distribution are represented by luminance and hue along a constant chroma in the HCL color space, respectively.
• Figure 2: Radiation pattern sensitivity to DMA configuration for different strengths of mutual coupling between meta-atoms. The mutual coupling strength between the meta-atoms is controlled by the number of vias (see Fig. \ref{['fig1']}). The scenario without mutual coupling is obtained by enforcing a "unilateral" approximation under which all couplings except for those from feed to meta-atoms are set to zero. (a-c) Radiation pattern sensitivity (partial derivative) with respect to one meta-atom's configuration. Magnitude and phase of the complex-valued sensitivity are represented by luminance and hue, respectively. The considered meta-atom is indicated. (d-f) Average of the sensitivity magnitude over all meta-atoms and for 1000 different random DMA configurations.
• Figure 3: Average magnitude of the radiation pattern sensitivity $\sigma$ vs. average accuracy $\zeta$ of a linear forward model.$\sigma$ is evaluated based on the partial derivative of the normalized radiation pattern with respect to the configuration of a given meta-atom, averaged over all meta-atoms, 1000 random DMA configurations, and 12 DMA topologies. $\zeta$ is evaluated similarly to a signal-to-noise ratio but using the prediction error of a multi-variable linear regression as "noise" (see Sec. II in rabault2024tacit for technical details), averaged over 12 DMA topologies.
• Figure 4: Radiation pattern synthesis for prototypical beamforming problem as a function of mutual coupling strength. The targeted radiation pattern is displayed in (a). The normalized radiation patterns obtained with an optimized DMA configuration are displayed for (a) no, (b) weak and (c) strong mutual coupling between the meta-atoms. The three considered DMAs in (b-d) are the same as in Fig. \ref{['fig2']}. Note the drastically different colorbar scales in (b-d).