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Reducing Simulation Effort for RIS Optimization using an Efficient Far-Field Approximation

Hans-Dieter Lang, Michel A. Nyffenegger, Heinz Mathis, Xingqi Zhang

TL;DR

The paper tackles the high computational cost of RIS optimization, which relies on multiport impedance or scatter matrices for every Tx/Rx position. It introduces a far-field extrapolation that builds the full $(N+2)\times(N+2)$ S-matrix from a single RIS simulation by modeling Tx–RIS and RIS–Rx couplings as far-field two-port links, using distances $d_{Tx-m}$, angles $\gamma_{Tx-m}$, and gains $G_m(\gamma)$ in the expressions for $S_{Tx-m}$ and $S_{Rx-m}$. The approach yields optimized capacitance values that closely match those from full-wave simulations and is corroborated by BRCS measurements, demonstrating practical validity. By reducing the need for repeated full-wave runs, the method can significantly speed RIS design and optimization in realistic NLOS scenarios.

Abstract

Optimization of Reconfigurable Intelligent Surfaces (RIS) via a previously introduced method is effective, but time-consuming, because multiport impedance or scatter matrices are required for each transmitter and receiver position, which generally must be obtained through full-wave simulation. Herein, a simple and efficient far-field approximation is introduced, to extrapolate scatter matrices for arbitrary receiver and transmitter positions from only a single simulation while still maintaining high accuracy suitable for optimization purposes. This is demonstrated through comparisons of the optimized capacitance values and further supported by empirical measurements.

Reducing Simulation Effort for RIS Optimization using an Efficient Far-Field Approximation

TL;DR

The paper tackles the high computational cost of RIS optimization, which relies on multiport impedance or scatter matrices for every Tx/Rx position. It introduces a far-field extrapolation that builds the full S-matrix from a single RIS simulation by modeling Tx–RIS and RIS–Rx couplings as far-field two-port links, using distances , angles , and gains in the expressions for and . The approach yields optimized capacitance values that closely match those from full-wave simulations and is corroborated by BRCS measurements, demonstrating practical validity. By reducing the need for repeated full-wave runs, the method can significantly speed RIS design and optimization in realistic NLOS scenarios.

Abstract

Optimization of Reconfigurable Intelligent Surfaces (RIS) via a previously introduced method is effective, but time-consuming, because multiport impedance or scatter matrices are required for each transmitter and receiver position, which generally must be obtained through full-wave simulation. Herein, a simple and efficient far-field approximation is introduced, to extrapolate scatter matrices for arbitrary receiver and transmitter positions from only a single simulation while still maintaining high accuracy suitable for optimization purposes. This is demonstrated through comparisons of the optimized capacitance values and further supported by empirical measurements.

Paper Structure

This paper contains 6 sections, 2 equations, 3 figures.

Table of Contents

  1. Introduction
  2. Far-Field Approximation
  3. Comparison
  4. Optimized Capacitance Values
  5. Measurements
  6. Summary & Conclusion

Figures (3)

  • Figure 1: Geometric illustration of the setup: The RIS facilitates an NLOS communication channel between the transmitter and the receiver antennas, positioned at angles $\beta$ and $\alpha$ w.r.t. the surface normal, respectively. The red path highlights the coupling of the Tx via RIS (at element $m=8$) to the Rx.
  • Figure 2: The RIS (a) and a comparison of the resulting capacitance values for (b) the approximation and (c) the full simulation (using FEBI with MLFMM in ANSYS HFSS), for $\alpha=0$°, $\beta=30$°.
  • Figure 3: Practical setup and results: (a) shows the realized $7\times2$-element RIS with electronics, (b) depicts the entire measurement setup with the Tx/Rx ridged horns and (c) presents a comparison of the resulting BRCS from simulation, approximation and measurement for a Tx at $\beta=30$° and the RIS optimized for an Rx at $\alpha=0$°. As reference, the BRCS of a plane copper reflector of the same dimensions is included, revealing that even such a small RIS can have a significant impact, exceeding $+15$ dB @ $\alpha=0$°.