Cross-Harmonic Ambiguity-Aligned Multiport Parameter Estimation for Time-Floquet RIS

Abstract: A time-Floquet reconfigurable intelligent surface (TF-RIS) periodically modulates its elements within a signaling interval, enabling frequency conversion and additional degrees of freedom compared with a conventional RIS. Time-Floquet multiport-network theory (TF-MNT) provides a physics-consistent model for TF-RISs that accounts for inter-element coupling, but its practical use requires estimating the underlying parameters when the TF-RIS design and radio environment are (partially) unknown. In this Letter, we propose a segmented estimation approach for constructing an accurate proxy TF-MNT model from end-to-end measurements. First, with the TF-RIS operated as a conventional RIS, we estimate conventional proxy MNT parameters independently at each considered time-Floquet harmonic. Second, under periodic time modulation, we align the inherent ambiguities among the per-harmonic conventional proxy MNT parameters, considering three measurement setups with different access to phase and harmonic information. Based on full-wave numerical simulations, we quantify the impact of the number of measurements and the noise level on the proxy-model accuracy. Finally, we demonstrate the performance loss incurred without the proposed ambiguity alignment in a canonical harmonic backscatter communications scenario.