  Mutual coupling is increasingly important in reconfigurable intelligent
surface (RIS)-aided communications, particularly when RIS elements are densely
integrated in applications such as holographic communications. This paper
experimentally investigates the mutual coupling effect among RIS elements using
a mutual coupling-aware communication model based on scattering matrices.
Utilizing a fabricated 1-bit quasi-passive RIS prototype operating in the
mmWave band, we propose a practical model training approach based on a single
3D full-wave simulation of the RIS radiation pattern, which enables the
estimation of the scattering matrix among RIS unit cells. The formulated
estimation problem is rigorously convex with a limited number of unknowns
un-scaling with RIS size. The trained model is validated through both full-wave
simulations and experimental measurements on the fabricated RIS prototype.
Compared to the conventional communication model that does not account for
mutual coupling in RIS, the mutual coupling-aware model incorporating trained
scattering parameters demonstrates improved prediction accuracy. Benchmarked
against the full-wave simulated RIS radiation pattern, the trained model can
reduce prediction error by up to approximately 10.7%. Meanwhile, the
S-parameter between the Tx and Rx antennas is measured, validating that the
trained model exhibits closer alignment with the experimental measurements.
These results affirm the accuracy of the adopted model and the effectiveness of
the proposed model training method.
