MORSE: Multipath Resilient One-Shot Localization for RIS via Spatial Spectral Encoding

Abstract

Reconfigurable intelligent surfaces (RIS) offer a promising way to control wireless propagation by engineering reflected paths. However, realizing this vision requires real-time knowledge of users' location relative to the RIS. This is challenging because RIS lack the capability to sense the channel, while existing approaches rely on beam search with high overhead. This paper presents MORSE, a novel one-shot localization framework that leverages the inherent frequency-dependent radiation patterns of RIS to create spatial-spectral encoding, where each spatial direction corresponds to a unique spectral signature. A user can then infer its angular position relative to the RIS by observing the received power spectrum. To ensure robustness in multipath environments, MORSE excites the RIS using an FMCW chirp, allowing the receiver to separate propagation paths by delay and isolate the spectral signature of the direct RIS-UE path. We implement MORSE on a 12×12 RIS prototype operating from 3 to 6 GHz and evaluate it through extensive over-the-air experiments in three environments. Our results demonstrate accurate single-shot joint azimuth and elevation angle estimation and robustness to multipath interference, enabling RIS adaptation in dynamic wireless environments.