Semi-Passive 3D Positioning of Multiple RIS-Enabled Users

TL;DR

This paper demonstrates that multi-user RIS-enabled localization can achieve submeter 3D positioning accuracy over large areas by leveraging LOS and NLOS signals, with optimized phase profiles to reduce interference.

Contribution

It introduces a novel multi-user RIS localization framework with a new estimator that reaches theoretical bounds and employs phase orthogonalization to mitigate interference.

Findings

Achieves submeter 3D localization accuracy.

Develops an estimator that attains Cramer-Rao bounds.

Uses phase orthogonalization to reduce inter-user interference.

Abstract

Reconfigurable intelligent surfaces (RISs) are set to be a revolutionary technology in the 6th generation of wireless systems. In this work, we study the application of RIS in a multi-user passive localization scenario, where we have one transmitter (Tx) and multiple asynchronous receivers (Rxs) with known locations. We aim to estimate the locations of multiple users equipped with RISs. The RISs only reflect the signal from the Tx to the Rxs and are not used as active transceivers themselves. Each Rx receives the signal from the Tx (LOS path) and the reflected signal from the RISs (NLOS path). We show that users' 3D position can be estimated with submeter accuracy in a large area around the transmitter, using the LOS and NLOS time-of-arrival measurements at the Rxs. We do so, by developing the signal model, deriving the Cramer-Rao bounds, and devising an estimator that attains these bounds. Furthermore, by orthogonalizing the RIS phase profiles across different users, we circumvent inter-path interference.