  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.
