  This study explores the use of non-line-of-sight (NLOS) components in
millimeter-wave (mmWave) communication systems for joint localization and
environment sensing. The radar cross section (RCS) of a reconfigurable
intelligent surface (RIS) is calculated to develop a general path gain model
for RISs and traditional scatterers. The results show that RISs have a greater
potential to assist in localization due to their ability to maintain high RCSs
and create strong NLOS links. A one-stage linear weighted least squares
estimator is proposed to simultaneously determine user equipment (UE)
locations, velocities, and scatterer (or RIS) locations using line-of-sight
(LOS) and NLOS paths. The estimator supports environment sensing and UE
localization even using only NLOS paths. A second-stage estimator is also
introduced to improve environment sensing accuracy by considering the nonlinear
relationship between UE and scatterer locations. Simulation results demonstrate
the effectiveness of the proposed estimators in rich scattering environments
and the benefits of using NLOS paths for improving UE location accuracy and
assisting in environment sensing. The effects of RIS number, size, and
deployment on localization performance are also analyzed.
