  Next-generation cellular networks will witness the creation of smart radio
environments (SREs), where walls and objects can be coated with reconfigurable
intelligent surfaces (RISs) to strengthen the communication and localization
coverage by controlling the reflected multipath. In fact, RISs have been
recently introduced not only to overcome communication blockages due to
obstacles but also for high-precision localization of mobile users in GPS
denied environments, e.g., indoors. Towards this vision, this paper presents
the localization performance limits for communication scenarios where a single
next-generation NodeB base station (gNB), equipped with multiple-antennas,
infers the position and the orientation of the user equipment(UE) in a
RIS-assisted SRE. We consider a signal model that is valid also for near-field
propagation conditions, as the usually adopted far-field assumption does not
always hold, especially for large RISs. For the considered scenario, we derive
the Cramer-Rao lower bound (CRLB) for assessing the ultimate localization and
orientation performance of synchronous and asynchronous signaling schemes. In
addition, we propose a closed-form RIS phase profile that well suits joint
communication and localization. We perform extensive numerical results to
assess the performance of our scheme for various localization scenarios and RIS
phase design. Numerical results show that the proposed scheme can achieve
remarkable performance, even in asynchronous signaling and that the proposed
phase design approaches the numerical optimal phase design that minimizes the
CRLB.
