Two-Timescale Joint Precoding Design and RIS Optimization for User Tracking in Near-Field MIMO Systems

Abstract

In this paper, we propose a novel framework that aims to jointly design the reflection coefficients of multiple reconfigurable intelligent surfaces (RISs) and the precoding strategy of a single base station (BS) to optimize the selftracking of the position and the velocity of a single multi-antenna user equipment (UE) that moves either in the far-or nearfield region. Differently from the literature, and to keep the overall complexity affordable, we assume that RIS optimization is performed less frequently than localization and precoding adaptation. The proposed procedure leads to minimize the inverse of the received power in the UE position uncertainty area between two subsequent optimization steps.The optimal RIS and precoder strategy is compared with the classic beam focusing strategy and with a scheme that maximizes the communication rate. It is shown that if the RISs are optimized for communications, their configuration is suboptimal when used for tracking purposes. Numerical results show that in typical indoor environments with only one BS and a few RISs operating on millimeter waves, high location accuracy in the range of less than half a meter can be achieved.Index terms-Reconfigurable Intelligent Surfaces, Bayesian tracking, MIMO, Optimization.

Constrained RIS Phase Profile Optimization and Time Sharing for Near-field Localization

“…Then, shifting away from snapshot estimation, in [32], a UE transmitting a narrowband signal is tracked through filtering (i.e., including 3D velocity as estimated variable, besides 3D position), while exploiting phase and amplitude observations accounting for the curvature-of-arrival of the impinging wavefront at a RIS in receiving mode in the NF region. Moreover, in [33] the authors present a tracking algorithm based on extended Kalman filter (EKF) to localize UEs, in a LoS scenario, with the aid of a reflective RIS at the millimeter wave (mmWave) frequency domain, while [34] addresses a joint RIS reflection coefficients and BS precoder optimization problem and estimates UE's trajectory in a single mobile UE multi-RIS MIMO scenario in the NF regime. It is true that [32]- [34] take the UE mobility into account, however, the estimation happens over multiple snapshots via a tracking filter and the mobility within individual snapshots is ignored.…”

“…Moreover, in [33] the authors present a tracking algorithm based on extended Kalman filter (EKF) to localize UEs, in a LoS scenario, with the aid of a reflective RIS at the millimeter wave (mmWave) frequency domain, while [34] addresses a joint RIS reflection coefficients and BS precoder optimization problem and estimates UE's trajectory in a single mobile UE multi-RIS MIMO scenario in the NF regime. It is true that [32]- [34] take the UE mobility into account, however, the estimation happens over multiple snapshots via a tracking filter and the mobility within individual snapshots is ignored. In summary, the literature lacks studies where the position and/or the velocity of a mobile UE is estimated, with a single epoch, via a reflective surface while exploiting the small-scale Doppler effects resulting from the mobility.…”

Multi-User Tracking in Reconfigurable Intelligent Surface Aided Near-Field Wireless Communications System

“…We adopt a simple Rice channel model to describe the links between RISs and users, and κ h ≥ 0 are the Ricean factors for (k-th)RIS-(m-th)user links. We assume that the LoS paths always exist, as the probability of the LoS condition is close to 1 in the nearfield region [37]. δ k,m;p ∼ CN (0, ρ 2 k,m;p ) denotes the random complex fading coefficients of the NLoS components.…”

“…If the user's initial location information is known, better positioning accuracy can be achieved through directional beamforming on the BS side [32]. Furthermore, works in [37][38][39][40] resolve the problem in a double-scale way, significantly reducing the RIS reconfiguration rate. Fewer works consider multi-user tracking problems.…”