3D Multi-Target Localization Via Intelligent Reflecting Surface: Protocol and Analysis

TL;DR

This work investigates 3D localization of multiple targets using a monostatic BS aided by multiple Intelligent Reflecting Surfaces (IRSs). It introduces a two-stage IRS-off/IRS-on localization protocol and an IRS-adaptive sensing scheme to disentangle echo paths and enable high-resolution DoA estimation. The paper derives CRBs and proposes MUSIC-based BS-target DoA estimation and on-grid IRS-beam scanning for IRS-target DoA, demonstrating that multi-beam IRS configurations are essential for sensing and achieving sub-meter accuracy. Extending to multi-target scenarios, it develops a pairwise IRS-based matching algorithm to reconstruct target locations with sub-meter precision, validated by numerical results. The approach offers a scalable, low-power sensing paradigm for environment-aware 6G networks with ubiquitous localization capabilities.

Abstract

With the emerging environment-aware applications, ubiquitous sensing is expected to play a key role in future networks. In this paper, we study a 3-dimensional (3D) multi-target localization system where multiple intelligent reflecting surfaces (IRSs) are applied to create virtual line-of-sight (LoS) links that bypass the base station (BS) and targets. To fully unveil the fundamental limit of IRS for sensing, we first study a single-target-single-IRS case and propose a novel \textit{two-stage localization protocol} by controlling the on/off state of IRS. To be specific, in the IRS-off stage, we derive the Cramér-Rao bound (CRB) of the azimuth/elevation direction-of-arrival (DoA) of the BS-target link and design a DoA estimator based on the MUSIC algorithm. In the IRS-on stage, the CRB of the azimuth/elevation DoA of the IRS-target link is derived and a simple DoA estimator based on the on-grid IRS beam scanning method is proposed. Particularly, the impact of echo signals reflected by IRS from different paths on sensing performance is analyzed. Moreover, we prove that the single-beam of the IRS is not capable of sensing, but it can be achieved with \textit{multi-beam}. Based on the two obtained DoAs, the 3D single-target location is constructed. We then extend to the multi-target-multi-IRS case and propose an \textit{IRS-adaptive sensing protocol} by controlling the on/off state of multiple IRSs, and a multi-target localization algorithm is developed. Simulation results demonstrate the effectiveness of our scheme and show that sub-meter-level positioning accuracy can be achieved.

Figures (12)

• Figure 1: System model of multi-IRS-assisted 3D multi-target localization.
• Figure 2: Localization protocol for a single target assisted by a single IRS.
• Figure 3: Normalized beam power distribution versus azimuth and elevation directions under different ${\bar{N}}_{\rm BS}$.
• Figure 4: True value versus approximate value in terms of average received power.
• Figure 5: Illustration of localization protocol for multi-target with multiple IRSs.