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 Cram\'{e}r-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.

翻译：随着新兴的环境感知应用，泛在感知有望在未来网络中发挥关键作用。本文研究了一个三维（3D）多目标定位系统，其中多个智能反射面（IRS）被用于创建虚拟视距（LoS）链路，以绕过基站（BS）和目标。为充分揭示IRS在感知中的基本极限，我们首先研究了单目标-单IRS场景，并提出了一种通过控制IRS开关状态的新型\textit{两阶段定位协议}。具体地，在IRS关闭阶段，我们推导了BS-目标链路方位角/俯仰角到达方向（DoA）的克拉美-罗界（CRB），并设计了一种基于MUSIC算法的DoA估计器。在IRS开启阶段，推导了IRS-目标链路方位角/俯仰角DoA的CRB，并提出了一种基于网格IRS波束扫描的简单DoA估计器。特别地，分析了由IRS从不同路径反射的回波信号对感知性能的影响。此外，我们证明了IRS的单波束无法实现感知，但可通过\textit{多波束}实现。基于获得的两种DoA值，构建了3D单目标位置。随后，我们将研究扩展到多目标-多IRS场景，并通过控制多个IRS的开关状态提出了一种\textit{IRS自适应感知协议}，并开发了一种多目标定位算法。仿真结果验证了所提方案的有效性，并表明可实现亚米级定位精度。