Based on the signals received across its antennas, a multi-antenna base station (BS) can apply the classic multiple signal classification (MUSIC) algorithm for estimating the angle of arrivals (AOAs) of its incident signals. This method can be leveraged to localize the users if their line-of-sight (LOS) paths to the BS are available. In this paper, we consider a more challenging AOA estimation setup in the intelligent reflecting surface (IRS) assisted integrated sensing and communication (ISAC) system, where LOS paths do not exist between the BS and the users, while the users' signals can be transmitted to the BS merely via their LOS paths to the IRS as well as the LOS path from the IRS to the BS. Specifically, we treat the IRS as the anchor and are interested in estimating the AOAs of the incident signals from the users to the IRS. Note that we have to achieve the above goal based on the signals received by the BS, because the passive IRS cannot process its received signals. However, the signals received across different antennas of the BS only contain AOA information of its incident signals via the LOS path from the IRS to the BS. To tackle this challenge arising from the spatial-domain received signals, we propose an innovative approach to create temporal-domain multi-dimension received signals for estimating the AOAs of the paths from the users to the IRS. Specifically, via a proper design of the user message pattern and the IRS reflecting pattern, we manage to show that our designed temporal-domain multi-dimension signals can be surprisingly expressed as a function of the virtual steering vectors of the IRS towards the users. This amazing result implies that the classic MUSIC algorithm can be applied to our designed temporal-domain multi-dimension signals for accurately estimating the AOAs of the signals from the users to the IRS.

翻译：基于其天线阵列接收的信号，多天线基站（BS）可应用经典的多重信号分类（MUSIC）算法来估计入射信号的到达角（AOA）。若用户与基站之间存在视距（LOS）路径，该方法可用于用户定位。本文考虑了一种更具挑战性的AOA估计场景，即在智能反射面（IRS）辅助的感知通信一体化（ISAC）系统中，基站与用户之间不存在视距路径，用户的信号仅能通过其到IRS的视距路径以及IRS到基站的视距路径传输至基站。具体而言，我们将IRS视为锚点，旨在估计用户入射至IRS信号的到达角。需注意，由于无源IRS无法处理接收信号，我们需基于基站接收的信号实现上述目标。然而，基站不同天线接收的信号仅包含经由IRS到基站视距路径入射信号的AOA信息。针对这一由空域接收信号引发的挑战，我们提出了一种创新方法，通过构造时域多维接收信号来估计用户到IRS路径的AOA。具体地，通过合理设计用户消息模式与IRS反射模式，我们证明所设计的时域多维信号可惊人地表示为IRS朝向用户的虚拟导向矢量的函数。这一重要结果表明，经典MUSIC算法可应用于所设计的时域多维信号，从而实现用户到IRS信号到达角的高精度估计。