Intelligent Reflecting Surface Can Be A Passive Anchor to Localize Near-Field and Far-Field Targets

This paper considers a bi-static localization archi-tecture in the sixth-generation (6G) integrated sensing and communication (ISAC) network, where a single-antenna user emits orthogonal frequency division multiplexing (OFDM) signals in the uplink, and a multi-antenna base station hears the echo signals from the targets to localize them. In particular, we focus on a challenging scenario where the line-of-sight (LOS) paths between the targets and the BS are blocked such that it is impossible to estimate the propagation delay and the angle-of-arrival (AOA) information of the targets regarding to the BS as in the conventional bi-static radar system. This paper proposes to leverage the intelligent reflecting surface (IRS) as a passive anchor to tackle the above challenge. Specifically, an IRS is deployed at a proper site with LOS paths to the targets such that the distance and AOA information of the targets regarding to the IRS is embedded in the signals received by the BS over the user-target-IRS-BS path. Moreover, an efficient signal processing method is devised such that based on the signals received by the BS, we can 1. detect which targets are in the far-field region and which are in the near-field region of the IRS; 2. estimate the distance and AOA information regarding to the IRS so as to perform localization. Numerical results are provided to verify the effectiveness of our proposed IRS-aided localization scheme exploiting IRS as a passive anchor for localization purpose.