THz SAR Assisted NLoS Passive Tag Identification

Communication through non-line-of-sight (NLoS) THz links in indoor environments has gained significant interest due to its potential to provide coverage for users experiencing line-of-sight (LoS) blockages. In self-localization and tracking systems, which utilize frequency-coded chipless RFID tags as anchors, LoS blockages between the reader and the tag can significantly reduce localization and tracking accuracy. This paper proposes an effective method for detecting blocked LoS tags using NLoS links enabled by synthetic aperture radar (SAR) imaging. To validate this approach, we conduct experiments with metal and acrylic objects acting as a reflector to establish an NLoS link using the tag. A trihedral corner reflector, designed with dielectric resonators at 85 GHz, serves as the RFID tag due to its high radar-cross-section (RCS) and retro-directivity. The proposed technique involves generating a high-resolution SAR image of the NLoS environment. From the SAR image, the tag coordinates relative to the imaging geometry are extracted and finally, the tag identification is performed by applying a time-gated window to the raw data based on the extracted coordinates. Moreover, the presented results demonstrate that the type of indoor object and its reflective signature significantly impact the backscattered spectrum, influencing the reliability of tag detection. Metal plates, with their flat spectra, have minimal influence on notch frequency detection compared to acrylic, which exhibits a more complex frequency response. This work provides a framework for high-resolution environmental mapping and accurate tag localization using NLoS links in complex indoor environments.