To detect multiple identifications (IDs) for cm-level high-precision positioning item-level tagging (ILT) systems in near-field ultra-highfrequency RFID applications, an electric-field-coupled multiple ID sensor is presented. The proposed ID sensor consists of four circular electric coupling parts, an impedance-matching network, and an SP4T switch for position control. By switching the electric-fieldcoupled sensing parts, cm-level position detection of densely aggregated RFID tags for a small-sized ILT can be achieved. The sensing distance of the proposed ID sensor is ∼7 mm and its physical size is 50 × 50 mm 2 (∼1/7λ 0 at 920 MHz).Introduction: RFID systems are widely used in many different applications including retail markets and supply chain management. Since an RFID tag provides information (e.g. product descriptor, serial number, inventory, and location in a warehouse) related to the product to which the RFID tag is attached or integrated, RFID applications have expanded from pallet or box tagging to item-level tagging (ILT) in recent years. However, in the case of ultra-high-frequency (UHF) RFID systems using an electromagnetic (EM) coupling, there exist critical problems such as a recognition rate of <100% (typically 95-98%) and a blind spot where the tag is non-responsive due to electromagnetic interference between multiple readers or multiple tags, in addition to the reflection, scattering, diffraction, and absorption of the radio frequency caused by environmental effects [1,2].
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