Theoretical Bounds for Enhanced Doppler-Based Motion Detection in UHF-RFID Readers

Radio Frequency Identification (RFID) is a widely used technology for identifying and locating objects equipped with low-cost RFID transponders (tags). UHF (Ultra High Frequency) RFID operates in frequency bands around 900 MHz and supports communication distances of up to 15 m between the reader and the tag. Reliable motion detection is therefore a highly relevant feature in modern logistics – for example, to determine whether a tag is actually placed on a conveyor belt or merely in its vicinity. A promising approach for accurate motion detection is the use of the Doppler effect. Some state-of-the-art UHF-RFID readers already support Doppler shift measurements. However, their measurement accuracy is insufficient for many applications. In this paper, we propose enhancements for the precise Doppler shift estimation using existing RFID systems - an essential step toward enabling RFID-based motion detection in future logistics. Further, we also derive the theoretical bounds for Doppler-based motion detection in UHF-RFID systems based on the Cramer-Rao Lower Bound. These bounds analyze the influence of tag signal strength, signal duration, and the intervals between multiple tag replies on the performance of motion detection and speed estimation algorithms. In addition, we establish theoretical limits that account for hardware constraints in current UHF-RFID readers. The results of this work provide valuable insights into the limitations of Doppler-based motion detection and support system-level performance optimization. They enable prediction of achievable performance based on reader noise figure, aiding in the design and tuning of RFID systems.