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University Research Tracks Everyday Activities via RFID

A team from the University of Michigan and the University of Washington have completed their testing of a UHF RFID system that measures received signal strength and phase responses to reader interrogation, to identify not only what tag is in a room but whether it has moved or interacted with a person.
By Claire Swedberg
May 29, 2019

A team of researchers from the University of Michigan and the University of Washington have completed the first round of testing for an RFID-based system that uses UHF RFID signals to identify a specific tag, and that uses fluctuations in these signals to understand changes in the field around that tag. The result could be a solution with which users could understand whether a tagged item has been moved or been interacted with, as well as if someone has approached or left the tag's vicinity.

The project, known as IDAct, has been underway throughout the 2018 to 2019 school year as researchers have worked to design a way to gain more information from a simple RFID tag read than just its ID number. The IDAct system takes advantage of received signal strength (RSSI), as well as RFID's built in "phase" functionality, according to Alanson Sample, an associate professor at the University of Michigan and a co-author of the project's white paper, titled "IDAct: Towards Unobtrusive Recognition of User Presence and Daily Activities." The question Sample posed was how to turn basic UHF tags into little sensors. "The idea I had," he says, "was to use a communication channel as a sensing mechanism."

RFID readers built into light fixtures read tagged items to track a person's daily activities in a test apartment.
The system leverages changes in the RSSI and phase, as reported by the RFID reader, as well as machine learning, to infer what is happening around a tag. Traditionally, RFID readers and the software that manages the collected read data focus on receiving a unique ID number from a passive RFID tag, but they can also use RSSI to determine approximately where that tag is located, or the direction in which it is moving.

IDActs take that a step further by using the RF phase measurement. UHF RFID readers randomly change their transmission frequency across 50 channels, from 902 to 928 MHz, in order to meet FCC regulations and minimize interference with other devices. These changes in frequency take place at 0.2-second intervals. The phase difference between channels can be dramatically increased with motion, or due to a person's presence.

The phase-sensing capability of RFID is what makes the IDAct system possible, Sample explains. One unique aspect of UHF RFID systems, he says, is that users can measure the phase of backscattered signals, which can be used to identify such details as a tag's exact location; other technologies, such as Wi-Fi or Bluetooth Low Energy (BLE), lack a phase response.

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