A Highly Integrated Passive Wireless Sensing System With Synchronized Data Streaming of Multiple Tags

Abstract

This article introduces a highly integrated passive wireless sensing system based on the extended electronic product code (EPC) Gen2 protocol, using sensor tags and a reader to support concurrent data streams from multiple sensors. Multiple subcarrier multiple access (MSMA) is used by assigning different subcarrier frequencies to the sensor tags for backscattering so that each tag has an independent channel for communication to achieve the simultaneous streaming of multisensor data. In order to improve the subcarrier frequency accuracy and further increase the number of communication channels, this article proposes a novel method of subcarrier frequency calibration based on the Gen2 protocol. This method enables a high-accuracy clock to be generated on-chip. At the same time, this article also proposes a method based on Gen2 protocol to start a group of tags and receive real-time data in parallel. The tag chip is fabricated in a <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$0.18{-}\mu \text{m}$ </tex-math></inline-formula> CMOS process with a die size of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$850\,\,\mu \text{m}\,\,\times 850\,\,\mu \text{m}$ </tex-math></inline-formula> , and the reader prototype is built using software-defined radio equipment. The test results show that the sensing system proposed in this article can collect and recognize concurrent data streams from multiple sensor tags synchronously. The measured sensitivity of the sensor tag is −14.9 dBm with a 3-axis accelerating sensor, and the calibration accuracy of subcarrier frequency is better than <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${\pm }{\mathrm {0.385}}$ </tex-math></inline-formula> kHz in the range of 40–640 kHz. With this accuracy, the tags can be allocated up to 78 subcarrier communication channels theoretically.