Article
Accurate Device-Free Tracking Using
Inexpensive RFIDs
Liyao Li, Chongzheng Guo, Yang Liu, Lichao Zhang, Xiaofei Qi, Yuhui Ren, Baoying Liu *
ID
and Feng Chen *
School of Information Science and Technology, Northwest University, Xi’an 710127, China;
liliyao@stumail.nwu.edu.cn (L.L.); guo_chongzheng@stumail.nwu.edu.cn (C.G.);
nwu_ly729@stumail.nwu.edu.cn (Y.L.); zhanglichao0331@gmail.com (L.Z.); qixf@nwu.edu.cn (X.Q.);
ryhui@nwu.edu.cn (Y.R.)
* Correspondence: paola.liu@nwu.edu.cn (B.L.); xdcf@nwu.edu.cn (F.C.)
Received: 30 June 2018; Accepted: 11 August 2018; Published: 27 August 2018
Abstract:
Without requiring targets to carry any device, device-free-based tracking is playing
an important role in many emerging applications such as smart homes, fitness tracking, intruder
detection, etc. While promising, current device-free tracking systems based on inexpensive
commercial devices perform well in the training environment, but poorly in other environments
because of different multipath reflections. This paper introduces RDTrack, a system that leverages
changes in Doppler shifts, which are not sensitive to multipath, to accurately track the target.
Moreover, RDTrack identifies particular patterns for fine-grained motions such as turning, walking
straightly, etc., which can achieve accurate tracking. For the purpose of achieving a fine-grained
device-free tracking system, this paper builds a trajectory estimating model using HMM (Hidden
Markov Model) to improve the matching accuracy and reduce the time complexity. We address
several challenges including estimating the tag influenced time period, identifying moving path and
reducing false positives due to multipath. We implement RDTrack with inexpensive commercial
off-the-shelf RFID (Radio Frequency IDentification) hardware and extensively evaluate RDTrack in
a lobby, staircase and library. Our results show that RDTrack is effective in tracking the moving target,
with a low tracking error of 32 cm. This accuracy is robust for different environments, highlighting
RDTrack’s ability to enable future essential device-free moving-based interaction with RFID devices.
Keywords: RFID; device-free tracking system; Doppler frequency shift; path matching model
1. Introduction
Have you ever heard news that jewelry stores or museums have been robbed? Has your house
been visited by a thief? It is common for some victims to go to a police station to report crimes with
respect to their property and security. Tracking [
1
] intruders and protecting private property from the
illegal encroachment of intruders is a major problem in museums, jewelry stores and warehouses where
the items contained therein are very valuable. In real life, we envision target tracking as an essential
means to support anti-intrusion protection. Existing approaches based on vision [
2
], ultra-sound [
3
]
and infrared [
4
] have been applied in target tracking. While promising, these techniques have various
limitations such as requiring line-of-sight, sensitivity to lighting, short communication distance, etc.
With the ubiquity of RF-enabled devices and infrastructure, some RF-based device-free target
tracking systems [
5
–
7
] have been proposed recently to overcome the above limitations in order
to support anti-intrusion protection. Device-free tracking systems hosted on inexpensive devices
leverage the Received Signal Strength Indicator (RSSI) [
5
,
8
] or Channel State Information (CSI) [
7
–
10
]
as fingerprints, which are easily corrupted by environmental changes. However, the equipment
Sensors 2018, 18, 2816; doi:10.3390/s18092816 www.mdpi.com/journal/sensors
