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mTrack: High-Precision Passive Tracking Using Millimeter Wave Radios
Teng Wei and Xinyu Zhang
University of Wisconsin – Madison
Near-field Wireless Tracking
Tracking objectives at mm-level accuracy
Turn any surface into interactive virtual touchscreen
Enable a new form of pervasive user-computer interface
Virtual Trackpad Interactive Display Tracking Whiteboard
State-of-the Art
C. Xu, etalSCPL: Indoor Device-free Multi-
subject Counting and Localization Using Radio
Signal Strength IEEE IPSN, 2013.
TagoramMobiCom
Radio-based tracking system
m-level dm-level cm-level mm-level
RF-IDrawSIGCOMM
PinLocMobiSys
H.Fang, 60GHz RSS Localization with
Omni-directional and Horn Antennas, Ph.D.
dissertation, 2010.
WiViSIGCOMM
WiTrackNSDI ?
Active
Passive
New Challenges
Weak signal intensity of passive reflection
Passive Fine-grained Tracking
Target does not modulate and emit signals
Irrelevant reflection from unintended objectives
Time-varying multipath reflection from background
Locating initial position with few number of devices
Especially from small objects, like pen
Costly to deploy substantial nodes
Overview the Basic Idea
60GHz laser-like directional beam ❶
Tx
RxPen
Rx2
Flexible beam-steering capability❷
5mm extremely short wavelength❸
Quasi-omni-directional illumination❺
Interactive diffusion from small objects❹
Understanding mmWave Passive Tracking
Feasibility Study
Tx
30cmPen 0.8cm
Rx
Diffusive Reflection
15~20dB
Tx
50cm60cm
Moving
Rx
Fine-grained Tracking
Tx
50cm
Rx
Initial Locating
Key Challenge: Background Reflection
Tx
Rx
Objects in the background
Background Reflection
Target Reflection
0
2𝜋
λ /2 λ 3 λ /2 2 λTarget movement
Phase ofReceived
Signal
Background Dominated
0
2𝜋
λ /2 λ 3 λ /2 2 λTarget movement
Phase ofReceived
Signal
Less than 2
Target Dominated
Rx
Target DominatedBackground Dominated
Naïve Solution
DC-filter the decoded symbols
Received signal Target
reflection
Backgroundreflection
I
Q
1, 0, 1, 0, …
Unmodulated
Modulated
Filter the received waveform (RFID)
Require target to modulate the reflect signal
I
Q
static background removed
Dual-differential Background Removal (DDBR)
Key Observation
Background reflection remains similar in consecutive samples
Differential cancels the background reflection
12[Δarg(�⃗�𝒕𝒓𝒈)𝑡 −1
𝑡 +Δarg (�⃗�𝒕𝒓𝒈)𝑡𝑡+1]=arg (�⃗�𝑟𝑒𝑐
𝑡+1− �⃗�𝑟𝑒𝑐𝑡 )−arg (�⃗�𝑟𝑒𝑐
𝑡 − �⃗�𝑟𝑒𝑐𝑡 −1 )
Lemma (DDBR): The average phase shift among three consecutive samples is
Average phase shift Diff. phase of sample differential
Sample differential
531
-1-3-4
Target movement
DDBR received signals
Phas
e
Advantage and Limitation
Handle time-varying background reflection
Simple computation of processing
Suitable for hardware implementation
Cons of DDBR
Vulnerable to the phase noise
60GHz COTS device has non-negligible phase noise
phase noise > phase shift
Pros of DDBR
Phase Counting and Regeneration (PCR)
Periodicity Pattern of Phase I (TD) II (BD) III (ITM)
0
2𝜋
λ /2 λ 3 λ /22 λTarget movement
Pha
se
0
2𝜋
λ /2 λ 3 λ /2 2 λTarget movement
Pha
se
0
2𝜋
λ /2 λ 3 λ /22 λ
Pha
se
Target movement
0 50 100 150 200 250 300 350
Case (I ) Case ( II )∧( III ) Case (I )
Sample index
30
-3
1030
-3
30
-5
PCR Algorithm
Reducing ITM to BDStep 1
Periodicity Counting Step 2
RegenerationStep 3
Input phase
Anchor Point Acquisition (APA)
Complementary to Tracking
Initial location for successive tracking
Prevent error accumulation
Calibrate tracking result
Discrete Beam Steering
Spline interpolation improves granularity of APA
reduce error
True direction
Background Reflection
Enhance 10dB
contrast
RSS subtraction improves contrast of APA
BG Pen
Touch Event Detection
e.g., start/pause of tracking
Detect touch gestures as control command
Gesture and Feature Space
Touch
LiftClick
Phase shift❶
Variance of phase shift❷
RSS❸
Event detection: Variance of phase shift
Event Classification: RSS
Decisiontreerule
Touch Lift Click
Implementation and Evaluation
Horn Antenna
MotorizedRotator
60 GHz RFFront-end (Rx)
High SpeedADC/DACWARP Board
PHY Extraction
Tracking
Locating
Touchdetection
AppsmTrack
60GHz SDR testbed Algorithm implementation
Testing objects
Metal-surfaced pen
Marker
Pencil
Passive Tracking
Rx 1Tx
Rx 2
Drywall
Cabinet
1m 1m⨉10cm10cm2m
1.5mExample trajectory of tracking
Error map over tracking region
Tracking Setup Result
Achieve high-precision tracking
1cm
3cm
Anchor Positioning and Event Detection
Event Touch Lift Click ND
Touch 94.0% 0 0 6.0%
Lift 0 93.5% 0 6.5%
Click 0 0 94.8% 5.2%
APA Performance
Randomly placed 30 positions
Beam-steering at step of
RSS: 12.3dB, 10.1dB and 4.7dBAverage error of 1.5 cm, 2 cm
and 6 cm
Event Detection
7 users
Each provides a 10-sampletraining set
20~50-sample testing set
Application: Trackpad
Experiment Setup
Example word Recognition Accuracy
Integrate mTrack into word-recognition application
Record hand-writing trace from mTrack
Export and control mouse of a PC
MyScript© Stylus for word detection
Conclusion
First RF-based system that achieves sub-centimeter scale passive object tracking
Implement on a configurable 60GHz radio testbed
Validate performance in a wireless trackpad setup
Resolve new practical challenges in passive tracking/locating
DDBR algorithm for addressing background reflection
PCR algorithm for mitigating phase noise issue
RSS interpolation and subtraction for improving granularity and contrast.
Questions?
Thank you