WiFi localization - DISCO · WiFi localization •Different techniques –RSSI based...
Transcript of WiFi localization - DISCO · WiFi localization •Different techniques –RSSI based...
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WiFi localization
Milan Pandurov
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Outline
• Indoor localization
• WiFi localization
• WiFi basics
• Localization approaches
• Prevention of localization
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Indoor positioning systems (IPS)
• Non radio technologies
– Magnetic
– Dead reckoning
– Known visual features (markers)
• Radio technologies
– Bluetooth
– Wi-Fi based positioning
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WiFi localization
• Different techniques
– RSSI based
– Fingerprinting based
– Angle of arrival based
– Time of flight based
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Basic Wi-Fi concepts
• WiFi channels
• MAC
• Channel estimation
• Multipath propagation
• MIMO
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Angle of arrival based localization (AoA)
• One approach to localize users
• Developed system: Ubicarse
“Accurate Indoor Localization With Zero Start-up Cost”
Swarun Kumar, Stephanie Gil, Dina Katabi, Daniela Rus
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Angle of arrival – how does it work
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Synthetic aperture
• Emulate antenna array
• Technique used in synthetic aperture radar (SAR)
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Circular SAR (1/2)
• Start with idea of emulation of circular antenna array
R
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Circular SAR (2/2)
• Channel model for kth snapshot of antenna:
• Relative power along direction α
α
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Ubicarse - solution
• Translation resilient SAR
• Create accurate indoor positioning system
– MIMO capabilities of modern devices
– Information of device’s orientation
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Translation-resilient SAR
Measurements at antennas
Relative channel
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Translation-resilient SAR
• Use same SAR formula for relative channel power:
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Accurate device localization
• Localization proces:
1. App asks user to twist device
2. Issues beacon requests to neighbor access points to estimate channels from them
3. Perform SAR to generate multipath power profiles
4. …
5. Show precise location
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Calculate location
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Object geotagging
• Localize devices with no radio devices attached
• Using camera & stereo vision algorithms
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How does Ubicarse perform
• Translation resilience
• Device localization
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Different approach
• What if we don’t want to:
– Have absolute position, but just absolute distance
– Have many access points
– Have to rotate device
• Different use cases:
– Smart home occupancy
– Geo-fencing
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Time of flight
• Time it takes for signal to propagate from transmitter to receiver
• Absolute time of flight
• High precision required
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Calculate absolute ToF
• Emulating ultra wideband radio with WiFi
• Developed system: Chronos
“Decimeter-Level Localization with a Single WiFi Access Point”
Deepak Vasisht, Swarun Kumar, Dina Katabi
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WiFi channel
• Can WiFi channels be combined to emulate ultra wideband radio?
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Chronos
• Indoor positioning system
• Calculates absolute distance by measuring ToF
• Sends data over different WiFi channels to emulate wide band radio
– Different frequencies have different properties
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Measuring time of flight
1. 2.
3.
• For a single channel
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Measuring time of flight
• Modulo measurements @2.4GHz
– ToF = modulo 1/f (0.4ns) – {0.1ns, 0.5ns, 0.9ns,…}
– Distance = modulo 12cm – {3cm, 15cm, 27cm,…}
• For a range of channels we get system of equations
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Measuring time of flight
• Chinese remainder theorem
• Problem to solve
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Multipath
• Generate multipath profile
• Choose shortest path – first peak
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Computing multipath profiles
• Signals reach receiver over p different paths
• Discrete Fourier transform?
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Phase offset
• Two phase offsets are created:
– PLL Phase offset
– Carrier frequency offset
• Recorder state information on transmitter
• Recorder state information on receiver
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Fixing phase offset
• Multiplying CSI at receiver and sender to recover wireless channel:
• Use that channel to calculate propagation time
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How does Chronos perform
• Test environment for measurement correctness
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Measurement correctness
Time of flight results Multipath profile results
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Location accuracy
Location accuracy Ranging accuracy
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Real use case performance
Room occupancy WiFi Geo-fencing
94% correct 97% correct
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Privacy concerns
• Can we use WiFi signal as covert channel?
• WiFi signals travel through walls
• Curious neighbor, or burglar?
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What can one hear?
• WiHear [link]
• E-eyes [link]
• See through walls with WiFi [link]
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Solution?
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Less radical solution?
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Usable solution?
• Privacy leakage lays in physical not in logical (data) layer
• Just distort physical layer (obfuscate)
• Developed solution: PhyCloak
“PhyCloak: Obfuscating Sensing from Communication Signals”
Yue Qiao, Ouyang Zhang, Wenjie Zhou, Kannan Srinivasan, Anish Arora
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What is sensitive data?
• Reflected signal from obstacles
• 3 degrees of freedom (3DoF)
– Amplitude gain
– Delay
– Doppler shift
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How to hide sensitive data
• Obfuscator needs to change 3DoF of reflected signal
• Build obfuscator to create another multipath signal
• Create signal in a way to cancel sensitive data out
Ox
S(t)
S(t)
r(t)
or(t)
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System goals
• Obfuscate Eve’s sensing • Preserve Carol’s sensing • Don’t degrade throughput in link Alice – Bob • Online self-channel estimation
Bob
Alice
Eve
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Obfuscating 3DoF
• Amplitude gain – Amplify received samples with different levels
• Doppler shift – Rotate nth sample by
• Delay – Delay to be forwarded signals – Done by rotating samples by fixed phase
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PhyCloak design
• High level block diagram
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Self channel estimation - problem
• Human movement near Ox causes strong residual noise
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Hiding Doppler shift
• By carefully choosing time to change phase Doppler shift can be hidden
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Results of obfuscation
• Signal to Obfuscation Ratio (dB)
• Hiding human motion
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Results of obfuscation
• Test scenarios:
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Obfuscation performance
• 3 Dof correlation vs SOR
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Obfuscation performance
• WiSee gesture detection accuracy
– Recognizes gestures: drag, push, dodge…
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Obfuscation performance
• WiSee NLOS/LOS performance
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QUESTIONS? Wi-Fi localization