hotemnets08-indoorloc
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Transcript hotemnets08-indoorloc
Towards Precise Indoor
RF Localization
Akos Ledeczi, Janos Sallai, Xenofon Koutsoukos, Peter Volgyesi
Vanderbilt University
Branislav Kusy
Stanford University
Miklos Maroti
University of Szeged, Hungary
Overview
•Objective – accurate indoor localization using radio interferometry (RI)
•Motivation – applications need location service, but GPS has many limitations
•Radio-interferometic ranging – how does it work ?
•Previous work – localization and tracking with RI andradio interferometric Doppler shifts
•Challenges – multipath, complex localization algorithms, long measurement duration
•Approach – lower carrier frequency, asymmetric architecture
•Work in progress – preliminary experimental results (time synchronization)
Motivation
Important applications
• require high accuracy localization
• ad-hoc deployable wireless sensors
• need to operate without human
intervention
Motivation
Important applications
Why radio-interferometry?
• require high accuracy localization
• ad-hoc deployable wireless sensors
• power constraints (lifetime of months on 2
AA batteries)
• need to operate without human
intervention
• GPS is often not applicable
• low cost – enables redundancy
Motivation
Important applications
Why radio-interferometry?
• require high accuracy localization
• ad-hoc deployable wireless sensors
• power constraints (lifetime of months on 2
AA batteries)
• need to operate without human
intervention
• GPS is often not applicable
• low cost – enables redundancy
Potentials of radio-interferometry
• Can be implemented with cheap HW
• More accurate than acoustic/ultrasonic/rf
TOF/TDOA
• Does not require line of sight
Motivation
Important applications
Why radio-interferometry?
• require high accuracy localization
• ad-hoc deployable wireless sensors
• power constraints (lifetime of months on 2
AA batteries)
• need to operate without human
intervention
• GPS is often not applicable
• low cost – enables redundancy
Potentials of radio-interferometry
• Can be implemented with cheap HW
• More accurate than acoustic/ultrasonic/rf
TOF/TDOA
• Does not require line of sight
Challenges
• Sensitive to RF multipath
• Measurements take a long time
• Localization algorithms are complex
Radio Interferometry
Interference
superposition of two waves (from one or two sources) resulting in new wave pattern
Applications
traditionally used in applied physics (geodesy, astronomy,…)
compute cross-correlation of a signal from a single source recorded by 2 observers
Problem
sensor hardware has insufficient processing power to compute correlation online
Solution
two transmitters slightly out of tune produce low frequency beat
use a simple peak detector to measure phase at receiver
2.5
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0.5
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-1
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two signals with slightly
different frequencies
-2
-2.5
observed beats: high carrier
freq, low frequency envelope
2
Radio-Interferometric Ranging
Senders (A, B) transmit simultaneously
•pure sinusoid waves at 400 MHz
•small freq difference (<1000 Hz)
Receivers (C, D) measure radio
interference
•sample RSSI (17 kHz)
•find beat frequency, phase offset
•time sync to correlate phase offsets
•result: (dAD-dBD+dBC-dAC) mod λc
dXY: distance between points X and Y
λc: average wave length of carrier freqs
Advantages
ΔφCD /(2fi) = (dAD-dBD+dBC-dAC) mod λc
q-range
•high accuracy (cm)
•long range (200m)
•low cost, low power HW
Previous work
RIPS
inTrack
computes
measures
redundancy
algorithm
in
relative spatial
map of stationary
nodes
Q-ranges
Roles
Heuristics/genetic
SenSys’04,
IPSN’06
location of a
mobile node
(transmitter)
Q-ranges
Refined search
EWSN’07
Analytical formula
(closed form)
MOBISYS’07
Extended Kalman
Filter (EKF)
SenSys’07
Frequencies
Roles
(one infrastructure
node is a transmitter,
rest are receivers),
Frequencies
mTrack
location and
instantaneous
velocity of multiple
mobile nodes
(receivers)
Q-ranges, beat
frequencies
(Doppler shift)
Roles
(two infrastrucrure
nodes transmitters,
rest are receivers)
Frequencies
dTrack
computes location
of a mobile node
(transmitter)
beat frequencies
(Doppler shift)
Roles
(one infrastructure
node is a transmitter,
rest are receivers)
Challenges I.
RF multipath
Distorts the phase of the beat signal
Caused by reflection from objects of similar or
larger size than the wavelength
Challenges I.
RF multipath
Distorts the phase of the beat signal
Caused by reflection from objects of similar or
larger size than the wavelength
Challenges I.
RF multipath
Distorts the phase of the beat signal
Caused by reflection from objects of similar or
larger size than the wavelength
Challenges II.
Localization algorithms
Computationally expensive
Highly redundant measurements
Measurement noise (due to multipath)
Requires PC-class hardware
Measurements are time consuming
1. Time synchronization
2. Calibration: tuning the transmitters to desired frequencies
3. Sampling the RSSI
4. Repeat steps 2 and 3 at multiple center frequencies
5. Report results (multihop routing)
Tradeoff
low computational power vs. measurement duration
Approaches I.
Low carrier frequency
Decrease multipath indoors
2.4GHz
–
0.125m
433MHz
–
0.69m
3MHz
–
100m
No modulo arithmetic needed if
wawelength
>
transmission range
Sufficient to measure phase at a single frequency
BUT:
Same velocity results in less Doppler shift
Antenna size increases
Limited unlicensed frequency bands at low frequencies
Redundant carrier frequencies
Find a consistent set in noisy measurements
Approaches II.
Redundant architecture nodes
Use spatial redundancy to mitigate measurement noise
Many possible measurement configurations possible
Allows for filtering out inconsistent q-ranges
Combine RSSI and RI measurements
Asymmetric architecture
Shift computation from tags to architecture
Inexpensive active tags transmitting pure sinusoids
computationally powerful architecture nodes
Possibilities
Increase beat frequency to shorten measurement time
(requires higher sampling frequency at receiver)
Use multiple sinusoids simultaneously
Eliminate calibration of beat frequency
Work in progress
Test platform: asymmetric architecture
Software defined radio (USRP/GNURadio)
Berkeley mica2 motes
Time Synchronization
SDR transmitter encodes a marking in its signal
SDR receivers use matched filter to find the position of the marking
Marking is a Hamming-windowed linear frequency modulated
(chirp) signal
Work in progress
Test platform: asymmetric architecture
Software defined radio (USRP/GNURadio)
Berkeley mica2 motes
Time Synchronization
SDR transmitter encodes a marking in its signal
SDR receivers use matched filter to find the position of the marking
Marking is a Hamming-windowed linear frequency modulated
(chirp) signal
Work in progress
Test platform: asymmetric architecture
Software defined radio (USRP/GNURadio)
Berkeley mica2 motes
Time Synchronization
SDR transmitter encodes a marking in its signal
SDR receivers use matched filter to find the position of the marking
Marking is a Hamming-windowed linear frequency modulated
(chirp) signal
Measurement results
Average jitter: 1 μs
Center freq:
Maximum jitter: 2 μs
Beat freq:
Phase:
1 degree
433MHz
1kHz
Questions
?
http://www.isis.vanderbilt.edu/projects/rips/