radio frequency t of distance measurement for low-cost wireless sensor localization
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Radio Frequency T OF Distance Measurement for Low-Cost Wireless Sensor Localization. Steven Lanzisera, David Zats and Kristofer S. J. Pister. Introduction 1. Paper was published in 2010 Localization is a hot topic Creating location-aware sensor networks - PowerPoint PPT PresentationTRANSCRIPT
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Radio Frequency TOF DistanceMeasurement for Low-CostWireless Sensor Localization
Steven Lanzisera, David Zats and Kristofer S. J. Pister
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Introduction 1.• Paper was published in 2010• Localization is a hot topic
– Creating location-aware sensor networks– Enabling mobile phones to host a lot of new
applications• Requirements
– Low-cost / low energy consumption is crucial– If we require a certain accuracy – we have to deal
with measurement errors
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Introduction 2.• Methods of localization
– Acustic (BeepBeep, we have seen it)– RF techniques (no GPS)– GPS
• The paper proposes an RF solution– Low cost / narrowband– No time-syncronization required– No base-station required– Approaches the Cramér-Rao bound in noisy environment– Accuracy – only in the order of a few meters (!)
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Abbreviations• RSS – Received Signal Strength• TOF – Time-of-flight• TWR – Two-way ranging• TWTT – Two-way Time Transfer• UWB – Ultra wide band• CMS – Code modulus synchronization• CRB – Cramér-Rao Bound• SNR – Signal-Noise Ratio• RMS – Root Mean Square• MSE – Minimum Squared Error• CDF – Cumulative Distribution• MSK – Minimum Shift Keying (FSK = Frequency ~)
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Localization problem• Localization consists of two parts
– Measure relationships between nodes– Using this information to determine position of
nodes• Received Signal Strength
– Well-studied method– Determines range based on signal strength– Very inaccurate
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Alternatives• Using Ultra Wide Band ranging
– UWB receivers are very complex and expensive• Narrowband solutions
– They usually require time synchronization, that adds complexity again
• A low cost – simple technology is needed with meter-level accuracy
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The presented system• Two-way ranging system• CMS (Code modulus synchronization)
– No time-sync required– Online measurement – Offline range extraction
• Works well in noise-limited environment• Mitigates the effects of multipath propagation
– Idea: take measurements on multiple frequencies• Approaches the Cramér-Rao bound• Room-level accuracy satisfied (~1-3m)
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Cramér-Rao bound• Statistics – estimation theory• Expresses a theoretical lower bound on variance of
estimators of a deterministic parameter• – unknown deterministic parameter
– number of measurements – probability density function of – expected value• Cramér-Rao bound
where
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Test-implementation• Commercially available accessories• 2.4GHz radio – Frequency Shift Keying• IEEE 802.15.4
– Standard which specifies physical layer and media access control for low-rate wireless PANs
– Zigbee, MiWi ... etc. • FPGA• Overall accuracy: 1m outdoor / 1-3m indoor
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Localization method 1.• Multilateration
– Determining the a 2D position with 3 reference nodes (reference nodes: fixed, known position)
• More nodes – better accuracy
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Localization method 2.• More reference nodes should be used than
strictly necessary• The geometry of the ref. nodes is important
– Collinear references do not work• This area is highly understood, the more
important part is determining the position from erroneous measurements
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Range estimation methods• RSS – constructive and destructive interference
make it unsuitably inaccurate• Time-of-Flight methods
– Speed of light = 299,792,458 m/s– 1 meter range accuracy = 3ns time resolution– Low-cost devices provide the same sampling
resolution as their clock frequency ~50ns– Cost, complexity and terrestrial environment (in
comparison with GPS) make TOF ranging unsuitable
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Types of errors to consider• Clock synchronization• Noise• Errors of samping artifacts• Multipath channel effects
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Clock synchronization• Usually a common time reference is required
in TOF systems• TWTT – Two-way time transfer
– mitigates the time offset, but not the frequency error (clock drift) – we have to deal with it!
A
BTs,A Tr,A
Tr,B Ts,B
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Noise 1.• We consider white noise• The accuracy depends on two components:
– Bandwidth (B)– Energy-to-noise ratio (Es/N0)
• CRB:
for most signals:ts – signal duration; SNR – Signal/noise ratio
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Noise 2.• Increasing the bandwidth increases tsB• Larger bandwidth – improved noise perform.• CRB can be closely approached if:
• Increasing the number of measurements improve the results in quadratic order
• Conclusion: noise alone does not prevent 1m accuracy if bandwidth is over a few MHz
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Noise 3.• Cramér-Rao bound as function of bandwidth• Basically, we increase power to increase Es/N0
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Sampling error 1.• Range binning
– Sampling rate: fs = 2B– Estimating the time of arrival– The space is divided into bins with c / fs width
• Sampling adds uniform uncertainity in each bin of :
• This will be (43m)2 if B = 2Mhz and fs = 1/B, BUT can be decreased to (1m)2 by making 1000 measurements
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Sampling error 2.• Tracking, filtering, averaging can eliminate this
error, but that is very unefficient• OR: Signal can be oversampled
– Usually the sampling error dominates the overall error, and not the CRB (the noise) – unless the sampling is very fast
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Sampling error 3.• (continued)
– In real systems usually 15dB < Es/N0 < 30dB, and noise is not a problem
– If we sample the signal above the Nyquist limit(fs > 2B) the entire information is captured and smaller sampling error is achieveable
– Interpolation can be done, but its complexity and power consumption is usually way out of the capabilities
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Multipath effects 1.• The signal reaches the receiver via different
paths – a path is called a channel• Impulse response of the channel:
• i=0 represents the direct path• Received signal:
(m(t) – transmitted signal)
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Multipath effects 2.• Noise does not effect multipath performance• We consider the two-path case
• For small periods, the , and are random variables, but they are freqency-independent over a given RF communication band
• We consider them constant for small periods
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Multipath effects 3.• A few MHz change in frequency dramatically
effects the multipath environment– Because of interference (constructive/destructive)
• Measured RSS (fixed transmitter/receiver)
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Multipath effects 4.• Delay spread: time between
first and last paths• Most of the signal
bandwidth is observable if
• Typical interpath delay, is more important• Indoors is usually between 5 and 10 ns• The estimate is blurred by the multipath effect• To resolve this problem we need B>100MHz, or at
least B>1/
T R
Delay spread
t
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Multipath effects 5.• Possible solutions to mitigate multipath effects:
– Increase bandwidth– Estimating channel impulse response– Multipath bias reduction
• The first two are well-studied• Using devices with larger bandwidth (UWB) is
expensive and they consume to much power• The achieveable accuracy appears to be around
30m with the second method – not sufficient
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The solution
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Ranging error mitigation• The paper presents two new methods to
mitigate all the errors– Code modulus synchonrization
• Combats sampling effects and poor time syncronization– Frequency diverse range estimation
• Improves range estimation accuracy
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Code modulus synchronization 1.• CMS uses a periodic signal, to modulate an RF
carrier, so large B*ts is possible (therefore noise is not a problem)
• First shaded region: C transmits the code to D• The phases are offset, but D knows the length
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Code modulus synchronization 2.• D samples and demodulates the signal, and
stores it• At this point D has a local copy of the code, but
it is shifted due to the clock phase offset• Now D sends back (two copies of) the code
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Code modulus synchronization 3.• C receives the transmission of D, and records it,
synchronized to its own local reference• The circular phase shift will be exactly undone this
way because of the round-trip nature of the system• C computes the cross-correlation and the measured
code-offset is the TOF
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Code modulus synchronization 4.• The received code can be interpolated to
improve resolution up to the noise limit• The system approaches the CRB even with a
single measurement• Multiple measurements can be averaged – this
helps achieving good noise-performance• Correlation and code-offset estimation can be
done offline after the RT part has ended
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CMS vs. TWTT 1.• CMS vs. TWTT
– Only one node performs the calculation → better sampling performance
BUT– The full processing gain of the system is not realized
at second node → Noise penalty– This means, that the second transmission (D→C)
contains noise from the first part (C→D)
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CMS vs. TWTT 2.• Only one node performs the calculation →
better sampling performanceBUT
• The full processing gain of the system is not realized at second node > Noise penalty
• This means, that the second transmission (D>C) contains noise from the first part (C>D)
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CMS vs. TWTT 3.
• - number of code copies averaged• The last factor represents the noise penalty of CMS
– For very low SNR, it is approximately ½ if no averageing is used ( = 1)
– For moderate to large values of , there is almost zero penalty
• Single measurement variance is also better• CMS is better to approach the CRB
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Frequency diverse range estimation 1.• Mitigates the multipath effect• Takes measurements on several carrier
frequences• The problem:
– Signal comes via two paths: one direct, more reflected– There is a delay and phase difference between them– Only the phase depends on the actual value– IEEE 802.15.4 uses MSK, a version of FSK– When changes to , the signal from the second
path have not arrived yet
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Frequency diverse range estimation 2.• Simulation shows, that this can result in either
positive or negative biases in range estimation
• According to the figure, we should make measurements over the same channel, with different phase relationships – averaging the value will reduce the overall bias
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Frequency diverse range estimation 3.
• Because the phase differencedepends on the , they usedifferent carrier frequencies
• The median of 16 estimateshad the best error performance,(compared to averaging): 80% below 3m error
• The demonstration environment implements this method
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Prototype 1.• Waldo device
– 2.4GHz radio– DA interfaces– FPGA (Verilog)– Microcontroller (C)
• Implementation– Bandwidth = 2MHz– Binary frequency shift keying: +/- 0.75MHz– Sampling: 5MHz digital demodulation– Demodulated data bandwidth limit: 2MHz with 16MHz
sampling – randge bins of 19m
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Prototype details• Ranging between node pairs
– Coordination / acknowledgement– 16 measurements – median is used– Maintaining CMS (2-period-length code 32 times)– Non-RT processing offline (linear regression to
estimate TOF)
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Tests 1.
• Better than 3m overall accuracy• Noise performance
– Verification with cable and simulated noise– Work within a factor of 2 of the CRB
Because of the limited dynamic range of the digital baseband processor
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Tests 2.• Ranging demonstrations (compared to RSS)
outdoor indoor
Received SignalStrength
CBS + Freq. Diverse Range Estimation
Error ratio (outdoor) 20% <1m 80% <1m
Error ratio (indoor) 50% <8m 50% <1m; 80% <3m
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Tests 3.
• Open area – 40×50m– Max distance: 70m– 4 static nodes– Simple MSE estimation– 80% of errors < 2m
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Conclusion
• CMS is a TWR method that approaches the CRB• Freq. diverse ranging estimation is a strategy
that improves ranging in multipath environments
• Overall accuracy: 1m outdoors, 1-3m indoors• Where Es/N0 is large, sampling error dominates
the noise-induced error, but CMS avoids this• Easy implementation, low costs, no UWB device
required
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Thank you for you attention!