Ranging and Distance Measurement in Sensor Networks

1
Ranging and Distance Measurement in Sensor
Networks

• Tufan C. Karalar
• Prof. Jan Rabaey
• University of California, Berkeley
• BWRC Winter Retreat
• January 13th 2005

2
Localization and need for ranging

• Given a set of reference points (artificial
  satellites, base stations etc.), determine the
  coordinates of unknown location

1. Measure distance to relate to reference points
2. Compute the coordinates.
Reference point 1
Reference point 1
r1
r1
Node being located
Located node
r3
r2
r3
r2
Reference point 2
Reference point 3
Reference point 2
Reference point 3
Savarse ICASSP01Karalar SIPS04
3
Challenges

• Be accurate even with small system physical
  dimensions (lt1m distance accuracy)
• Consume low power ( lt 30mW of max power)
• Require inexpensive components

4
Different Views
RX
TX
t

• Time domain view h(t) C d(t- t0)
• Radar, GPS, MITs Cricket,UWB ranging systems
• Frequency Domain Phase view ?H(?) ? t0
• Sonars, Acoustic MIMO systems
• Frequency Domain Amplitude view H(?) 1/da
• 802.11 based localization systems

5
Different views compared
Choose time domain view
6
Performance bounds
H. Van Trees Estimation Detection and Modulation
Theory, Vol. 3 p.299
Cramer Rao Lower Bound(CRLB) on the Time of
flight estimate

• Sampled Systems
• Max. range error due to delay quantization
  150m/Fs(Msps)
• 256 128Msps sampling rate is enough for 0.5 to
  1m accuracy

7
Wideband signals
Pulse based signals
Multi tone signals

• Easy implementation involving only an FFT
• No need for cyclic extensions
• No random data, no real Peak to Average power
  ratio problem at the PA

• Requires spectral estimation with
• FFT,
• SVD decomposition
• Least squares computation
• Expensive implementation

Choose Multi tone signals
8
Synchronization

• Signals with different speeds
• RF trigger, acoustic channel estimation
• Two way time transfer
• Reverse and forward transmission to overcome
  clock offsets

Choose two way time transfer
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Synchronize Two Way Time Transfer
Time of Flight (TOF)
Clock Offset (OS), 1 leads 2
Forward Rx _at_ TFR
Forward Tx_at_ TFT
Transceiver 1
Transceiver 2
Reverse Tx_at_TRT
Reverse Rx_at_TRR
TFRTFT OS TOF
TRRTRT OS TOF
TOF ½ TRR TRT TFR TFT

• Can Measure TOF irrespective of clock offset!

US Naval Observatory, Telstar Satellite, circa
1962 http//www.boulder.nist.gov/timefreq/time/two
way.htm
Multiband OFDM ranging proposal IEEE
802.15-04/050r0, JAN 2004
10
Proposed scheme
TRR
TFR
TFR TOF OS TRR TOF OS TOF 0.5(TFR
TRR) OS 0.5(TFR - TRR)
Assumption Time of Flight (ToF) does not exceed
the periodicity (T)
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Proposed system specifications

• Fs128Msps, T 1µs
• 128 point FFT
• 64MHZ Total signal BW
• 500kHz carrier spacing
• 6bit ADC
• 2.4GHz ISM band operation
• PLL loop has 800kHz BW, -60dB in-band
  attenuation. Due to multi band approach PLL spec
  is important.

12
Receiver block diagram
Down convert the signal
LO
FFT
DEMUX
BUFFER
Sample
r(t)
. . .
BPF
ADC
. . .
. . .
/Pilot(?) (Remove Pilot seq.)
Time of arrival
IFFT
First Strongest tap
Range
. . .
. . .
. . .
. . .
K
Time of arrival in reverse dir
13
Transmitter block diagram
LO
MUX
IFFT
GeneratePilotsequence
BUFFER
PA
BPF
DAC
. . .
. . .
Up convert the signal
14
Performance

• 5-tap Rician channel, with exponential delay
  profile
• 100ns channel delay spread
• Actual range varies 1.5-13m

15
Baseband power estimates
Digital FFT power estimate
A/D power estimate
16
Prototyping

• Aims to verify functionality of the algorithm on
  the field
• FPGA for tasks performed in digital
• COTS analog blocks for analog tasks
• Reuse two boards that has been designed by other
  groups in BWRC

2.4GHz RF board from MCMA group
Baseband Board from TCI project
17
More on prototyping

• Could not use bluetooth transceivers due to
  integrated GMSK modulators
• Could not use 802.11 front ends due to BW
  limitations (20MHz)

• Baseband board includes
• Virtex300E FPGA (300k CLB)
• 125Ms/s 8bit DAC
• 100Ms/s 8bit ADC
• RF board includes
• 2.4 GHz Frequency synthesizer
• Integrated LNA, mixer, IF amplifiers
• On board Continuous time LPF
• IQ modulator, PA, RF amps, RX/TX switch

18
Conclusions and future work

• Conclusions
• An RF signal based TOF measurement scheme for
  ranging in sensor networks is proposed
• Analysis of the proposed system is presented
• A system prototype is described

• Future work
• Compete the system prototype and demonstrate its
  functionality
• Determine possible practical problems and causes
• Integrate the range measurement and triangulation
  blocks