[PPT] - FLIGHT: Clock Calibration Using Fluorescent Lighting Zhenjiang Li*, PowerPoint Presentation

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FLIGHT: Clock Calibration Using Fluorescent Lighting

Zhenjiang Li*, Wenwei Chen, Cheng Li, Mo Li, Xiang-Yang Li, Yunhao Liu

Nanyang Technological University, Singapore Hong Kong Universiy of Science & Technology, Hong Kong Tsinghua University, China Illinois Institute of Technology, USA

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Time Synchronization

! A variety of applications

! Phone-to-phone gaming

Virtual Table Tennis [Sensys’11] Virtual Sword Fight [MobiSys’12]

Precise 3D localization

2/35

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Time Synchronization

! A variety of applications

! Body-area networks

Fall Down Detection [SenSys’08-Demo]

E v e n t

r

d e r i n g & d e t e c t i

n

3/35

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Time Synchronization

! A variety of applications

! MAC-layer protocol design

B C D A Active State Dormant State

Time slot alignment

4/35 Opportunistic flooding [MobiCom’09]

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Time Synchronization

! Design challenges (1/2)

! Initial clock offset

Embedded Devices Smart phones, Tablets,...

Initial offset: seconds or even more !

5/35

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Time Synchronization

! Design challenges (2/2)

! Clock uncertainty

✓ Inherent clock uncertainty

CMOS oscillator 5-100 ppm

✓ Ambient environments

Temperature Humidity

✓ Internal factors

Supply voltage Pressure ... ...

1000 2000 3000 4000 5000 6000 3.2766 3.2766 3.2766 3.2767 3.2767 3.2767 3.2767 3.2767 3.2768 3.2768 3.2768 x 10

4

Time Elapsed (s) Clock Frequency (Hz)

50 32767.70 32767.80 32767.9 32768.0 1820 1840 1860 32766.90 32767.00 32767.10 32766.90 32767.00 32767.10 3620 3640 3660 32766.1 32766.2 32766.3 32766.4 Vcc = 2.1V Vcc = 2.4 V Vcc = 3.0 V

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Time Synchronization

! Calibration vs. synchronization ! Clock Calibration

✓ Similar speeds ✓

Various apps.

Time synchronization = Initial offset cancelation + Clock calibration

7/35

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Time Synchronization

! The state of the art

! Communication-based solutions

! RBS[OSDI’02], TPSN[SenSys’03], and FTSP

[SenSys’04]

✓ High power consumption & overhead ✓ Easy to implement

8/35

Internet Wireless Sensor Networks Sink Node

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Time Synchronization

! The state of the art

! External signal source based solutions

GPS module FM station [MobiSys’11] Electromagnetic radiation [Sensys’09]

✓ Less communication overhead ✓ Hardware support

9/35

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Idea Overview

! Key observations (1/3)

! Fluorescent lighting

Alternating Current (AC) 50 or 60 Hz Light Intensity (100 or 120 Hz)

P e r i

d

i c a l S i g n a l !

10/35

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Idea Overview

! Key observations (2/3)

! Availability of fluorescent light

Airport Data center Home Supermarket Office Hospital

Indoor Environments

11/35

Energy efficient 30~50W 24hr < 0. 1 USD

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Idea Overview

! Key observations (3/3)

! Mobile devices

! Sensor motes, smart phones,

tablets, notebooks,...

Light sensor / Camera

12/35

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Idea Overview

! FLIGHT

Sampling light sensor

13/35 Oscilloscope

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Idea Overview

! FLIGHT

S a m p l i n g l i g h t s e n s

r

Phase differences

Periodical Pattern !

14/35

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Design Overview

! System architecture of FLIGHT

Calibration Interval Prediction

Error Estimation Interval Calculation Interval Updating

Light Sensor Period Extraction Logic Time Maintenance Logic Time Control (Get / Set) Interval adaptation Clock calibration Logic time interface

Sampling Window Config. Filtering α(t) correction

Logic time updating

15/35

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Design Overview

! Period extraction

! Where is the delimiter?

Rotation

16/35

Facing the lamp Opposite to the lamp

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Design Overview

! Period extraction

! Roaming

100 m 480 m 230 m

Bright place Dark place

17/35

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Design Overview

! Period extraction

! Frequency domain Self-correlation, folding,...?

✓ Long computation delay ✓ Energy consumption ✓ Buffer occupancy

50 100 150 200 250 300 350 −5 −4 −3 −2 −1 1 2 x 10

4

Frequence (Hz) FFT

Feasible cutoff frequency region

18/35

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Design Overview

! Period extraction

! Filtering solution Advantages of filtering

✓ Less computation ✓ Energy efficient ✓ Less buffer occupancy First-order: Second-order: ...

20 40 60 80 100 170 175 180 185 190 195

Sampling Index Light Intensity (mV)

rder=1

20 40 60 80 100 175 180 185 190

Sampling Index Light Intensity (mV)

rder=2

20 40 60 80 100 178 180 182 184 186 188

Sampling Index Light Intensity (mV)

rder=4

20 40 60 80 100 182 183 184 185 186 187

Sampling Index Light Intensity (mV)

rder=6

19/35

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Design Overview

! Logic time maintenance

Define frequency ratio

20/35

t t

L IM - I1 = I Light periods: Clock ticks:

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Design Overview

! Logic time maintenance

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t t

Light periods: Clock ticks: Logic time:

t

1 / (α·fg)

. . . . . . . . .

n m ✓ Two clocks ✓ Granularity

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Design Overview

! Calibration interval

! Energy concern

Native clock Calibration Interval α(τ1) α(τ2)

ˆ ˆ

. . .

τ1 + t cl(τ1) + t / (α(τ1)·fg)

ˆ

Logic time τ1

calibration 1 calibration 2

τ2 1 / (α(τ2)·fg)

ˆ

Time synchronization: a common starting point + Clock calibration

22/35

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Design Overview

! Calibration interval

23/35

. . . . . .

L L-1 2 1 t t I1 IM

Clock ticks Light periods τ

✓ However...

✓ Buffer concern ✓ Uncertain delay of computation

✓ Long sampling window

✓ Robust to sampling jitters ✓ Better accuracy of freq. ratio

Parallelize sampling & filtering & delimiter searching operations

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Design Overview

! Calibration interval

! Filter order vs. sampling rate

24/35

SLIDE 25

Experimental Evaluation

! Experiment setup

TinyOS 2.1x TelosB mote (32.768KHz)

✓ Kernel: NesC

Beacon node

25/35

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Experimental Evaluation

! Clock offset

! Initial offset = 0

200 400 600 800 1000 1200 1400 −600 −400 −200 200 400 600 800 1000

Time (min) Logic time error (µs)

Max logic time error Avg logic time error

26/35

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Experimental Evaluation

! Distance to lamps

27/35 The rear of the laboratory

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Experimental Evaluation

! Distance to lamps

4 5 6 7 −1000 −800 −600 −400 −200 200 400 600 800 1000

Logic Time Error ( µs) Different Distances (m)

4 5 6 7 100

Light Intensity (mV)

Error−10min Error−20min Error−30min Light Intensity

28/35

✓ Lamps can be sparsely

used

✓ Each lamp can coope-

rate a large number of nodes

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Experimental Evaluation

! Mixed with other types of light

29/35

✓ Adding an additional

direct component (e.g., 800mV + 90mV)

✓ Sunlight, Filament

light, LED light

200 400 600 800 1000 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

Logic Time Errors (µs) CDF

Sunlight Filament light LED light

✓ Direct signals

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Experimental Evaluation

! Clock offset with roaming

200 400 600 800 1000 1200 1400 −800 −600 −400 −200 200 400 600 800 1000

Time (min) Logic Time Error (µs)

Period 1 Period 2 Period 3

Periods 1 & 2: roaming inside the academic building Period 3: roaming in the laboratory 30/35

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Experimental Evaluation

! Clock offset in two different buildings

31/35 Two nodes are separated by more than 100m Within [350,550] min, two nodes are locally roaming

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Experimental Evaluation

! Energy consumption

10 20 30 10 20 30 10 20 30 30 60 100 50 100 150 200 250 300 350

Average Power (µW)

FLIGHT(min) ROCS(min) FTSP(sec) WizSync(min)

10 15 20 25 30 35 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5

Calibration Interval (min) Average Power (µW)

✓ ROCS [MobiSys’11] ✓ WizSync [RTSS’11] ✓ FTSP [SenSys’04]

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Discussion & Future Work

! Main features

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✓ No extra hardware support ✓ Energy efficient ✓ Robust to network disconnection

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Discussion & Future Work

! Limitation

! Indoor environments

! Future works ! Other mobile / embedded

platforms

✓ Applicability ✓ Advanced techniques

! Fingerprint of indoor

environments

✓ One strong indication ✓ Automatically control

LBS, GPS, ...

34/35

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