FLIGHT: Clock Calibration Using Fluorescent Lighting
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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 MobiCom 2012 MengLin, 2012 1
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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 - PowerPoint PPT Presentation
Transcript of FLIGHT: Clock Calibration Using Fluorescent Lighting
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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, SingaporeHong Kong Universiy of Science & Technology, Hong Kong
Tsinghua University, ChinaIllinois Institute of Technology, USA
MobiCom 2012
MengLin, 2012
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Outline
• Introduction of time synchronization
• Design Overview
• Performance Evaluation
• Discussion
• Conclusion
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Time Synchronization
• A variety of applications– Phone-to-phone gaming• Precise 3D localization
– Body-area networks• Event ordering and detection
– MAC-layer protocol design• Time slot slignment
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Time Synchronization
• Design challenges– Initial clock offset– Clock uncertainty• CMOS oscillator• Clock drift rate of 30-50 ppm
– Ambient environments• Temperature, humidity, …
– Internal factors• Supply voltage
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Time Synchronization
• Calibration vs. Synchronization– Clock calibration mainly ensures that different
clocks advance with a same speed– Clock synchronization ensures the absolute clock
values of different nodes to be consistent– Clock synchronization = Initial offset cancelation +
clock calibration
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Time Synchronization
• The state of the art– Communication-based solutions• High communication overhead and power drain
– External signal source based solutions, such as• Power lines => signal decay• FM radio => power consuming• Wi-Fi => channel contention and collisions• All requiring hardware support
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Idea Overview
• Key observations– Fluorescent lighting• Twice of the AC
– Can be available in most indoor environment
– Light sensor / camera on sensor motes, smart phones,…
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Empirical Measurement Study
• To evaluate stability and accuracy of Fluorescent lighting
• Single-lamp experiment in the laboratory
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Empirical Measurement Study
• Multi-lamp experiment in the laboratory
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Design Overview
• System architecture of FLIGHT
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Design Overview
• Period extraction– To exploit the sensitivity of the detected light
intensity• Moving across different floors• Rotating the sensor node
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Design Overview
• Period extraction– Frequency domain – FFT + low-pass filter– Use filter to make maximum point unique in one
period
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Design Overview
• NotationNotation Meaning
cn(t) Native clock at time t (tick)
cg(t) Global reference clock at time t (tick)
cl(t) Logic clock at time t ()
fn(t) Frequency of the native clock at time t
fg Frequency of the fluorescent light
α(t) Frequency ratio: fn(t)/ fg
is the instant rate of the native clock at time
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Design Overview
• The concept of periodical calibrations
– Calibration interval• Computation and energy concern
– Needs to precisely compensate the clock frequency and eliminate drift
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Design Overview
• Logic time maintenance
Define frequency ratio:
L: the number of light periods detected in the calibration window τ Ij: the sample index where 1 ≤ j ≤ M: the observed native clock frequency in τ based on the generated global reference
calibration window size τ
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Design Overview
• Logic time maintenance– Eliminate logic time drift between two consecutive
calibrations– Update logic time
: the finish time of the ith calibration: the number of light periods detected in the ith calibration: the number of light periods between the ith and i+1th calibration
𝑡=𝑡𝑐 𝑗
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Design Overview
• Calibration interval– Long sampling window
• Robust to sampling jitters• Better accuracy of freq. ratio
– However...• Buffer concern• Uncertain delay of computation
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Performance Evaluation
• Experiment setup– One beacon node placed in the middle of the
laboratory to trigger each node logging its current logic time
– 12 sensor nodes distributed in the lab
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Performance Evaluation
• Calibration interval– Filter order vs. sampling rate
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Performance Evaluation
• Static case– Average error less than 600 , 80% < 200 – Maximum error < 950 , 80% < 350
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Performance Evaluation
• Distance to lamps– Only open one lamp in the rear of the laboratory• Light intensity > 25mv => logic time error < 600• Each lamp can sparsely used to cooperate many nodes
– Logic Intensity > 50mv with 6m coverage away from lamp
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Performance Evaluation
• Mixed with other types of light
Sun light, 80% of time error < 600us
LED light, 80% of time error < 900us
Filament light, 80% of time error < 610us
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Performance Evaluation
• Three dynamic cases(1/3)– Time error with controlled mobility
Error < 1000us, 80% < 400us
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Performance Evaluation
• Dynamic case(2/3)– roam in the office, classroom, and laboratory– Outside the lab [period1 & 2], error is larger– Due to the mobility, surrounding environmentand uncovered by light
(3/3) – roam in two different buildings which are 150m away from each other– Within [350,550] min, two nodes are locally roaming– Avg error = 400 us; 1000us when moving
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Performance Evaluation
• Energy consumption– ROCS [MobiSys’11]– WizSync [RTSS’11]– FTSP [SenSys’04]
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Discussion
• Main features– No extra hardware support– Energy efficient– Robust to network disconnection
• Limitation– lighting availability– Exposure to the lighting– Noise interference
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Conclusion
• Utilizing fluorescent lighting as external signal source to perform synchronization is stable and energy saving
• The frequency of external signal source determines the granularity of logic time
• Nice comparison and organization but many notations are confusing without clearly description
