FLIGHT: Clock Calibration Using Fluorescent Lighting

1. 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

2. Outline • Introduction of time synchronization • Design Overview • Performance Evaluation • Discussion • Conclusion

3. 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

4. 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

5. 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

6. 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

7. 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,…

8. Empirical Measurement Study • To evaluate stability and accuracy of Fluorescent lighting • Single-lamp experiment in the laboratory

9. Empirical Measurement Study • Multi-lamp experiment in the laboratory

10. Design Overview • System architecture of FLIGHT

11. Design Overview • Period extraction • To exploit the sensitivity of the detected light intensity • Moving across different floors • Rotating the sensor node

12. Design Overview • Period extraction • Frequency domain – FFT + low-pass filter • Use filter to make maximum point unique in one period

13. Design Overview • Notation is the instant rate of the native clock at time

14. Design Overview • The concept of periodical calibrations • Calibration interval • Computation and energy concern • Needs to precisely compensate the clock frequency and eliminate drift

15. Design Overview • Logic time maintenance L:the number of light periods detected in the calibrationwindow τ Ij: the sample index where 1 ≤ j ≤ M : the observed native clock frequency in τ based on the generated global reference calibrationwindowsize τ Define frequency ratio:

16. Design Overview • Logic time maintenance • Eliminate logic time drift between two consecutive calibrations • Update logic time : the finish time of the ithcalibration : the number of light periods detected in the ithcalibration : the number of light periods between the ith and i+1th calibration

17. Design Overview • Calibration interval • Long sampling window • Robust to sampling jitters • Better accuracy of freq. ratio • However... • Buffer concern • Uncertain delay of computation

18. 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

19. Performance Evaluation • Calibration interval • Filter order vs. sampling rate

20. Performance Evaluation • Static case • Average error less than 600 , 80% < 200 • Maximum error < 950 , 80% < 350

21. 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

22. 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

23. Performance Evaluation • Three dynamic cases (1/3) • Time error with controlled mobility Error < 1000us, 80% < 400us

24. 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 environment and 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

25. Performance Evaluation • Energy consumption • ROCS [MobiSys’11] • WizSync[RTSS’11] • FTSP [SenSys’04]

26. Discussion • Main features • No extra hardware support • Energy efficient • Robust to network disconnection • Limitation • lighting availability • Exposure to the lighting • Noise interference

27. 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