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 1

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

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 3

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 4

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 5

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 6

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,… 7

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

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

10. Design Overview System architecture of FLIGHT 10

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

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

13. Design Overview Notation 13 NotationMeaning c n (t) Native clock at time t (tick) c g (t) Global reference clock at time t (tick) c l (t) f n (t) Frequency of the native clock at time t fgfg Frequency of the fluorescent light α(t)Frequency ratio: f n (t)/ f g

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

15. Design Overview Logic time maintenance 15 Define frequency ratio: calibration window size τ

16. Design Overview Logic time maintenance – Eliminate logic time drift between two consecutive calibrations – Update logic time 16

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

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 18

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

20. Performance Evaluation 20

21. Performance Evaluation 21

22. Performance Evaluation Mixed with other types of light 22 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 23 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 24

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

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

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 27