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Ultra-Low Power Time Synchronization Using Passive Radio Receivers

Ultra-Low Power Time Synchronization Using Passive Radio Receivers. Yin Chen † Qiang Wang * Marcus Chang † Andreas Terzis †. * Dept. of Control Science and Engineering Harbin Institute of Technology. † Computer Science Department Johns Hopkins University. Motivation.

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Ultra-Low Power Time Synchronization Using Passive Radio Receivers

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  1. Ultra-Low Power Time Synchronization Using Passive Radio Receivers Yin Chen†Qiang Wang* Marcus Chang† Andreas Terzis† *Dept. of Control Science and Engineering Harbin Institute of Technology †Computer Science Department Johns Hopkins University

  2. Motivation • Message passing time synchronization • Requires the network be connected • Requires external time source for global synchronization • Is there a low-power and low cost solution?

  3. How did we disseminate time information in history?

  4. Time Ball

  5. Since half a century ago, we started to use RF time signals.

  6. Current Day Time Sources Radio Controlled Clocks & Watches LF Time Signal Radio Stations This work will test DCF77 and WWVB

  7. Contributions • Ultra-low power universal time signal receiver • Evaluations on time signals availability and accuracy in sensor network applications • Applications using this platform The antenna is 10 cm in length Smaller ones are available but we have not tested on our receiver

  8. WWVB Radio Station • Located near Colorado, operated by NIST • Covers most of North America

  9. WWVB Time Signal • 60 kHz carrier wave • Pulse width modulation with amplitude-shift keying • NIST claims • Frequency uncertainty of 1 part in 1012 • Provide UTC with an uncertainty of 100 micro seconds

  10. WWVB Signal Propagation • Part of the signal travels along the ground • Groundwave : more stable • Another part is reflected from the ionosphere • Skywave : less stable • At distance < 1000 km, groundwave dominates • Longer path, a mix of both • Very long path, skywave only

  11. WWVB Code Format • Each frame lasts 60 seconds • Each bit lasts 1 second 60 seconds 2010-5-24 06:11:00 UTC Marker bit Bit value = 0 Bit value = 1

  12. Time Signal Receiver Design • Requirements • Low power consumption • High accuracy • Low cost • Small form factor

  13. Core Components • CME6005 • 40-120 kHz, can receive WWVB, DCF77, JJY, MSF and HBG • less than 90 uA in active mode and 0.03 uA when standby • PIC16LF1827 • 600 nA in sleep mode with a 32 KHz timer active • 800 uA when running at 4 MHz • Costs (as of 2010) • CME6005: $1.5 • PIC16LF1827: $1.5 • Antenna: $1 • Total: $4 Most of the time Reading bits & Writing to the uart Drop-in replacement of GPS Time in NMEA format & 1-pulse-per-second

  14. Decoder Loop • Every second • MCU decodes the next bit from the signal receiver • Every minute • MCU decodes the UTC time stream • MCU sends the time stream to the uart

  15. Power Consumption

  16. Experiment Configurations • Multiple motes, each connected to a receiver • One master mote • All motes are wired together • Master mote sends a pulse through a GPIO pin every 6 seconds • All motes timestamp this pulse as the synchronization ground truth • For WWVB, the distance is 2,400 km (indoor & outdoor), mainly sky wave • For DCF77, the distance is 700 km (indoor), mainly ground wave Near the edge of the coverage map

  17. Outdoor Experiment

  18. Availability

  19. WWVB Outdoor WWVB Indoor DCF 77 Indoor

  20. Accuracy • The differences of the time readings at the motes when the master mote sends the pulses Clock frequencies vary more in outdoor experiment

  21. Comparison with FTSP • FTSP sync accuracy depends on resync frequency • Because clock frequency varies over time

  22. Clock Frequency Variations Motes were placed together under a tree.

  23. Power Consumption • What happens as sync interval T increases? • Schmid et al. observed that FTSP syncs in the millisecond range when using T = 500 seconds interval Time signal receiver Sync error in milliseconds range FTSP

  24. Qualitative Observations • Steel frame buildings completely shield the time signal • Brick buildings allow signal reception • Laptops (when using AC power), oscilloscopes can easily interfere the time signal within a few meters • We used a portable logic analyzer connected to a laptop running on its battery

  25. Applications • Synchronous MAC Protocols • Latency Reduction • Sparse Networks • Drop-in Replacement for GPS • Network-Wide Wakeup • Failure-Prone Sensor Networks

  26. Synchronous MAC Protocols • Modify LPL • Sleep interval is divided into slots

  27. Summary • Lower power consumption in the millisecond range • Support sparse networks • Provides an appropriate solution to the milliseconds and seconds range • GPS is an overkill • RTC drifts a few minutes per year even with temperature compensation

  28. Thank you!

  29. Signal Generator • 50 meters coverage

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