Ultra-Low Power Time Synchronization Using Passive Radio Receivers Yin Cheny, Qiang Wangz, Marcus Changy, Andreas Terzisy Johns Hopkins University, Harbin Institute of Technology IPSN 2011 Presenter: SY

Time Synchronization Protocols GPS, RBS, FTSP, PulseSync, Power line, etc. Desired properties – Low-power consumption – Support for large scale networks – Accuracy that is independent of network size – Access to UTC time – Support for disconnected operation – Low latency – Operate in both indoors and outdoors universal time signal receiver Their accuracy is in ms GPS, RBS, FTSP, PulseSync Power line

Outline Time Signal Radio Stations Universal Time Signal Receiver Evaluation Applications And Conclusion

Time Signal Radio Stations Broadcast time signal periodically Some of the time signal radio stations

WWVB Radio Station Location: Fort Collins, Colorado National Institute of Standards and Technology (NIST) Coverage Area grp40/vb-coverage.cfm

DCF77 Location: Mainflingen, Germany German national meteorology institute (PTB) Coverage area

LF Radio Waves Two components – Ground wave follows the Earth’s curvature – Sky wave Earth’s ionosphere For 50 kW DCF77 transmitter – within 600 km  ground wave dominates – km  ground wave and sky wave equal magnitude – beyond 1100 km  only the refracted sky wave

WWVB Data Frame One-bit-per-second time code Each frame contains 60 bits Data frames transmitted back-to-back – 60 frames per hour Start-of-a-second time stamps 06:11 UTC on the 144th day (May 24) of 2010 DST bits indicate that the daylight saving time is active

Outline Time Signal Radio Stations Universal Time Signal Receiver Evaluation Applications And Conclusion

Universal Time Signal Receiver Antenna CME6005 radio chip Microcontroller PIC16LF mm 33 mm analog time signals digital outputs Interrupt: Record timer value Interrupt: Record timer value Interrupt: Record timer value Calculate interval Interrupt: Record timer value Calculate interval Power Consumption: 90uA one-pulse-per- second (1PPS) UART output output Power Consumption: 0.6uA (sleep) 800uA (Active) Power Consumption: 0.6uA (sleep) 800uA (Active)

Power Consumption

Compare Power Consumption FTSP Universal Time Signal Receiver Compare

Synchronization Error

Signal Availability Hourly decoded ratio

Signal Availability

Some Notes Interference – Steel frame buildings completely shield – Brick buildings allow signal reception – CRT screens can interfere from 5-10 meters – Laptops can interfere within one meter Local Time Signal Generators – Up to 50 meters radius

Synchronous MAC Protocols Implemented a time-scheduled version of LPL running CTP on top Duty cycles

Other Applications Latency Reduction Sparse Networks Drop-in Replacement for GPS Network-Wide Wakeup Failure-Prone Sensor Networks

Conclusion Implemented universal time signal receiver – Very low power – Millisecond accuracy – Wide availability