Energy-Efficient Rate-Adaptive GPS-based Positioning for Smartphones

1. Energy-Efficient Rate-Adaptive GPS-based Positioning for Smartphones Jeongyeup Paek USC Annenberg Graduate Fellowship Program The Second Annual Research and Creative Project Symposium April 29, 2010

2. Problem • Many emerging smartphone applications require position information to provide location-based or context aware services. • GPS is often preferred over GSM/WiFi based methods. • But, GPS is extremely power hungry! • Can drain phone battery in few hours. Average Error GPS: 23.2m WPS: 36.8m GSM: 313.6m GSM WPS GPS Average Error GPS: 8.8m WPS: 32.44m GSM: 176.7m Off (0.06) On (0.44) GSM WPS GPS

3. GPS (In)accuracy Actual Path taken A B Never went here Incorrect GPS Path C GPS may provide less accurate positioning in urban areas, especially for pedestrian use Then… can we sacrifice a little accuracy in exchange for significant reduction in energy usage?

4. RAPS : Rate-Adaptive Positioning System • An energy-efficient positioning system that adaptively duty-cycle GPS only as often as necessary to achieve required accuracy based on user mobility and environment • Design Goal • Reduce the amount of energy spent by the positioning system while still providing sufficiently accurate position information • Challenge • Determine when and when not to turn on GPS efficiently using the sensors and information available on a smartphone

5. RAPS Components • Movement Detection • Use duty-cycled accelerometer with onset detection algorithm to efficiently measure the activity ratio of the user • Velocity Estimation • Use space-time history of the past user movements along with their associated activity ratio to estimate current user velocity • Unavailability Detection • Use celltower-RSS blacklisting to detect GPS unavailability (e.g. indoors) and avoid turning on GPS in these places • Position Synchronization • Use Bluetooth-based position synchronization to reduce position uncertainty among neighboring devices When to turn on GPS When to NOTturn on GPS

6. Activity Detection • Use accelerometer to detect user motion • Binary sensor to detect non-movement • Measure activity ratio • Duty-cycle it for energy efficiency • 5 min accelerometer consumes more energy than 1 min GPS Activity Off (0.062) On (0.141) Operating point: 12.5%

7. Velocity Estimation • Use history of user positions • Associate average velocity and activity ratio to particular space and time • Use these information to estimate current user velocity • Using this velocity, calculate uncertainty and decide when to turn on GPS C D B A

8. Celltower-RSS Blacklisting • Use celltower information to detect GPS unavailability • Signatures exist for indoor places that you go often Turn on GPS only when available! Good Variable Bad

9. Bluetooth Position Synchronization • Use Bluetooth to synchronize position information with neighboring nodes • Cheaper than GPS • Short communication range (~10m) • Bluetooth is widely being used Save energy by lowering overall uncertainty and reducing the number of GPS activations

10. Evaluation Home • Energy savings achieved by RAPS • Contribution of individual components • Methodology • 6 phones with 5 different schemes • around USC campus area Shopping ~3 miles 0.4 miles Class Library Lunch Periodic GPS with 20 seconds interval Lab Class USC

11. Benefits of RAPS - Lifetime • RAPS’s lifetime is 3.87 times longer than that of Always-On • Each of its components contribute to this saving 34:41 31:53 BSP – 10.8% 3.87 times longer lifetime! Blacklist – 59.0% Lifetime (hours) 16:42 16:19 Accel – 1.5% 8:57 History – 28.5% Tested Schemes

12. Conclusion and Future Work • RAPS is a rate-adaptive positioning system for smartphone applications • GPS is generally less accurate in urban areas, so it suffices to turn on GPS only as often as necessary to achieve this accuracy • Uses collection of techniques to cleverly determine when to and when not to turn on GPS • Increases lifetime by factor of 3.8 relative to Always-On GPS Thank you