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

1. Energy-Efficient Rate-Adaptive GPS-based Positioning for Smartphones Jeongyeup Paek, Joongheon Kim, Ramesh Govindan CENS Talk April 30, 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 Relatively less clear view of the sky Actual Path taken No received GPS signals Distance (meter) A B Never went here Incorrect GPS Path Samples (70 locations) 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. Periodic Duty-Cycling • There exist uncertainty • Key challenge is to decide T • Accuracy vs. Energy trade-off exists • May introduce significant uncertainty …. T Uncertainty (distance) Energy

5. 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 • Trade-off position accuracy for reduced energy • Challenge • Determine when and when not to turn on GPS efficiently using the sensors and information available on a smartphone

6. 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 NOT to turn on GPS

7. Activity Detection • Use accelerometer to detect user motion • Binary sensor to detect non-movement • Measure activity ratio • Onset detector for identifying activity • 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%

8. 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

9. Celltower Data for Movement Detection? • Celltower and RSS data cannotreliably measure user movement

10. Celltower-RSS Blacklisting • However, it can detect GPS unavailability • Signatures exist for indoor places that you go often Turn on GPS only when available! Good Variable Bad

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

12. Bluetooth Position Synchronization • Use Bluetooth to synchronize position information with neighboring nodes • Cheaper than GPS • Little uncertainty • Short communication range (~10m) • Bluetooth is widely being used • Saves energy by 43% in 2-node example • TX costs ~3.07 Joule, RX costs ~1.58 Joule • GPS activation for 60sec costs ~22 Joule • (22 * 1 + 3.07 + 1.58) / (22 * 2) • More the merrier! • Energy cost is amortized over the number of nodes in neighborhood • For 5 nodes, 74% reduction in energy Off On Listen & connect RX Off Listen Bluetooth Slave On Device Discovery TX Listen Off Off Bluetooth Master

13. Evaluation • Benefits of RAPS • Energy savings achieved by RAPS • Contribution of individual components • Comparison to periodic GPS strategy • Flexibility • Integration with WPS • Pervasiveness of GPS errors • GPS vs. AGPS • Different platforms

14. Benefits of RAPS Home • Energy savings achieved by RAPS • Contribution of individual components • Methodology • 6 phones with 5 different schemes • around USC campus area (in & outside buildings) • 34 hours Shopping ~3 miles 0.4 miles Class Library Lunch Lab Class Periodic GPS with 20 seconds interval USC

15. 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

16. Benefits of RAPS - Reasons Avg. GPS Activation Interval Expected Avg. Power Usage 630.9 588.5 GPS Interval (seconds) Estimated Average Power (W) 259.5 135.4 (20) Tested Schemes Tested Schemes

17. Did BPS work? GPS Activation: BPS Communication: • Contributed 10.8% of the total RAPS lifetime savings • Defers GPS activation • Bi-directional  natural incentive for sharing BPS Disabled BPS EnabledNode 1 BPS EnabledNode 2

18. Did Celltower-RSS Blacklist work? • Contributed 59% of the total lifetime increase • Significantly increase the average interval between GPS activations • For majority of cell towers, GPS position fixing never fails • For a smaller number of cell towers, mostly those that the user visits often, GPS failures do occur Observation Count Success Ratio (%) Observed Cell-towers Do not turn on GPS when not available!

19. Comparison to Periodic GPS • RAPS consumes, • 48% less power compared to periodic GPS with comparable average uncertainty • 22% less power compared to periodic GPS with comparable success ratio • Success Ratioachieved by RAPS • Average Distanceachieved by RAPS Distance (meter) 72.2% Success Ratio (%) 85.8 m Periodic Interval (Seconds) Periodic Interval (Seconds) 1.47 times longer lifetime 1.14 times longer lifetime

20. Is RAPS flexible? • Integration with WPS, a WiFi Positioning System • Energy cost include WiFi scanning and data communication with database server • Known to be less accurate than GPS • Yes!! • Consumes less energy • Faster position fix and turn off time • Lower accuracy Power (Watt) Time (Seconds)

21. Are GPS Errors Pervasive? AGPS vs. GPS Four different types of phones Relatively less clear view of the sky Relatively less clear view of the sky Distance (meter) Distance (meter) Samples (35 Locations) Samples (35 Locations) Negligible difference (difference only in FTTF) Similar (except G1)

22. 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 • Future Work • Parameter settings (e.g. accelerometer duty cycling) • Privacy and security for position sharing • User study (variety, consistency, handling behavior, etc.) Thank you