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. 2 WPS GSM GPS Average Error GPS: 23.2m WPS: 36.8m GSM: 313.6m WPS GPS GSM Average Error GPS: 8.8m WPS: 32.44m GSM: 176.7m Off (0.06) On (0.44)

3. GPS (In)accuracy 3 Actual Path taken Incorrect GPS Path Never went here A A B B C 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? Distance (meter) Samples (70 locations) No received GPS signals Relatively less clear view of the sky

4. Periodic Duty-Cycling There exist uncertainty – Key challenge is to decide T Accuracy vs. Energy trade-off exists May introduce significant uncertainty 4 T Uncertainty (distance) Energy ….

5. RAPS 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 5

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 6 When to turn on GPS When NOT to turn on GPS

7. Activity Detection accelerometer 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 7 Activity Off (0.062) On (0.141) Operating point: 12.5%

8. Velocity Estimation history 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 8 A B D C

9. Celltower Data for Movement Detection? Celltower and RSS data cannot reliably measure user movement 9

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

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 11 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 74% – For 5 nodes, 74% reduction in energy 12 Off Bluetooth Master Bluetooth Slave On Listen & connect Device Discovery RX TX Listen Off

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 13

14. Benefits of RAPS 14 Lab Class LibraryLunch Shopping Home USC ~3 miles 0.4 miles Class HistoryAccel. Cell-RSS Blacklist BPS RAPS (2) RAPS-B RAPS-BC RAPS-BCA Always-On Periodic GPS with 20 seconds interval 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

15. Benefits of RAPS - Lifetime 3.87 times RAPS’s lifetime is 3.87 times longer than that of Always-On – Each of its components contribute to this saving 15 34:41 31:53 16:42 16:19 8:57 Lifetime (hours) Tested Schemes 3.87 times longer lifetime! BSP – 10.8% Blacklist – 59.0% Accel – 1.5% History – 28.5%

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

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

18. 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 Did Celltower-RSS Blacklist work? 18 Observed Cell-towers Success Ratio (%) Observation Count 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 19 Distance (meter) Periodic Interval (Seconds) Success Ratio (%) 85.8 m Average Distance achieved by RAPS 72.2% Success Ratio achieved by RAPS 1.47 times longer lifetime 1.14 times longer lifetime

20. Is RAPS flexible? WPS 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 20 Power (Watt) Time (Seconds) Avg. Positioning Interval Avg. Power Usage Avg. Uncertainty RAPS - GPS465.1 sec0.064 W85.8 m RAPS - WPS387.3 sec0.035 W122.9 m

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

22. Conclusion and Future Work RAPS 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.) 22 Thank you