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

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

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?

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 Challenge – Determine when and when not to turn on GPS efficiently using the sensors and information available on a smartphone 4

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 5 When to turn on GPS When to NOT turn on GPS

Activity Detection accelerometer 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 6 Activity Off (0.062) On (0.141) Operating point: 12.5%

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 7 A B D C

Celltower-RSS Blacklisting GPS unavailability Use celltower information to detect GPS unavailability – Signatures exist for indoor places that you go often 8 GoodBadVariable Turn on GPS only when available!

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

Evaluation 10 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

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 11 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%

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 12 Thank you