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Towards Collaborative Localization of Mobile Users with Bluetooth

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Location awareness is a key requirement for many pervasive
applications. Collaborative localization techniques are interesting
because they help to improve accuracy and coverage indoors and improve
power consumption by duty-cycling GPS outdoors. We use Bluetooth
for collaborative localization of mobile personal devices. Specifically,
we embed information in Bluetooth device names to improve latency
of information exchange between participating nodes. We identify
and demonstrate on real hardware two problems in the Bluetooth stack
that negatively impact localization accuracy: a) device name caching
that introduces significant device-specific delays in transmitting information
between nodes, and b) poor accuracy of time synchronization in
modern mobile devices. Our solution is to append additional time information
to the device name and track time o↵sets between nodes. We
verify experimentally that this helps to both detect outliers and correct
for time-synchronization errors and thus mitigate localization errors.

Published in: Engineering
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Towards Collaborative Localization of Mobile Users with Bluetooth

  1. 1. Towards Collaborative Localization of Mobile Users with Bluetooth Alexandre Barreira CSIRO ICT Centre, Brisbane, Australia Philipp Sommer Brano Kusy Raja Jurdak UTC/Georgia Tech.
  2. 2. Localisation • Indoors • Specialized tracking devices • Infrastructure deployment cost • Setup phase • Outdoors • GPS! • Reasonably accurate … • …yet energy expensive • Collaborative Bluetooth Localisation • Can help both • Built-in to smart phones/laptops • No infrastructure/setup in office environments • More energy-efficient than GPS
  3. 3. • Problem • Protocol imposes pairing/piconet association • Solution • Avoid expensive handshake • Use friendly name to share location info – up to 248 characters • Embed location info • Indoors: coordinates • Outdoors: GPS • Problem • Infrastructure setup • Solution • Use only existing infrastructure with bluetooth • Laptops • Desktops • Use office directory to map names to locations Bluetooth Localization Overview
  4. 4. Infrastructure-based Bluetooth Localisation X Bluetooth Coverage Gaps
  5. 5. Collaborative Bluetooth Localisation Can fill coverage gaps X X X
  6. 6. Infrastructure-based Bluetooth Localisation X Sparse coverage
  7. 7. Collaborative Bluetooth Localisation X Can provide denser coverage
  8. 8. Bluetooth neighbor discovery Use frequency hopping to transmit and listen to neighbors A B C
  9. 9. Bluetooth neighbor discovery A has list of neighbor MAC addresses A B C Neighbor Address MACB MACc
  10. 10. Bluetooth neighbor discovery A requests friendly name of each neighbor in second step A B C name? (name, RSSI, class)
  11. 11. Bluetooth neighbor discovery for localization name = (LOCx, LOCy, LOCz) A B C name? (name, RSSI, class) Neighbour Location RSSI class B C 2,3,4 4,3,5 -75 -66 Phone Desktop
  12. 12. RSSI to bound distance
  13. 13. Device Name Caching • Discovery phase every several seconds •Varies per device/manufacturer • In the meantime, node keeps neighbor location information •Risks stale neighbor list •Risks inaccurate location •Smart phone OS limits control •No methods to flush cache •Caching strategies vary per device model/OS version
  14. 14. Rejecting cached device names • Include timestamp into device name • Receiver can estimate time offset between remote device and local clock name = (LOCx, LOCy, LOCz, t) A B C name? (name, RSSI, class) Neighbou r Locatio n time Min offset RSSI class B C 2,3,4 4,3,5 20 35 19 13 -75 -66 Phone Desktop
  15. 15. Simple Approach to Reject Cached Names • Assumption: mobile phone clocks remain stable over short time intervals • Set (or learn) lower bound for time offset with each neighbor • IF a name with offset>lower bound+c • Discard this name
  16. 16. Rejecting cached device names • Include timestamp into device name • Receiver can estimate time offset between remote device and local clock name = (LOCx, LOCy, LOCz, t) A B C name? (name, RSSI, class) Neighbou r Locatio n time Min offset RSSI class B C 2,3,4 4,3,5 20 35 19 13 -75 -66 Phone Desktop
  17. 17. Rejecting cached device names • Include timestamp into device name • Receiver can estimate time offset between remote device and local clock name = (LOCx, LOCy, LOCz, t) A B C name? (name, RSSI, class) Neighbour Locatio n time Min offset RSSI class B C 2,3,4 4,3,5 20 35 19 13 -75 -66 Phone Desktop
  18. 18. Experiments • 2 Samsung Nexus S phones • Both running Android 2.3.3 • Both phones • continuously update their Bluetooth device names once every second with the current local time • perform periodic Bluetooth device inquiries • Local clocks of the devices are only loosely synchronized with a clock offset of 9.5 seconds. 0 500 1000 1500 2000 2500 3000 3500 Time [s] 0 5 10 15 20 25 30 35 Time[s] Time Difference Sender-Receiver Lower Bound for Clock Offset Latency Sender-Receiver (after Correction)
  19. 19. Summary • Collaborative Bluetooth localization • Indoors • Fill coverage gaps • Increase density • Outdoors • Saves on using GPS frequently • Simple method to avoid device name caching • Establish pairwise clock offsets • Discard names that diverge from these offsets • Open issues • Learning and adapting pairwise offsets • Bounding uncertainty with high mobility • Versatile localization algorithms
  20. 20. Thank you Thank you Dr. Raja Jurdak CSIRO ICT Centre Principal Research Scientist Research Group Leader Phone: +61 (0)7 3327 4059 Email: raja.jurdak@csiro.au Web: http://jurdak.com University of Queensland Adjunct Associate Professor

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