PinPoint: An Asynchronous Time Based Location Determination System

1. PinPoint: An Asynchronous Time Based Location Determination System Moustafa Youssef, Adel Youssef, Chuck Rieger, UdayaShankar, Ashok Agrawala Presented by Sofia Nikitaki

2. Motivation • Location-aware applications • Software-based • Standard protocols • Rapidly deployable • No calibration • Energy-efficient • Number of messages

3. Review • Motivation • Related Work • PinPoint Technology • PinPoint Hardware • Results • Conclusions

4. Time-based Localization • Problem • Clock synchronization • Solutions • Use synchronized clocks: GPS • Expensive • Use echoing (round trip time measured by same clock) • Time measurement is not precise • Affected by processing time • Use two different signals: Cricket • RF for synchronization • Ultrasound for ranging • PinPint

5. The PinPoint Technology • Time of arrival based • Software only solution: can work with standard protocols (WiFi, WiMax) • Efficient (constant number of messages per node to locate all other nodes • Works both indoors and outdoors

6. PinPoint Technology • 3 Phases • Measurement phase • Information Exchange phase • Computation Phase

7. PinPoint Technology: Measurement Phase • "node  message (ID, transmit)  records the receive timestamp (the messages sent by other nodes) • All messages are one way and broadcast • Inter-node distances, hence spatial layout (from trilateration) • Inter-node clock drifts and offsets, hence ability to carry out synchronous action with other nodes

8. Clock Model • Node’s clock is assumed to have drift stable over short periods. • Clock time τ is related to the real time t by Where: • a, b constant, measurement phase. • B drift rate (no worse than 100 parts permillion) • t is measured with a nanosecond resolution

9. O(n) messages for n nodes Let ta1, tb1: tx, rxtsof first A msg tb2, ta2: tx, rxtsof first B msg ta3, tb3: tx, rxtsof second A msg ta4, tb4: tx, rxtsof second B msg PinPoint TechnologyGlobal

10. PinPoint Technology:Information Exchange Phase • "node  message • containing its receive timestamp for messages transmitted by other nodes • O(n) messages

11. PinPoint Technology:Computation Phase • "node computes • spatial coordinates • clock attributes of every other node Redundant information used to reduce errors No communication takes place • Accuracy: few feet • Synchronized clocks

12. Calculations for Node Pair A and B • Drift ratio • Propagation delay • Remote clock reading

13. Youssef14PinPoint Estimators • Clock model • Each node has its offset (a)and drift rate (b) from the global time • t= b(a+ t) • local_time= drift_rate* (offset+ global_time) • Distance estimate: bbd • Bbis in the order of hundred parts in a million

14. PinPoint Hardware

15. PinPoint Hardware • Altera Cyclone 1C20 FPGA development kit • computation processor and 3 ns timestamping clock • Maxim 2820 radio with Maxim 2242 RF power amp • communication modules for transmitting the signal used for timestamping • MaxStream 9xStream radio modem • Information exchange

16. PP2: Timestamping • 2.4 GHz QPSK modulation • PP2 baseband signal • Received signal processed as follows • Zero-crossings detected and time stamped • Many more zero-crossings detected than transmitted (noise, multi-path) • Choose “longest chain” of zero-crossings as true signal • Do least-squares fit to obtain arrival time of signal • Works well indoors (detects first signal in multi-path)

17. Distance Vs Reported Clock Ticks • 4-6 feet accuracy using 3 ns clock

18. Indoors Evaluation-First Testbed • 4.18 feet average error • 8.42 feet at 95% • 37 feet range

19. All locations NLOS 4.95 feet average error 11 feet at 95% 74 feet range Indoors Evaluation-Second Testbed

20. Outdoors Evaluation • 6.85 feet average error • 13.03 feet at 95% • 146 feet range

21. Mobility Evaluation • Can track the user in realtime • PinPoint exchange cycle is much faster than user mobility rat

22. Conclutions • A time-based ranging technology • asynchronous clocks • no echoing • constant number of messages per node • Can synchronize clocks • Can work with standard protocols • Does not require calibration • 4-6 feet accuracy (1.8288 m)

23. Thank You