1 pinpoint: an asynchronous time based location determination system moustafa youssef, adel youssef,...
TRANSCRIPT
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PinPoint: An Asynchronous Time Based Location Determination System
Moustafa Youssef, Adel Youssef, Chuck Rieger, UdayaShankar, Ashok Agrawala
Presented by Sofia Nikitaki
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Motivation
Location-aware applications Software-based
Standard protocols Rapidly deployable
No calibration Energy-efficient
Number of messages
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Review
Motivation Related Work PinPoint Technology PinPoint Hardware Results Conclusions
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Time-based Localization Problem
Clock synchronization Solutions1. Use synchronized clocks: GPS
Expensive2. Use echoing (round trip time measured by same
clock) Time measurement is not precise Affected by processing time
3. Use two different signals: Cricket RF for synchronization Ultrasound for ranging
4. PinPint
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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
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PinPoint Technology
3 Phases Measurement phase Information Exchange phase Computation Phase
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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
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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 per
million) t is measured with a nanosecond resolution
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PinPoint TechnologyGlobal O(n) messages for n
nodesLet ta1, tb1: tx, rx ts of
first A msg tb2, ta2: tx, rx ts of
first B msg ta3, tb3: tx, rx ts of
second A msg ta4, tb4: tx, rx ts of
second B msg
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PinPoint Technology:Information Exchange Phase
node message containing its receive timestamp for
messages transmitted by other nodes
O(n) messages
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PinPoint Technology:Computation Phase
node computes spatial coordinates clock attributes of every other node
Redundant information used to reduce errorsNo communication takes place
Accuracy: few feet Synchronized clocks
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Calculations for Node Pair A and B
Drift ratio
Propagation delay
Remote clock reading
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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 Bb is in the order of hundred parts in a
million
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PinPoint Hardware
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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
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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)
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Distance Vs Reported Clock Ticks
4-6 feet accuracy using 3 ns clock
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Indoors Evaluation-First Testbed
4.18 feet average error 8.42 feet at 95% 37 feet range
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Indoors Evaluation-Second Testbed
All locations NLOS 4.95 feet average
error11 feet at 95% 74 feet range
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Outdoors Evaluation
6.85 feet average error 13.03 feet at 95% 146 feet range
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Mobility Evaluation
Can track the user in realtime PinPoint exchange cycle is much
faster than user mobility rat
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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)
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Thank You