Assist. Prof. Peerapong Uthansakul, Ph.D. School of Telecommunication Engineering Suranaree...

Assist. Prof. Peerapong Uthansakul, Ph.D.School of Telecommunication Engineering

Suranaree University of Technology

Motivation Background on time delay distribution Time delay measurement Positioning technique Results and discussions Conclusion

uthansakul@sut.ac.th2

positioning methods based on WLAN infrastructure can be classified into four methods◦ Time of Arrival (TOA) or Time Difference of

Arrival (TDOA)◦ Angle of Arrival (AOA)◦ Received Signal Strength Indicator (RSSI)◦ cell-ID.

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Among those four parameters, TDOA or TOA are the best for indoor positioning systems in term of sensitivity to physical environment.

This is because the accuracy of positioning is distorted the least by multipath signals and distance between access point and user in comparing with AOA and RSS methods [16].

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However, they gain less attraction than the system using RSS.

This is due to the fact that RSS measurements can be obtained relatively effortlessly and inexpensively without the need for hardware and firmware modification [19].

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In [17-18], Gunther and Hoene attempt to measure time delay distribution without any modifications.

But their experiments were done under arranged scenarios. ◦ Non-operating access point => not

practical◦ Point-to-point measurement=>not

practical◦ No other active user => not practical

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In this work, the following contributions can be found.◦ Time delay measurements are investigated

under operating WLAN system.◦ The relation between measured time delay

distribution and distance is presented. ◦ The utilizing method of measured data for

positioning purpose is proposed.

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WLAN is based on CSMA/CA Time delay of successful transmission

Time delay

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Background on time delay distribution

2 ACKframecs TSIFSTDIFSTT

scejc TjTNNTD

Probability of time delay

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Background on time delay distribution

0 5 10 15 20 25 30 35 400

0.05

0.1

0.15

0.2

0.25

0.3

0.35

Time delay, D (ms)

Pro

babi

lity,

Pj(D)

First transmission attempt, j=0

First retransmission, j=1

Second retransmission, j=2

Third retransmission, j=3

DDPDPR

jj 0for)()(

0

The method to collect the time delay of successful transmission is done by ping command from computer notebook.

The measurements are performed at 4th level of C-Building at SUT.

There are 4 access points on this level. The user knows IP address of all access

points.

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Time delay measurement

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Time delay measurement

At each location, 100 trial time delays are collected in order to compute the probability of time delays.

there is no modification needed at both access point and user hardware.

As a result, it can be directly implemented to any existing WLAN system without extra costs.

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Time delay measurement

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Time delay measurement

0 5 10 15 200

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

Time delay, D (ms)

Pro

babi

lity

of s

ucce

ssfu

l tra

nsm

issi

on

First transmission attempt, j=0

First retransmission, j=1

Second retransmission, j=2

Relation between distance and time delay distribution ??

By using average time delay of first transmission attempt (j = 0), a good agreement can be met.

The results are confirmed by measuring 5 times a day, 2 different days.

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Time delay measurement

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Time delay measurement

1 2 3 4 51

2

3

1 2 3 4 5

1.5

2

1 2 3 4 51

2

3

User location

Ave

rage

tim

e de

aly

(ms)

AP1

AP2

AP3

In this work, the empirical approach is adopted to find an appropriated parameter for translating time delay into distance.

Two steps of positioning technique:◦ Determine conversion between time delay

and distance◦ Apply triangle locating

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Positioning technique

Relation between distance and time delay

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Positioning technique

0 0.5 1 1.5 20

5

10

15

20

25

30

35

40

45

50

t (ms)

d

(m)

Measured data

Approximate Line

td 57.28

Triangle locating

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Positioning technique

Measurement area

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Results and discussions

The average error is 3.96 m

Outside Measurement area

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Results and discussions

The average error is 3.12 m

This paper has been demonstrated the new technique for WLAN positioning system.

The proposed technique provides the most convenient method to know the position of user without any extra cost of firmware and hardware.

The measurement results confirm the success of using proposed method.

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Conclusion

Thank you for your attention !

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End