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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
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.
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].
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].
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
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.
WLAN is based on CSMA/CA Time delay of successful transmission
Time delay
Background on time delay distribution
2 ACKframecs TSIFSTDIFSTT
scejc TjTNNTD
Probability of time delay
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.
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.
Time delay measurement
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.
Time delay measurement
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
Positioning technique
Relation between distance and time delay
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
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.
Conclusion
