1
Device-Free Localization
Ossi Kaltiokallio
Department of Automation and Systems Technology Aalto
University School of Science and Technologywww.autsys.tkk.fi
-7 -6 -5 -4 -3 -2 -1 0 1-1
0
1
2
3
4
5
6
7
Overview
2
• Device-free localization (DFL) based on distributed processing of RSSI • Objective • What enables the technology?• How a person can be detected and localized? • DFL application briefly• Experimetal results and DEMO(s)
Objective
3
• Device-free localization (DFL) system• Re-deployable, remotely configurable, and easy to use
• Operates in real-time• Target position is estimated with minimal latency
• Distiributed processing of RSSI • Communication overhead is minimized• Limited resources of the node• Enables large scale WSNs
• Energy efficiency• Radio management
Electromagnetic spectrum
4
5
What enables the technology?
6
Sources of RSSI variability
• multipath fading and shadowing, antenna differences, node orientation, surrounding environment, distance, transmission power, etc.
Sources of Information (1/3)
7
• Fading, deviation of the attenuation • Fading may either be due to multipath propagation or shadowing • Presence of reflectors around communication link
• creates multiple paths that TX can traverse• result RX ”hears” multiple copies of the transmitted
signal, each propagated via different path
• Each signal experiences difference in attenuation, delay and phase shift
• Result can be either destructive or constructing
50 100 150 200 250-70
-68
-66
-64
-62
-60
-58
Sample Number [n]R
SS
I [d
Bm
]
Link i-j
Sources of Information (2/3)
8
160
183
135
210
• RSSI measurements of one link ...
50 100 150 200 250-84
-82
-80
-78
-76
-74
-72
-70
-68
-66
Sample Number [n]R
SS
I [d
Bm
]
Link i-(j-1)
Sources of Information (3/3)
9
160
183
135
210
• ... and the link next by, RSSI measurements are quite different
Positioning via RSSI…
10
• Single link• The distributed algorithm is capable of detecting LoS
crossings• Accuracy very limited• Possible false alerts• What if accuracy is required? the key lies in nodes
number
• Multiple links• Many nodes simultaneously can detect the person• Data aggregation coordinates of the person can be
estimated if nodes positions are known a priori• False alerts can be filtered out
Master’s thesis approach (1/2)
11
Master’s thesis approach (2/2)
12
13
Application in a nutshell
DFL System
WSN Matlab
Communication
Distributed algorithm
Positioning Tracking
SITUATION AWERNESS
SINK
13
Results of Master’s thesis
14
Problems and Solutions
15
• Algorithm designed to only detect LoS crossings that are caused by shadowing
• Increse sensitivity multipath fading events also detected
• LoS crossings visualized with a line • The person affects the RSSI also elsewhere model
alerts with an ellipsoid
• Transmission interval (16 ms)• Operations of the nodes optimized (10 ms) more
measurements, better results
Alert model
16
17
… and the story continues
• Each alert is visualized with an ellipsoid• Ellipsoid’s tend to cluster around the actual position
• Why not just send all RSSI measurements to the sink?• Tracking accuracy enhanced with a Kalman filter
-8 -6 -4 -2 0
0
2
4
6
8
x coordinate [m]
y c
oo
rdin
ate
[m
]
Known path Estimated path Nodes
Experimental results (1/3)
18
-8
-6
-4
-2
0
x c
oo
rdin
ate
[m
]
100 200 300 400 500 600 700 800 900 1000
0
2
4
6
8
Time [TDMA cycle]y
co
ord
ina
te [
m]
Estimated position
Known position
Experimental results (2/3)
19
Test 1a Test 2c Test 7a Test 3a
Node interval [m] 1 1.5 1.5 2
Monitored area [m2] 16 36 36 (obstructed)
64
RMSE 0.19 0.20 0.22 0.23
Min RMSE 0.17 0.17 0.20 0.19
Max RMSE 0.22 0.21 0.23 0.26
Max error 0.63 1.07 1.19 1.14
Alerts [%] 16.43 14.56 16.35 11.69
10 20 30 40 500
1
2
3
4
5
6
7
8
9
Transmission interval [ms]
Exp
ecte
d S
yste
m L
ifet
ime
[day
s]
Radio ON-OFFRadio always ON
• Demo • Processing the RSSI in a distributed fashion reduces
communication overhead– Alerts account only for around 10-17 % of the total
number of packets (area always occupied)
20
• Radio management:– Radio enabled for
TX: 2.30 ms, RX: 4.05 ms– An additive increment in
network lifetime
Experimental results (3/3)