Proc. the 2005 International Conference on Pervasive Systems and Computing (PSC-05),
pp. 47-53, Las Vegas, Nevada, USA, June 27-30, 2005.
Positioning Technique of Wireless LAN
Terminals Using RSSI between Terminals
Teruaki Kitasuka
Kenji Hisazumi
Faculty of Information Science
and Electrical Engineering,
Kyushu University,
Fukuoka 816-8580, Japan
Japan Science and
Technology Agency,
Japan
Abstract—
The IEEE 802.11 wireless LAN spreads very widely. The
wireless VoIP technologies will accelerate spreading. In the
environment in which almost all people take along their own
wireless terminal, location information of terminals can be
used as them of persons, and can support human collaborative
work.
We focus on the positioning technology of IEEE 802.11
devices. There are many researches of the technology already.
Our target is providing precise relative location of IEEE 802.11
terminals with short site calibration process and no additional
hardware. RSSI requires no additional hardware. However
RSSI is affected by obstacles, multipath fading, etc.
In this paper, we describe the design and implementation
of a prototype of proposed positioning system called WiPS.
To improve the accuracy of relative location, WiPS uses RSSI
between terminals. WiPS can be work well, if LOS path between terminals are available and distance between terminals
are less than several meters. WiPS does not need LOS path
between access point and each terminal. We show the result of
empirical study using a prototype. The order of terminals can
be obtained precisely, when terminals are in line.
Tsuneo Nakanishi
and Akira Fukuda
Faculty of Information Science
and Electrical Engineering,
Kyushu University,
Fukuoka 816-8580, Japan
Average positioing
error range
Real position
Conventional positioning
result (an example)
WiPS positioning result
WiPS reduce interterminals distance error
by using RSSI between
terminals
Fig. 1.
Benefit of WiPS
1. Introduction
Wireless LAN (IEEE 802.11) based location sensing
technologies are focused on, with an increase in demand
of wireless LAN ready laptop PC, PDA and wireless
LAN VoIP phone. The benefit of these technologies is
the wireless LAN device can play roles of both communication device and positioning device. RADAR[1] is
the first research of wireless LAN positioning probably.
Several systems are already released commercially such
as Ekahau Positioning Engine[2], AirLocation[3], and
AeroScout[4].
Location sensing technology is very important for an
infrastructure of a ubiquitous computing environment.
Outdoor location sensing technology such as GPS is
already developed and widely used. For indoor location
sensing, there are several technologies, but they are not
widely used currently. If low-cost and smart location
sensing technology is developed, it will be used widely
and can be a popular way of positioning. In this paper,
we use IEEE 802.11b wireless LAN device as locatoin
sensing device.
As indoor location sensing, media of applied technologies are ultrasonic, infrared, camera, GPS pseudolite,
etc. These technologies have to pay high installation
cost and/or maintenance cost. For example, ultrasonic
sensor based systems have to install many ultrasonic
receivers on a ceiling and connect them to a server. All
of these technologies require installing exclusive devices
of positioning in the room or in a building.
We proposed WiPS (Wireless LAN based indoor
Positioning System)[5], [6]. WiPS is different from
conventional wireless LAN based positioning systems.
Major differences are that no additional hardware and
no site-calibration process are required. A strong point
of WiPS is that relative position of terminals can be
obtained more precisely than other wireless LAN based
positioning systems (Fig. 1). The mechanism providing
this benefit is derived from measuring RSSI between
terminals. For example, a person happens to meet his/her
colleague and they discuss their research. One of them
wants to give a document file to other. If there are many
passers-by around them, they can still find colleague’s
location by WiPS. Conventional positioning systems
contain several meters error in measured positions and
measure the position of each terminal separately. Therefore relative position between terminals has much error,
about 2 times larger than absolute positioning error. In
Keywords: Positioning technologies, Location sensing,
IEEE 802.11, RSSI, Relative position
the crowded place, this error degrades the effectiveness
of position information. For another example, when you
attend a meeting, WiPS can provide the precise seating
arrangement.
This paper is organized as follows: Sec. 2 summarizes
current positioning technologies using IEEE 802.11 series. In Sec. 3, an implementation and features of WiPS
are described. In Sec. 4, results of simple experiments
are described. The outdoor, indoor, and close range
experiments are shown. In these experiments, three or
seven PDAs are used. “Flag game” demonstration is also
explained. In the demonstration, WiPS determines that
user’s hands are up or down periodically. We conclude
this paper in Sec. 5.
2. Related Works
There are many location system using IEEE 802.11
standards in the research and commercially. IEEE 802.11
location system has a benefit of single device solution
of both data communication and location sensing.
The first paper of IEEE 802.11-based user location and
tracking system is RADAR[1]. RADAR measures RSSI
at access points to determine the location of a mobile
terminal. There are other RSSI-based systems such as
Ekahau Positioning Engine[2], CMU-TMI[7] and so on.
Except for The Wall Attenuation Factor (WAF) propagation model of RADAR, site calibration is required
to activate location system. The site calibration is the
process to learn the radio propagation statistics of the
specific place. This process requires man-hour. 1 hour
per 1,200 m2 is needed for site calibration on the Ekahau
Positioning Engine. The process of site calibration is the
most important disincentive of installation.
Instead of RSSI, TDOA (Time difference of arrival)
method is applied to IEEE 802.11-based positioning systems. There are several TDOA-based location systems
such as AirLocation[3] and AeroScout[4]. TDOA-based
systems are requires additional hardware to measure the
time difference. Access points or receivers which support
TDOA have to be installed. The clocks of these access
points and receivers have to synchronize with each other
precisely for TDOA. Since the radio propagation speed
is 3.00 × 108 m/s, radio signal propagates 0.3 m in 1 ns
(1 GHz).
Finally, there are many positioning technologies[8] are
developed and spread, such as GPS and ultrasonic-based
positioning systems[9].
3. WiPS
3.1 Overview
WiPS (Wireless LAN based indoor Positioning
System) is the location sensing technique which we
are developing[5], [6]. WiPS determines the location
of wireless terminals. On each wireless terminal, WiPS
Gn Reference host
G1
Mobile host
A
Connections between
reference host and A
Connection between
mobile host and A
B
D
C
G2
G3
Fig. 2. Basic mechanism of WiPS. To determine the position of A,
conventional method uses only RSSI to APs ‘G1,’ ‘G2,’ and ‘G3’.
WiPS uses RSSI to not only APs but also other terminals ‘B,’ ‘C,’ and
‘D’.
client software runs and measures RSSI (received signal
strength indicator) of packets sent by other mobile
terminals. All RSSI information is aggregated by the
WiPS server. The WiPS server calculates positions of
all mobile terminals.
Conventional wireless LAN based positioning systems, which use RSSI, use only RSSI between access
point and mobile terminal. These systems determine
location of each terminal separately. On the other hand,
WiPS uses RSSI between mobile terminals. Fig. 2 shows
the difference. For this difference, WiPS can determine
more precise relative position of terminals.
In WiPS, an operation for calibration is reduced. Conventional systems need site calibration, but WiPS does
not need it with a little degradation. The process of site
calibration is that calibrator handles a mobile terminal,
and walks around the site to collect the relationship
between RSSI and location. The site calibration should
be done, if the site layout is changed. WiPS is planed to
overcome site calibration by RSSI measurement between
mobile terminals. There is another calibration which is
called terminal antenna calibration. The antenna calibration is still remained in WiPS.
WiPS can offer precise relative location between mobile terminals. Therefore an application which requires
relative location is suitable to WiPS. We pick up flag
game as one of this kind of applications.
3.2 Radio Propagation Model
In the current implementation of WiPS, free space
propagation model is used. The reason of selecting this
fundamental model is to make WiPS free from site calibration and other site specific configuration. Since WiPS
utilize RSSI between mobile terminals, relative distance
between sender and receiver is shorter than conventional
WiPS Client viewer
TCP
WiPS Server
WiPS Client daemon
TCP
NETLINK I/F
802.11 WLAN driver
802.11 WLAN device
JavaVM
PDA
Server
Other PDAs’ packet
Fig. 5.
Software organization of WiPS experimental system.
Fig. 3.
Relation between RSSI and distance on the free space
propagation model
PDA
PDA
AP
PDA
Server
PDA
Legend
Wireless link
Wired link
Fig. 4. Configuration of WiPS experimental system. PDAs are the
targets of positioning, and a server calculates PDAs’ positions using
RSSI.
RSSI-based positioning. Shorter distance implies less
influence of obstacles to RSSI in our experiment.
Free space propagation model assumes the ideal
propagation condition that there is a line-of-sight path
between transmitter and receiver. It does not consider
any effect of multipath fading and other path loss. In
indoor space, free space model is not precise, but still
appropriate to determine distance, if there are many
mobile terminals in the space.
In the free space model, relation between RSSI and
distance is described as
µ
¶
4πd
Pr (d) = P0 − 20 log10
[dBm],
(1)
λ
where
P0
=
λ
=
empirical constant
c
3 × 108 [m/s]
=
.
f
2.4[GHz]
The graph of Pr (d) is shown in Fig. 3. P0 is set to
31.0[dBm] in our implementation. Same IEEE 802.11
devices are used in all mobile terminals.
3.3 Implementation
We describe a current implementation of WiPS. Fig.
4 shows the configuration of our implementation. PDAs
in Fig. 4 are used as mobile terminals. A server communicates with PDAs and calculates PDAs’ location.
AP (access point) provides only network connectivity
between PDA and the Server. AP does not act as
positioning reference. The specifications of PDAs are:
• OS: Embedix based on linux-2.4.18-rmk7-pxa3
• CPU: Intel XScale (PXA250 400MHz)
• IEEE 802.11 I/F: IEEE802.11b CF Type II. PrismII
chip set.
• IEEE 802.11 driver: wlan-ng ver. 0.1.12 (modified)
Fig. 5 shows the software organization. Each PDA
runs IEEE 802.11 I/F in an infrastructure mode to communicate with a WiPS server through AP. IEEE 802.11
I/F of each PDA is set into a promiscuous mode, to
measure RSSI of other PDA’s packets. Each PDA sends
packets to only AP. However, in promiscuous mode, a
WLAN driver in each PDA can receive the packets not
only from access point to itself, but also from other
PDA to access point. To enable promiscuous mode, a
command ‘wlanctl-ng eth0 dot11req mibset
mibattribute=p2PromiscuousMode=true’ are
used. The driver retrieves a RSSI value and a source
MAC address from a packet. The driver carries a RSSI
value and a source MAC address to WiPS client daemon.
WiPS client daemon has a connection to WiPS server,
and reports pairs of average RSSI values and source
MAC address to WiPS server. Reporting format of WiPS
client daemon is shown in Table I. In the Table, RSSI
item is used to report an average RSSI value “RSSIAve” of packets which are sent from a specific source
PDA. “MAC-Address” is the source MAC address of
the packets. “num-of-pkts” is the number of packets to
average. The “ts1” and “ts2” fields are the timestamp of
receiving first packet and last packet of the averaging,
respectively.
A notebook PC is used as a WiPS server. WiPS server
software is developed on Java platform. The server PC
consists of J2SE 1.4.2, Windows 2000, Intel Pentium III
600 MHz, 256MB RAM. WiPS server receives the RSSI
TABLE I
DATA FORMAT FROM W I PS CLIENT DAEMON TO W I PS
Format
MAC MAC-Address
SIGNAL MAC-Address RSSI-Ave num-of-pkts ts1 ts2
4. Experiment
In this Section, three experiments are described. The
demo application is also introduced briefly.
4.1 Preliminary
The preliminary experiment is held in the open air, on
a roof of a building and uses three PDAs. The experiment
is used to determine P0 value of Equation (1). The results
of 2 m distance between PDAs are used to determine P0
value.
Three PDAs are put in vertex positions of equilateral
triangle. Each PDA is on a tripod at 1.1 m height from
floor of the roof. The distance between PDAs is modified
from 1 m to 10 m. The cases of 1, 2, 3, 4, 6, 8, 10 m
of the distance is measured. In each case, WiPS run 5
min. and an average of 5 min. is used as a result.
Fig. 6 shows the result. Fig. 6 (a) is the relationship between real and measured distance between PDAs. In Fig.
6 (a), WiPS (◦ mark) is the distance measured by WiPS.
RSSI (+ mark) is the distance using bare RSSI and
Equation (1). From Fig. 6 (a), the dispersion of measured
distance of WiPS is smaller than bare RSSI. For each
pair of PDAs, each PDA measures RSSI. WiPS averages
these two RSSI measurements. Therefore dispersion gets
WiPS
RSSI
10
measured distance [m]
reports from all PDAs, and calculates PDAs’ location
using free space model (Fig. 3). Step of calculation in
WiPS server is the following:
1) Makes a distance list of all pairs of PDAs. The
distances are calculated from last averaged RSSI
values with Equation (1) and Fig. 3.
2) Determines initial locations of all PDAs in heuristic manner.
3) Modifies locations of all PDAs cyclically to satisfy
distance contraint, until convergence.
RSSI value contains error which is attributable to multipath fading, antenna characteristics, and other fluctuations. For this error, the distance list of all pairs of PDAs
also contains error. To reduce the effect of this error,
WiPS server employs iterative approximation algorithm,
described in [5], [6].
The current implementation uses only RSSI between
mobile terminals. Each RSSI between an access point
and a mobile terminal is no used, since an access point
and mobile terminal has different antenna charactaristics
and current WiPS can not handle heterogeneous charactaristics of antennas.
8
6
4
2
0
0
2
4
6
8
10
real distance [m]
(a) Measured distance between terminals
received signal strength [dBm]
Item
Own MAC address
RSSI
SERVER
15
RSSI
free space model
10
5
0
-5
-10
-15
-20
-25
-30
0
2
4
6
distance [m]
8
10
(b) Relationship between distance and RSSI
Fig. 6.
Results of outdoor measurement.
smaller in WiPS. Long distance measurement tends to be
dispersed.
Fig. 6 (b) shows the relationship between real distance
and RSSI value. Dashed line in the Fig. is the Equation
(1) and RSSI shows the actual measured values. From
Fig. 6 (b), the free space model does not fit this environment so much. However, in the indoor experiment
(Sec. 4.2), the order of terminals which are in line can
be obtained precisely.
4.2 Order Determination
The order determination experiment is held indoors.
Fig. 7 (a) is the layout of the experiment. Seven PDAs
are put on the desk in line. Two PDAs of both edge act
as references of positioning. We give the positions of
two references into WiPS. Fig. 7 (b) is the screenshot
of WiPS server. In Fig. 7 (b), last two bytes of MAC
address is used as ID of each PDA. The measured order
of seven PDAs is correct, comparing it with Fig. 7 (a).
Table II and Fig. 8 are the detail of measured results.
Table II shows the measured distance between each
pair of PDAs with standard deviation of distance. For
1
TABLE II
R ESULT OF INDOOR MEASUREMENT. R ELATIVE DISTANCES
( UPPER ) AND STANDARD DEVIATION ( LOWER ). ( UNIT: METER )
Reference PDAs
Positioing PDAs
Desk
2
3
4
5
6
7
1.5 m
25 cm
(a) Layout of PDAs
PDA
1
(61:DD)
2
(65:6D)
3
(65:91)
4
(68:8D)
5
(69:07)
6
(69:DA)
7
(69:D1)
1
0.63
0.01
0.73
0.01
0.77
0.01
0.85
0.01
0.96
0.03
1.50
-
2
0.63
0.01
-
3
0.73
0.01
0.10
0.01
-
0.10
0.01
0.14
0.01
0.21
0.01
0.33
0.02
0.87
0.01
0.06
0.01
0.12
0.01
0.23
0.02
0.77
0.01
4
0.77
0.01
0.14
0.01
0.06
0.01
0.08
0.01
0.19
0.02
0.73
0.01
5
0.85
0.01
0.21
0.01
0.12
0.01
0.08
0.01
0.12
0.02
0.66
0.01
6
0.96
0.03
0.33
0.02
0.23
0.02
0.19
0.02
0.11
0.02
-
7
1.50
0.87
0.01
0.77
0.01
0.73
0.01
0.66
0.01
0.54
0.03
-
0.54
0.03
measured distance [m]
2
WiPS
RSSI
1.5
1
0.5
0
0
0.5
(b) Screenshot of WiPS server
example, when PDA No. 4 which is the center of seven
PDAs is focused attention on, PDAs No. 3, 2, and 1
satisfy the closer order to No. 4 in measured distance.
PDA No. 5, 6, and 7 also satisfy it. In this experiment,
WiPS can determine the right order of seven PDAs.
However, measured distance is not precise to compare
with real distance, e.g., the real distance between PDAs
No. 3 and 4 is 25 cm, but measured distance is only 6
cm. We discuss this phenomenon using Fig. 8. Fig. 8
takes the same form as Fig. 6, e.g., there are two 125
cm distant pairs of PDAs – No. 1 and 6 pair, and No.
2 and 7 pair – and their measured results are put on
the 25 cm of real distance. From Fig. 8 (a), measured
distances calculated from bare RSSIs (+ marks) place a
disproportionate emphasis on closer than real distance.
However, some of measured distances of WiPS (◦ mark)
places better than them of bare RSSIs.
Fig. 8 (b) shows the difference between free space
model and measured relationship between RSSI and distance. It can be seen that there is a mismatch of assumed
model to calculate distance from RSSI. In addition,
RSSIs of the same distance are varied. Although this
mismatch and variation, WiPS can determine the right
order.
2
(a) Measured distance between terminals
Indoor experiment.
received signal strength [dBm]
Fig. 7.
1
1.5
real distance [m]
25
RSSI
free space model
20
15
10
5
0
-5
-10
-15
-20
0
0.5
1
1.5
real distance [m]
2
(b) Relationship between distance and RSSI
Fig. 8.
Results of indoor measurement.
4.3 No Reference Situation
No PDAs are specified as references of positioning.
Therefore, WiPS determine relative position of each pair
of PDAs autonomously. Very high density situation is
tested.
Fig. 9 (a) is the layout of this experiment. Seven PDAs
are used. They are placed about 20 cm apart, in 50 cm
square area. IDs of anterior PDAs are 69:D1, 69:07,
65:91, 69:DA from left to right, and IDs of posterior
PDAs are 65:6D, 61:DD, 68:8D from left to right.
We show a screenshot of the measurement result in
Fig. 9 (b). The result is segmented three parts; left,
(a) Condition of crowded case measurement. (IDs of anterior PDAs
are 69:D1, 69:07, 65:91, 69:DA from left to right, and IDs of
posterior PDAs are 65:6D, 61:DD, 68:8D from left to right.)
Fig. 10.
(b) Transient result of crowded case measurement.
Fig. 9.
Crowded case experiment.
center, and right parts at a glance. This Fig. is rotated
by hand, but the relative position is not modified. The
left part contains two PDAs of 65:6D and 69:D1. These
two PDAs are placed in most left side in Fig. 9 (a). The
center part contains three PDAs of 69:07, 65:91, and
61:DD. Three PDAs in the center part are placed center
part in real position, however relative position between
three PDAs is not precise. The right part contains two
PDAs of 68:8D and 69:DA. These are most right PDAs
in anterior and posterior in Fig. 9 (a).
As a result, WiPS can determine relative position
with a little inaccuracy. A major error is the relative
position of three center PDAs. Other relative position is
determined precisely.
4.4 Demo Application: Flag Game
We make a demo application of WiPS. The application
is “Flag game.” Flag game is a Japanese classical game
for children. A child holds two colored flags, with a flag
in one hand and another in the other hand. Another child
indicates which flag takes up or down, e.g. “take up red
flag,” “take down blue flag,” “do not take up both flags,”
etc. First child have to follow the instruction of later
child quickly.
Our demo application adopts Flag game as a motif.
Two participants play the Flag game. Three PDAs are
used for the game. A responder holds two PDAs instead
of flags. Another PDA is put on the desk in front of
the responder. WiPS determines the distances between
Screenshot of the flag game
all pairs of three PDAs. Demo application determines
the position of the responder’s hands using the distance
between PDAs in a hand and on the desk.
A screenshot of demo application “Flag game” is
shown in Fig. 10. The place of each PDA is selected
in lower pain of the window. Left and right indicator
show the states of both hand.
5. Conclusion
In this paper, we describe the implementation of
WiPS. The results of several experiments are shown.
WiPS is implemented by off-the-shelf PDAs with CFtype IEEE 802.11b wireless LAN cards as positioning
target and also as positioning reference. Wireless LAN
driver on the PDA is modified to record RSSI values
of packets sent by other PDAs. WiPS server aggregates
RSSI values from WiPS clients to determine locations
of clients. To run WiPS server, only Java runtime and
network connectivity is required.
We evaluate WiPS by three experiments. In the experiments, three or seven PDAs are used. First, we measure
the relationship between RSSI and distance in the open
air environment, to determine the constant variable of
radio propagation model. The second experiment shows
that WiPS can determine the order of PDAs precisely,
when seven PDAs are in line at intervals of 25 cm.
Although the radio propagation model does not fit the
real environments, WiPS work well enough to find the
sequence of PDAs’ position. The last experiment shows
the result in the case of no references of positioning.
We show only a transient result. In this result, relative
positions of PDAs are retrieved almost exactly. We also
introduce the demo application “Flag game.” In this
demo, WiPS measure the movement of human arm. The
person holds PDAs in his/her hands. WiPS detects each
hand is up or down.
Our goal is development of positioning system which
is freed from site calibration and site configuration.
When the positioning system can be used anywhere,
human communication is enforced naturally by mobile
terminals. IEEE 802.11-based cell-phone accelerates the
needs of positioning system. As future works, we will
evaluate WiPS in more practical environment to confirm
the effectiveness. We will plan to propose smart and
automatic calibration mechanism, which needs no manhour and adjusts the environmental changes. Finding
suitable application of WiPS is also future work.
Acknowledgment
The work reported in the paper was partially supported by the grants of MIC Strategic Information and
Communications R&D Promotion Programme, and JSPS
KAKENHI (Young Research: 17700067).
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