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device is located at or nea the place 'L'. That is, we
canestimate the indoor location of a device with a specic WiFi
ngerprint if a appropriate Wi-Fi radio map for the indoor site is
available.
B. Open Radio Map ConstructionOne of essential preliminary steps
for a open radio map
based indoor navigation service is the constrction of anindoor
radio map. The indoor radio map is the collection ofWi-Fi gerints
on pre-designated places or points ofbuildings or indoor spaces.
Since we assume that the placeinformation of predesignated places
or points ae known atcollecting time, the data is collected in a
form of pair. The data for a radiomap construction can be collected
by trained experts or general users through paricipato approach.
Well-trained expers ae eligible for collecting good quality data in
smallareas, whereas general users should be involved in collecting
low or medium quality data in large aeas at low cost.We have used a
paly picipato approach. That is, theradio map data for rooms ad
laboratories are collected by
general users, and for corridors by trained experts
Location RecognitionOnce the indoor radio map is constructed,
the location of
a device with a specic Wi-Fi gerprint ca be identiedbased on the
information of the radio map. There are severaldierent ways of
deciding the location of a device with acerain Wi-Fi ngerprint. k
method identies k nearestneighbor Wi-Fi ngerprints of the target
Wi-Fi gerprint[8]. The center of the k neaest neighbor Wi-Fi
ngerprintsis regarded as the location of the device. Euclidean
distanceis usually used in nding the k nearest neighbor Wi-Fi
ngerints. Probabilistic methods computes the mean advaiace of Wi-Fi
ngerints for every location using the
data collected in training stage [9]. Once the mean and variance
of Wi-Fi ngerints of every spot or location aeavailable, we can
compute the probability for a device witha specic Wi-Fi gerprint to
be in the location. The location with the highest probability is
regarded as the locationof the device.
Meanwhile the Wi-Fi ngerprints captured by one deviceshows
slightly different pattes om those captured byother devices in
terms of the number of signals and theirstrengths. This may
severely aect to the accuracy of location recognition. In order to
cope with this problem, weneed to adjust the Wi-Fi gerprints of one
device to aother. Since there ae dozens of devices and mobile
phones, weprepare a adjustment table for the adjustments. The
ad
justment table is updated whenever a new device is introduced.
Therefore when we collect Wi-Fi ngerprints for aradio map
construction, the device information should bestored together with
the Wi-Fi gerprints.
III. INDR A VIGAN
A Source and Destination SpeccationsSource d destination
specication is two essential ele
ments for a indoor navigation service. Like outdoor navi-
gation, destination specication is rather simple
adstraightforward in a indoor navigation as well. Store
name,telephone number, room number, etc. or their combinationsare
used for the specication of a destination. The destination is
usually specied maually by users. Once the destination is
determined by a user then a navigation systemidenties the location
in a map d marks the destination on
the map. Thus the navigation system should have someways or
mechanisms for mapping specied destinations tolocations on a
map.
Unlike destination specication, source specication isnot the
role of users. The source or the current location of amobile device
must be recognized by the system in real time.The radio map and
location recognition techniques in theabove ae used for this. The
device captures an Wi-Fi ngerprints, ad its location is estimated
based on the capturedWi-Fi gerprint and marked on the radio map.
The devicemay capture Wi-Fi gerprints several times until it
getsmore reliable Wi-Fi ngerint. However the indoor location
recognition of a device based on Wi-Fi ngerprints isnot accurate
enough to mak the location in high precision
with a small-size circle on a map. Thus we use a big adgray
transparent circle to speci the area where the deviceis estimated
to be located. The size of the circle may chageaccording to the
expected accuracies of the estimations.
F309(eRo A
' 2 dFigure 1. APa of3d Foor, Main uiding, KST ICC Munji
Campus.
B. Path FindingOnce a source ad a destination are specied and
marked
on a map, nding a shortest path om the source to the destination
follows in the next step. In order to perform thepath-nding, the
map is convered into a graph where predesignated points on the map
are represented by nodes, adlinks between the points ae represented
by edges. We as
sign a point to the center of each room or a small area anduse
many number of points to represent corridors. The dooror entrance
information is usel in constructing such agraph. Then a
shorest-path nding algorithm is applied fora given source and a
destination. Figure 1 shows a par ofthe map of 3rd oor, main
building, KAIST Munji Campusad an associated graph.
Among several algorithms for solving the shorest-pathnding
problem, an algorithm invented by Bellman-Ford isoen used for
path-nding in navigation [10]. We use
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Bellma-Ford's path-ding algorithm for the system. It is asimple
dynamic programming based algorithm with singlesource ad
single-destination. Unlike other algorithms,Bellman-Ford path-nding
algorithm allows not only positive but also negative weights in a
graph, as long as there isno negative cycle. Once the shorest path
is identied, thepath om the source to the destination is drawn in
line on
the map of the building so that the users ca gure out
theircurrent locations, directions to move, ad destinations to
go.
IV. MEMENTATION
A Radio Map Construction ToolWe developed a tool to help the
construction of a radio
map. The tool provides utilities to capture Wi-Fi ngerprints and
to speci the points or places where the Wi-Fingerints are captured
on a map. The tool also providesutilities to impor maps. A map
drawn by users or a CAmap c be impored by the tool. Aer capturing
ngerprints, users have to speci the points where the ngerprints
captured either by ataching place information to the
ngerprints or marking the points on the map. For rooms orclosed
small spaces, ataching place information mayenough for specication.
The center of the space is considered as a representative point in
that case. However for thelarge open space or narrow long space
like a corridor, making the point where the ngerprints captured on
the map ismore suitable. The tool generates a graph based on
themarked points and representative points. In order to maethe
graph more complete, the tool may need a help omusers in speciing
corridor information in the graph.
Figure 2. A path is drawn on a Googe android phone.
B. Radio Map ConstructionWi-Fi ngerints to construct an open
radio map were
collected on 3rd oor, main-building, KAIST Munji Capus, Korea.
We collected Wi-Fi ngerprints for rooms adlaboratories through a
picipato approach, whereas forcoidors by two master students i.e.
by trained users. 4000Wi-Fi gerprints were collected in total.
Client VI ImplementationA Google adroid phone was used for a
client I imple
mentation. It was developed in android developing package
with Eclipse. Figure 2 shows a path drawn on a map inGoogle
adroid phone. From the tests for arbitrarily chosensources ad
destinations, the navigation system recognizedthe source locations
and successlly drew the paths in mostcases.
V. UMMRY
In this presentation, we introduced an open radio mapbased
indoor navigation system. Although we conrmedthat the open radio
map based indoor navigation service waspossible at the limited
indoor spaces we tested, there stillremains a long way to go to me
the system complete interms of coverage and precision. everheless,
the openradio map based indoor navigation service deserves to
bestudied her because there is a high potential that it canprovide
a usel service in an indoor space with complexstrctures. We need to
continue to accumulate Wi-Fi gerprints and associated maps all over
the places for the service.Once the level of service that the
system can deliver to general users reaches to a acceptable state,
the integration ofindoor ad outdoor navigation service would be the
appaent next step.
COWEDGEMENT
This work was suppored by the , Korea, under theITC suppor
program supervised by the IPA (IPA-2009-(C 1090-0902-0036)).
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