A Hierarchical Architecture for Indoor Positioning Services

PROCEEDINGS OF THE 2nd WORKSHOP ON POSITIONING, NAVIGATION AND COMMUNICATION (WPNC’05)
& 1st ULTRA-WIDEBAND EXPERT TALK (UET'05)
A Hierarchical Architecture for Indoor Positioning Services
Li-Der CHOU
and
Chun-Yen CHANG
Department of Computer Science and Information Engineering, National Central University
Chungli, Taoyuan, Taiwan 32001 , R.O.C.,
Email: cld@csie.ncu.edu.tw
Abstract –Wireless technologies are capable of supporting mobility for users, and thus are
popularly adopted to access the Internet today. As long as the position of a user is identified,
location-based services, such as tour guide systems and mobile shopping, can be delivered to the
user and improve the convenience in life. In indoor environment, the GPS signal cannot be
received, and the positioning service requires the aid of indoor wireless technologies, such as
wireless LAN, Bluetooth, Infrared and RFID. By integrating these heterogeneous wireless
technologies, a hierarchical architecture for the indoor positioning service is proposed to
enhance the positioning accuracy. The hierarchical architecture is implemented in the
environment of laboratory and the accuracy is about 1 meter.
1. Introduction
Today many location-based services have been developed in indoor and outdoor environment,
such as tour guide systems [1] [2], mobile shopping, or listing neighboring entertainments, restaurants
and scenic spots. All of the location-based services depend on the position of the users. The Global
Position System (GPS) [3] signals cannot be received in the indoor environment, and the technologies
of the indoor positioning service [4] are not so easy and direct as the outdoor positioning ones.
Therefore, how to estimate accurately the position through these wireless technologies becomes a key
technical issue in the indoor environment.
In general, the positioning service requires the aid of indoor wireless technologies. There exist
many kinds of indoor positioning technologies by detecting signals of heterogeneous wireless
technologies, such as wireless LAN (WLAN), Bluetooth, infrared and RFID [5] [6]. The
characteristics of accuracy, signal transmission distance and deployment cost for the indoor
positioning technologies are quite different. For example, the transmission distances of the infrared
and the RFID signals are limited from several centimeters to meters only, but the coverage of the
access points for Bluetooth and WLANs can be up to 100 meters. The indoor positioning technologies
based on infrared and RFID are more accurate than those based on Bluetooth and WLANs. However,
infrared signal transmitters [7] [8] [9] and RFID transponders are usually not deployed as widespread
as Bluetooth and WLANs, and can not be accessed in some parts of the indoor environment. Table 1
gives the comparisons among the heterogeneous positioning technologies, where the items for
comparison include adaptive range, indoor accuracy, indoor Signal Error Rate, power consumption,
hardware cost and implementation complexity. Thus the heterogeneous indoor positioning
technologies should be adopted and cooperated, so as to match the features and the accuracy
requirement of the provided indoor location-based services.
In the paper a hierarchical architecture is proposed to improve the accuracy of indoor positioning,
according to the hand-held equipment of users and the actual indoor environment. The proposed

This research was supported in part by the Service-oriented Information Marketplace Project, the Ministry of
Economy of the Republic of China under grant 93-EC-17-A-02-S1-029, and by National Science Council of
the Republic of China under contract NSC 93-2219-E-008-002.
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Table 1.
Comparisons among heterogeneous positioning technologies
Technology
Items
Range
GPS
GSM
WLAN
Bluetooth
Wide area Wide area Micro area Micro area
Accuracy
Signal Error Rate
No signal
indoors
No signal
indoors
Infrared
RFID
Pico area Pico area
Low
Low
Low
High
High
Low
Mid
Low
Lowest
Lowest
Power Consumption
Low
Low
High
High
Low
Low
Hardware Cost
High
High
High
High
Low
Low
Implementation Complexity
High
High
High
High
Low
Low
architecture is capable of crosschecking hierarchically the indoor positioning results generated by the
heterogeneous positioning technologies, and then performing the adjustment so as to achieve the
accuracy of the estimation. Besides, the proposed architecture is implemented in the environment of
laboratory, and the accuracy is up to 1 meter.
2. Proposed hierarchical positioning architecture
Assume that the hand-held devices carried by the mobile users are capable of receiving signals
transmitted from heterogeneous wireless technologies, such as WLANs and infrared. Figure 1 shows
the architecture of the proposed hierarchical indoor positioning service. As an example, the mobile
device is equipped with appropriate network interfaces to receive the ID signals transmitted by WLAN
access points and infrared signal transmitters. The accurate positions of the WLAN access points and
the infrared signal transmitters are given in advance. A program, called the location client, is installed
Fig. 1.
Architecture of the proposed hierarchical indoor positioning service
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Location Based Service
Module (LBSM)
Database
RP Info
Location Info
Location Module
RSSI
Location Info
IRID and
RSSI
IRID
NDIS Interface
RS232 API
Wireless Lan
Card
Raw IR
Location
Info
Positioning
Engine
Signal Collector Module
Location Client
Fig. 2.
Location Server
Functional structure of the location client and the location server
in each mobile host. As the ID signals are received, the location client of the mobile host delivers the
signal strengths and IDs of all detected WLAN access points, or just the ID of the infrared signal
transmitter, to the location server. The location server will estimate the position of the mobile host, and
then deliver the estimated position back to the location client. Then the location-based services
corresponding to the estimated position will be retrieved to serve the mobile user. The estimated
positions will be stored in the database, so that the location server is able to trace the historical moving
path of each mobile user. As a mobile host receives the ID signal of an infrared signal transmitter, the
distance error of the estimated position is only several centimeters to several meters from the infrared
signal transmitter. However, as the ID signal of a WLAN access point is received, the distance error of
the estimated position is about 10 to 100 meters from the WLAN access point. Obviously, the position
information provided by the infrared signal transmitter is more accurate than that provided by WLAN
access points, because the transmission distance of the infrared positioning devices is much smaller
than the WLAN access points. In general, WLAN access points can be deployed to cover the whole
indoor environment easily, but it is difficult for infrared signal transmitters. Therefore, the positions
estimated by the location server are almost based on the signal information of WLAN access points.
Once the infrared signal is detected, the position of the mobile host will be adjusted immediately to the
position of the infrared signal transmitter, because the infrared technology is more accurate.
Obviously, the proposed hierarchical positioning architecture is based on the client/server model.
The functional structure of the location server and the location client is shown in Fig. 2. The location
client implemented in the hand-held devices consists of signal collector module, location module, and
location-based service module. The signal collector module is responsible for detecting the positioning
signals for heterogeneous wireless technologies. The location module is responsible for delivering the
collected signal information, such as Received Signal Strength Indicator (RSSI) and Infrared ID
(IRID), to the location server, and receiving the estimated position from the location server. After
receiving the estimated position, the location-based service module immediately offers the
corresponding services or actions for the mobile user. The location server consists of positioning
engine and database. The positioning engine is responsible for estimating the position of the mobile
user according to the signal information delivered from the location client. Basically, the position can
143
PROCEEDINGS OF THE 2nd WORKSHOP ON POSITIONING, NAVIGATION AND COMMUNICATION (WPNC’05)
& 1st ULTRA-WIDEBAND EXPERT TALK (UET'05)
be estimated according to the measured RSSI values of at least three access points. Then the
positioning engine performs the crosscheck and the adjustment for the estimated position, according to
the signal information of other heterogeneous wireless technologies, so as to improve the accuracy.
Another mission of the positioning engine is to transmit the estimated position back to the location
module of the location client. The database is responsible for recording the estimated positions, so that
the moving path of each mobile user can be traced and monitored.
3. Implementation
The proposed hierarchical positioning architecture is implemented in the laboratory, where two
positioning technologies, infrared and WLAN, are implemented. There are three access points of IEEE
802.11b wireless LANs and four infrared signal transmitters deployed in the laboratory, as shown in
Fig. 3, where the transmitted signals of the three WLAN access points cover the whole laboratory. The
infrared signal transmitters and the WLAN access points transmit their unique ID number periodically.
To improve the accuracy of the estimation, the RSSI values of the WLAN access points in specific
reference points are measured and stored in the database of the location server in advance. Figure 4
shows the furnishings of the laboratory and the selected reference points of WLAN, the blue spots. If
the signal collector module of the location client detects the WLAN signals only, the location server
estimates the position by comparing the RSSI values of all detected WLAN access points with those of
the reference points. Once the signal collector module detects the signal of an infrared signal
transmitter, the corresponding position of the infrared signal transmitter will be reported as the
position of the mobile user to reduce the distance error. Moreover, the reported position of the infrared
signal transmitter will be adopted to adjust the positioning information of the WLAN reference points
stored in the database.
Moreover, the historical position information stored in the database can be analyzed and retrieved
to display the moving path of the mobile user in a graphical interface. Thus the system administrator is
able to trace every mobile user and know where the hot indoor spots are. The location-based services
can be appropriately and dynamically adjusted according to the location of the hot indoor spots. In Fig.
5, the red lines represent the moving path that a specific mobile user passes through. The time that the
mobile user visited that position is also displayed in the graphic interface. Experimental shows that the
distance error of the proposed hierarchical architecture can be reduced to 1 meter.
Infrared Signal
Transmitter
Database
IRID
AP
AP
AP
Fig. 3
RSSI
.
.
.
.
.
Estimative
Position
RSSI
IRID
Positioning
Engine
RSSI
RSSI
Infrared signal transmitters and WLAN access points are deployed in the laboratory
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PROCEEDINGS OF THE 2nd WORKSHOP ON POSITIONING, NAVIGATION AND COMMUNICATION (WPNC’05)
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Fig. 4
The furnishings of the laboratory and the reference points of WLAN
Fig. 5
The graphical interface of the moving path for a specific mobile user
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& 1st ULTRA-WIDEBAND EXPERT TALK (UET'05)
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