A Hierarchical Architecture for Indoor Positioning Services

Transcription

1 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, ROC, 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 E

2 Items Table 1 Technology Comparisons among heterogeneous positioning technologies GPS GSM WLAN Bluetooth Infrared RFID Range Wide area Wide area Micro area Micro area Pico area Pico area Accuracy Signal Error Rate No signal indoors No signal indoors 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 142

3 Location Based Service Module (LBSM) Database Location Info RP Info Location Info Location Module Location Info NDIS Interface IRID RS232 API IRID and Positioning Engine Wireless Lan Card Raw IR Fig 2 Signal Collector Module Location Client 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 () 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

4 be estimated according to the measured 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 80211b 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 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 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 IRID Database AP IRID Estimative Position AP Positioning Engine AP Fig 3 Infrared signal transmitters and WLAN access points are deployed in the laboratory 144

5 PROCEEDINGS OF THE 2nd WORKSHOP ON POSITIONING, NAVIGATION AND COMMUNICATION (WPNC 05) 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 References [1] L-D Chou, C-H Wu, S-P Ho, C-C Lee, and J-M Chen, Re qui r e me ntan a l y s i sa nd Implementation of Position-aware Multimedia Museum Guide Systems, Proceedings of AINA 2004 International Conference on Advanced Information Networking and Applications, Fukuoka, 145

6 Japan, Mar 2004 [2] L-D Chou, C-C Lee, M-Y Lee, C-Y Chang, A Tour Guide System for Mobile Learning in Museum, Proceedings of IEEE WMTE 2004 The Second IEEE International Workshop on Wireless and Mobile Technologies in Education, Chungli, Taoyuan, Taiwan, ROC, Mar 2004 [3] P Enge, and P Misra, Special Issue on GPS: The Global positioning System, Proceedings of the IEEE, pp 3-172, Jan 1999 [4] J Werb, C Lanzl, Designing a Positioning System forfinding Things and People Indoors, IEEE Spectrum, 71-78, Sept 1998 [5] A Ward, A Jones, and A Hopper "A New Location Technique for the Active Office," IEEE Personal Communications, Vol 4, No 5, pp 42-47, Oct 1997 [6] Radio Frequency Identification RFID: A Basic Primer, v111, an AIM white paper, Sept 1999 [7] L-D Chou, C-H Wu, S-P Ho, and C-C Lee, Position-Aware Multimedia Mobile Learning Systems in Museums, Proceedings of IASTED WBE 2004-International Conference on Web-based Education, Innsbruck, Austria, Feb 2004 [8] L-D Chou, C-C Lee, M-Y Lee, G-Y Chang, C-H Liao, and K-F Jian, Design and Implementation of Location-based Multimedia Tour Guide Systems, Proceedings of TANet 2003 Taiwan Area Network Conference, Taipei, Taiwan, ROC, pp , Oct 2003 [9] L-D Chou, C-C Lee, M-Y Lee, and C-Y Chang, Position-Aware Active Learning Based on Connectionless Positioning in Indoor Environment, Positioning, Navigation and Communication, Series in Hannoversche Beiträge zur Nachrichtentechnik, pp , Shaker Verlag Publishers, Germany, ISBN: , Mar