Omni-directional Antenna Model Based Wi-Fi Positioning Method for Urban Area

J-H. Song, G-I. Jee

Abstract:	This paper presents omni-directional antenna based Wi-Fi positioning method and test results in urban area. The range accuracy of calculated by received Wi-Fi signal strength is improved using omni-directional antenna based path-loss model. The car based test results show that proposed algorithm improve the position accuracy about 30% than previous non-directional antenna model. The GNSS is dominant and powerful method to obtain navigation solution. Because GNSS provides good navigation solution to user when GNSS signal is available. In urban area, GNSS cannot provide accurate and continuous solution as open sky condition due to signal attenuation and blockage. Therefor we need another navigation method to backup GNSS. Recently, with rapid development of smart portable device, many communication company provide Wireless LAN (WLAN) services. So it is easy to detect the Wi-Fi signals in urban area. Utilizing these many Wi-Fi signals, navigation solution can be calculate. Generally, Wi-Fi positioning technique can be divided into fingerprinting and Wi-Fi trilateration. In both case, positioning is calculated using received signal strength of Wi-Fi signal. In first technique, the measured signal strengths at a certain point are compared with the signal strengths values of predefined signal strength map database. The amount of data and time are need to construct the predefined signal strength map. Also it have to be updated when AP information is changed. To update signal strength map database lots of time and effort takes. In second technique, the Wi-Fi signal strength is converted to transmission range using path-loss model and position is calculated with trilateration method. This method need less information such as AP position, transmitted signal strength and antenna gain and doesn’t have to build date base. Also, it can be easily integrated with any other range based positioning method, especially GNSS. But it is not easy to obtain the accurate range from signal strength. In this paper, we use the trilateration method to calculate the position and propose the new range calculate method. Generally, path-loss value is calculated with non-direction antenna model. In this model, transmitted signal strength is attenuated same with distance for all direction. In fact, signal strength attenuation is more affected by vertical position. If receiver is located beneath of the AP, signal strength attenuation is larger than that of horizontal point which has same distance. In this paper, we utilize this antenna pattern, i.e. omni-directional antenna model to improve the range and position accuracy. There are two problem to apply this method. First, the signal attenuation is represented as function of AP elevation angle and AP tilt angle in this model. Therefor these value must be known. In this problem, we assume AP position is known and user position is unknown and will be estimated. Actually, AP elevation angle can be represented as function of relative distance of user and AP position. Since in our problem, the receiver position is unknown parameter, we cannot calculate the AP elevation angle simultaneously. Therefor we suggest the new measurement model and estimate the AP elevation angle with position. Also, AP tilt angle is constant and can be estimated with successive measurement. Second, the omni-directional model has two model and to apply this model we have to know which model is more proper for current epoch. Likewise, there are ambiguity for measurement model. Most simple method is predict the measurement model using previous estimated state. But this method can be diverge when estimated state have large error. The Interactive Multiple Model (IMM) method is utilized to solve this problem. The simulation and car navigation test are performed to verify and evaluate the proposed method. First, the proposed method are verified by simulation with Monte-Carlo method. After, car navigation test are performed in urban area. The Novatel SPAN GPS RTK/IMU device is used as reference and the Wi-Fi RSSI is obtained using notebook wireless LAN card.
Published in:	Proceedings of the 26th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2013) September 16 - 20, 2013 Nashville Convention Center, Nashville, Tennessee Nashville, TN
Pages:	1665 - 1669
Cite this article:	Song, J-H., Jee, G-I., "Omni-directional Antenna Model Based Wi-Fi Positioning Method for Urban Area," Proceedings of the 26th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2013), Nashville, TN, September 2013, pp. 1665-1669.
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