WLAN TOA Ranging with GNSS Hybrid System for Indoor Navigation

B. Li, K. O’Keefe

Abstract:	There are several factors that challenge the development of indoor positioning such as limited market demand and incomplete infrastructure. However, with the increasing popularity of wireless communications network infrastructure, and an increasing interest in location-aware services, there is a need for an accurate location-finding technique for indoors. WLAN constitute the main infrastructure to be utilized for wireless location finding. Its market is growing rapidly as the flexibility, connectivity, mobility, and low cost of this technology meets the needs of consumers. The goal of WLAN is to provide local wireless access to fixed network architectures; therefore it is the prefect infrastructure for building indoor positioning techniques. The WLAN signals are defined by the IEEE 802.11 standard, which is used to implement WLAN networks in the 2.4, 3.6 and 5.4 GHz frequency bands. The WLAN based TOA ranges research started in late 90s. McCrady et al 2000 first proposed a TOA based on RF samples, which can achieve a horizontal error probable of 2 meters at 50% probability. More recently innovative TOA-based ranging techniques have been proposed by Ciurana et al 2007. This approach is based on RTT (Round Trip Time) measurements using standard IEEE 802.11 link layer frames. Ranging results obtained using the proposed technique show an encouraging achievable ranging precision with meter level error. The approach described in by Ciurana et. al 2007 shows that WLAN packets can be measured using today’s inexpensive commercially available equipment to determine the distance between two wireless nodes for location sensing applications. Furthermore, a software based approach was proposed by Hoene et al 2008. Unlike other two-way TOA range estimation methods, this method proves that Round-Trip-Times of IEEE 802.11 MAC packet sequences (e.g., RTS, CTS, DATA and ACK) can be used instead of traditional packet pair ping requests (e.g.,DATA and ACK), in order to range between WLAN nodes. However, the application of such a direct ranging approach has never been tested with multiple users in a network, especially at the system level, nor has the integration of WLAN ranges with GPS measurements been demonstrated. We propose a new method for performing simultaneous localization and mapping (SLAM) using range-only WLAN TOA observables combined with available GPS pseudorange observations for 2D indoor and urban canyon navigation. Hybrid location methods (employing WLAN fingerprinting, cellular finger printing and/or GPS) claim that the hybrid location accuracy is about 10 m (open sky) and the position errors in indoor environments could be up to 200 m since GPS signal is mostly unavailable. In comparison, we have conducted several indoor ranging tests to evaluate the performance of WLAN ranging and have found it possible to obtain 4.54 meter range precision at one sigma level. The corresponding accuracy is of course highly dependent on the level of multipath present. However, with this precision of range from a number of overlapping WLAN nodes combined with GPS pseudorange measurements, 5 meter or better position accuracy can be achieved. Moreover, this can be further improved by network adjusting and map matching techniques based on available floor plans. At the mobile user level, positioning algorithms can be categorized into either a Beacon-based state, where the WLAN access point position is used as reference allowing users to position in an absolute local level frame; and a Beacon-free state, where each mobile user has its own reference frame, only adding nearby users and access point positions to expand the frame as they are encountered by the user. In this paper, the second approach is implemented in an extend Kalman Filter SLAM technique where the filter combines the observations from all available sensors to estimate the position of the user and nearby access points in near real time. Both the Beacon-based state and beacon-free state estimations will be compared with a true trajectory obtained using a tactical grade GPS/INS solution and with a commercially available hybrid solution. Both approaches (Beacon-based and Beacon-free) can either be implemented in a centralized processor, or in a decentralized architecture where each user is doing its own processing. This paper will also compare the performance of a centralized solution using all available data to a distributed solution using only measurements made by the individual mobile user in conjunction with information about state estimates received from other elements in the network. Our results will demonstrate that the proposed location/tracking algorithm can achieve 5 meter or better location accuracy in urban canyon and indoor environments where this is not possible using either standalone high sensitivity GPS or WLAN. The details of the integration process, the efficiency and accuracy of the positioning engine with associated confidence level as well as the floor plan matching technique will be discussed in this paper.
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:	416 - 425
Cite this article:	Li, B., O’Keefe, K., "WLAN TOA Ranging with GNSS Hybrid System for Indoor Navigation," Proceedings of the 26th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2013), Nashville, TN, September 2013, pp. 416-425.
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