Taking Positioning Indoors
Wi-Fi Localization and GNSS
Solving the challenge of robust, reliable positioning in GNSS signal-challenged environments has long represented a kind of Holy Grail for product designers, systems integrators, and service providers. One promising approach is to combine GNSS with terrestrial systems that use existing wireless infrastructures. This article describes how Skyhook Wireless has succeeded with GPS and wireless local area networks — better known as Wi-Fi. As a result, its technology is now incorporated into a wide range of mobile “connected” platforms, including smart devices such as Apple’s iPhone and iPod.
Wireless local area networks (WLANs), popularly known as Wi-Fi, were originally designed for data applications. Over the past decade or so, WLAN infrastructure has been implemented for high-speed wireless Internet access in homes, “hot-spots,” university campuses, and corporate buildings. Hundreds of millions of Wi-Fi access points (APs) are deployed in major urban areas worldwide.
Today, in the lead author’s lab on the third floor of the Atwater Kent Laboratory at the Worcester Polytechnic Institute, we can read the addresses of 48 Wi-Fi access points within range of our Wi-Fi–capable devices.
Wardriving or access point mapping is a term commonly used for the process of locating Wi-Fi APs while moving around an area and building a database that can be leveraged later for Wi-Fi localization.
In 2000, three years after release of the first IEEE 802.11 WLAN standard, articles describing the use of Wi-Fi signals for indoor geolocation appeared in the research literature. During the past few years, Wi-Fi positioning or localization has found its way in metrowide positioning systems.
GPS was not designed for indoor applications and does not perform well in indoor and dense urban areas. Wi-Fi localization complements GPS positioning by providing robust indoor coverage, reduction in time to fix, reduced power consumption, and resistance to interference. GPS complements Wi-Fi by providing outdoor coverage and a universal coordinate reference frame.
Emerging “smart” devices, such as Apple’s iPhone, use Wi-Fi localization technology to complement GPS and cell tower localization in numerous everyday consumer applications, particularly in metropolitan areas. Applications range from social networking to tagging photos or videos with the corresponding location information. This animation shows the density of daily Skyhook location requests in New York City, per square kilometer per hour.
In this article, we describe the evolution of the Wi-Fi localization technology with particular emphasis on its recent application in smart devices. We describe how this technology evolved out of time-of-arrival (TOA)-based GPS technology and how these two technologies are intertwined to address the needs of rapidly expanding consumer applications.
Wi-Fi: From Data Transmission to Localization
These Internet applications are commonly used in indoor areas, where extensive multipath conditions require robust methods to achieve high data rates. As a result, WLANs introduced the first popular commercial application of spread spectrum technology, orthogonal frequency division multiplexing (OFDM), and more recently multi-input multi-output (MIMO) antenna systems.
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The Emergence of Wi-Fi Localization
About the same time, venture capitalists started funding startup companies such as PinPoint in Woburn, Massachusetts, and WhereNet, based in Santa Clara, California. Both were seeking to develop and implement indoor geolocation technologies with accuracies comparable to those required for SUO/SAS.
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Wi-Fi Localization: TOA versus RSS
Although TOA-based Wi-Fi localization uses an existing infrastructure, designers still need to modify the mobile devices’ hardware to extract the TOA estimate from a received Wi-Fi signal. Moreover, implementation of a precision TOA-based system faces the same multipath challenges encountered previously, demanding complex algorithms and solutions.
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RTLS: Indoor Tracking Using Wi-Fi Signals
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WPS: A Software GPS
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Evolution of Hybrid Localization
Smart phones have cellular network connections as well as Wi-Fi chipsets. Wi-Fi signals from hot spots, home routers, and public access and enterprise wireless networks cover most of the indoor and urban areas where Internet applications are commonly used. In locales such as interstate highways, where Wi-Fi signals may not be available all the time, less accurate cell-tower localization can complement this coverage.
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Database Collection and Algorithms
In general the distribution of the actual Wi-Fi access points in metropolitan areas forms a stochastic process with particular spatial and temporal characteristics, because the number of the access points and their locations are constantly changing. During any given time interval, new access points are installed and some old access points are re-located or even disestablished.
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RTLS is currently combined with GPS to provide accurate indoor tracking and coarser outdoor tracking when the asset or personnel is moving between two specific building destinations. WPS is integrated with GPS to provide for a comprehensive coverage in numerous everyday consumer applications. To extend the Wi-Fi localization applications to the military and public safety, we need to understand the effects of electromagnetic and radio frequency interference in this technology to have an optimum solution for its integration with GPS techniques.
For the complete story, including figures, graphs, and images, please download the PDF of the article, above.
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