Abstract
Determination of indoor position based on fine time measurement (FTM) of the round trip time (RTT) of a signal between an initiator (smartphone) and a responder (Wi-Fi access point) enables a number of applications. However, the accuracy currently attainable—standard deviations of 1–2 m in distance measurement under favorable circumstances—limits the range of possible applications. An emergency worker, for example, may not be able to unequivocally determine on which floor someone in need of help is in a multi-story building. The error in position depends on several factors, including the bandwidth of the RF signal, delay of the signal due to the high relative permittivity of construction materials, and the geometry-dependent “noise gain” of position determination. Errors in distance measurements have unusal properties that are exposed here. Improvements in accuracy depend on understanding all of these error sources. This paper introduces “frequency diversity,” a method for doubling the accuracy of indoor position determination using weighted averages of measurements with uncorrelated errors obtained in different channels. The properties of this method are verified experimentally with a range of responders. Finally, different ways of using the distance measurements to determine indoor position are discussed and the Bayesian grid update method shown to be more useful than others, given the non-Gaussian nature of the measurement errors.
Highlights
OverviewDetermining position accurately indoors, where GPS is not reliable, has many potential applications and has been of interest for some time [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15]
The contributions of the research presented here are as follows: This paper introduces: (1) “frequency diversity”—a method for doubling the accuracy of fine time measurement (FTM) round trip time (RTT) distance measurements; (2) the “position-dependent error” texture surface—a new way of understanding the nature of the errors in FTM RTT distance measurement; (3) analysis of the unusual properties of the errors in distance measurement in terms of properties of super-resolution algorithms; (4) recognition of the serious impact of signal delay in common building materials resulting from their high relative permittivity—arguably more important than possible multi-path effects
The high permittivity of building materials biases the distances measured by FTM RTT
Summary
Determining position accurately indoors, where GPS is not reliable, has many potential applications and has been of interest for some time [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15] We start by briefly discussing methods for indoor position determination. This is followed by an exploration of the error sources in indoor position determination, those for FTM RTT. Different attempts at getting more accurate distance measurements using uncorrelated error contributions are discussed and the frequency diversity method introduced. Experimental results confirm that frequency diversity can double the accuracy of indoor position, given that there are six non-overlapping 80 MHz channels available in the 5 GHz band. Various methods for determining position from distance measurements are explored and the Bayesian grid update method shown to be well suited to the task given the unusual nature of the error in distance measurement
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