Next Generation Indoor Positioning System Based on WiFi Time of Flight (2013)

WiFi coverage in the indoor environment is ubiquitous. Thus, it is no surprise that it is often used to obtain positioning information. However, performance to date has not been satisfactory, and many potential use cases have been left on the planning table. In this paper we introduce the WiFi Time of Flight (ToF) protocol, a new time-based range measurement protocol, providing high accuracy positioning information. We describe the development stages from protocol conception, through simulation, algorithm construction and performance, to real life range and position results.
Concept
The basic concept of WiFi ToF is to determine distances by measuring travel times, similarly to GPS. However, some major differences between the technologies need to be addressed. For example, GPS satellites are synchronized using atomic clocks as opposed to WiFi whose Access Points (APs) are not synchronized at all. In ToF we compensate for lack of synchronization by measuring round trip delays. Moreover, while GPS systems typically have Line of Sight (LoS), WiFi environments often exhibit significant multipath channels. As a result, our solution includes an algorithm for detecting the LoS component of the signal, which is possible due to the higher bandwidth (BW) of the WiFi signal. In fact, the higher position accuracy desired in indoor environments motivated us to develop an algorithm precise enough to provide a finer position accuracy than that of GPS. 
The following goals were therefore set: (1) define a simple roundtrip transaction for timing measurements; (2) get the exact reception and transmission times of the training frame; (3) get the channel information in order to detect the LoS component. 
Simulation
To develop and test the algorithm for detecting the LoS component in a typical indoor channel, we developed a ray tracing simulation. We generated channels similar to those we would expect in a real life environment, including dry walls, wooden doors, glass walls, concrete, etc. In this controlled environment we were able to simulate various room structures, AP positions, BW, SISO vs. MIMO performance, tracking performance, non-isotropic antennas and more. The range and position results of the simulation set our expectations for the ToF system.
Measurement process
The ToF basic protocol is similar to the timing offset measurement described in 802.11v. An accurate Time of Departure (ToD) and Time of Arrival (ToA) is calculated at both ends and after combining all measurements the ToF is determined. The time stamp used for calculating the ToD or ToA is the time of departure or arrival of the first sample of the training frame used for channel estimation. Since there are some unknown delays within both WiFi transceivers, in both the analog and digital domains, the setup is calibrated prior to measuring the ranges. As for detecting the LoS path in the channel, we used the MUSIC algorithm, which is commonly used for this task and gave good results as expected. 
System description
The first deployment of the WiFi ToF system was in our office area. We used several ToF units with some acting as access points and others as mobile devices. We collected measurements in both stationary and dynamic scenarios, and compared the results to the simulation outcome.