We’ve all come to rely on using GPS to track and direct us when we’re outside in our cars and bikes, but when we get inside a large indoor location, such as an airport, or a conference hall, we’re still, on the whole, having to rely on paper maps. To move us all on, the Wi-Fi Alliance has announced an interoperability certification program to enable Wi-Fi for indoor location. The Wi-Fi CERTIFIED Location™ feature is based on the Fine Timing Protocol from IEEE 802.11-2016 and delivers metre-level accuracy for indoor device location. This feature enables the use of Wi-Fi for use cases such as indoor navigation, asset tracking and network management.
In this blog post, we will look in detail how the Wi-Fi Location protocol works and go on to describe hardware support for location estimation in Imagination’s Ensigma Explorer platform.
Previous methods for Wi-Fi Location
Up to now, indoor location using Wi-Fi has so far mainly relied on signal strength measurements, to either provide an estimate of distance by measuring a reduction in signal strength or matching a pattern of received signal strengths to measure patterns at known points.
However, signal strength measurements are quite variable, limiting the inherent accuracy of these methods. In order to get a good accuracy, a site survey needs to be carried out to measure the signal strengths at each location. This is a very time-consuming process and needs to be repeated every time the equipment is changed.
Measuring location using Wi-Fi – how it works
Wi-Fi signals travel through the air at a predictable rate – the speed of light. Therefore, the time between a transmission leaving an access point (AP) or station (STA) and arriving at another AP or STA can be converted to distance by multiplying by the speed of light. This is called a ‘time-of-flight measurement’.
The challenge with ‘time-of-flight’ is difficulty in getting both the devices in the measurement synchronised to the same clock reference; within a nanosecond or so. This problem is normally avoided by making a ‘round-trip-time’ measurement.
In practice the ‘round-trip-time’ is several orders of magnitude greater than the ‘time-of-flight’ and varies over time, making the raw measurement inaccurate.
- If the STA can also measure accurate timestamps, it can determine the time it took to turn around the frame with an accuracy of several nanoseconds.
- If the protocol enables timestamps to be transmitted between devices, all four timestamps can be collected in one device and the calculation can proceed.
- The distance calculation depends on all four timestamps (or two time differences) being in one place. One device has to send its timestamps to the other, enabling that second device to make the calculation.
The new Wi-Fi Location protocol: Fine Time Measurement
With the new protocol, the access point sends a frame, and the mobile device receives the frame. The mobile device responds with an acknowledgement (Ack) and the access point receives the Ack. The access point then sends a frame containing its two timestamps.
Ensigma Explorer hardware support for Fine Timing Measurement
The accuracy of Wi-Fi Location depends on the accuracy of the timestamp (1ns of uncertainty is equivalent to 30 cm of distance). Thus, it is very important to have accurate time stamps. Imagination’s Ensigma Explorer Wi-Fi is a high performance connectivity IP that supports IEEE 802.11ac 2×2. The IP supports highly accurate time stamps that are crucial to maintain the accuracy of Wi-Fi location measurements.
High-resolution timers record the WLAN packet arrival and departure times with very low uncertainty. A 48-bit counter with high resolution operates at 320 MHz. The snapshot of the counter can be taken either by configuring a trigger register (in software) or using hardware.
Given the ubiquitous nature of Wi-Fi, the promise of Wi-Fi for indoor location holds great promise. A team of researchers at MIT have in fact used a combination of angle and time-of-flight measurement to get accuracies within 10 centimetres. The Ensigma Explorer hardware platform is set to provide high accuracy location measurements and this feature has been already been licensed to one of our lead customers in the localisation space and we expect that this is an area that will grow in importance in the near future.
This post is written by Narayanan Raman and Richard Edgar.
About Narayanan Raman
Narayanan Raman is Senior Business Development Manager, Connectivity for Imagination Technologies.
About Richard Edgar
Richard Edgar is Director of Communications Technology for Imagination Technologies. Richard has worked in the Wi-Fi industry for far too long at various semiconductor and equipment manufacturers developing various short-range wireless solutions. Richard represents Imagination in various industry bodies for short range wireless technologies. When not obsessing about wireless technologies, Richard enjoys church bell ringing, archery and shouting at his children.