What are location determination algorithms?

Q. How do location determination algorithms work for wireless LANs? How accurate are they? Can one detect mobility using these algorithms?

Q: How do location determination algorithms work for wireless LANs?  How accurate are they? Can one detect mobility using these algorithms? – Ravi, India

A: This is a timely question, Ravi, as location services are becoming an increasingly important feature in WLANs.  They enable IT staff to restrict access to a wireless network based upon the user’s physical location, which helps to enhance security. For this to be effective within an enterprise, location-tracking schemes should be accurate to within three meters. This is significantly different from the cellular world, where location-tracking schemas produce varying levels of accuracy – from 50 meters to 300 meters – depending upon the type of technique used. 

In the enterprise, most location-tracking systems are based on RF triangulation or RF fingerprinting techniques. RF triangulation, which is the less accurate of the two techniques, calculates a user’s location based upon the detected signal strength of nearby access points (AP). It naturally assumes that signal strength is a factor of proximity, which is true a majority of the time. Using RF triangulation, I have measured accuracies of 10 meters or better with 80% confidence. In my opinion, it is a useful tool. But it might not be accurate enough for all applications, such as securing access down to a specific cubicle.

RF fingerprinting compares a client’s view of the network infrastructure with a database that contains an RF physical model of the coverage area. This database is typically populated by either an extensive site survey or an RF prediction model of the coverage area. In my experience, RF fingerprinting can be extremely effective. For example, I have seen some systems that have measured user location with an accuracy of 3 meters with 80% confidence. Furthermore, the Microsoft Radar project has reported accuracies of approximately 3.5 meters. It is my understanding that new technologies are emerging that leverage RF fingerprinting to attain accuracies within 1 meter, making them extremely effective tools for location tracking. The tradeoff, of course, is that RF fingerprinting must work in conjunction with RF prediction and site survey tools, which have been expensive in the past. However, as WLAN systems ship with site survey tools, this becomes less of an issue.

In regards to the last part of your question pertaining to mobility, there are several algorithms developed that can predict user trajectory and speed based on real-time trending analysis. I would expect that these will be incorporated in commercially available products in the very near future, along with other location tracking features.