We’re currently researching on how we can technically achieve the localization, so we can have a better look on how our project will be done.
We were investigating a method where smartphones are listening to ultra-sonic beacons produced by known stations and then deduce their absolute position.
This has some great advantages : first the fact that it’s a software only solution for the user makes it very cheap and really scalable, but also we can control the emitting power of the beacons and so reduce the number of them (also cost effective) as we reduce the scalability problem of multiple emitters on a very small bandwidth (it will enable a faster localization).
After some investigating by reading some scientific papers (for instance ), the problem will more likely to be around the sensing rather than on the emission : not a lot of microphones can listen to frequencies higher than 20kHz, and cheaper microphones often stop at 17kHz. For a comparaison, audible frequencies are between 20Hz – 20kHz but some people can listen as up as 25kHz.
Although we may need to measure the frequency response because maybe we could manage to achieve a 1kHz bandwidth by pushing the beacon signal louder. I’ve searched on some MEMS microphone datasheets for the frequency response diagram, but none seems to display frequencies higher than 20kHz, although 19->20kHz may be usable for our needs (we need to do some tests). A 1kHz bandwidth should enable us to achieve a theorical bitrate of ~50bps using Shannon formula with a 60dB SNR (a value derivated from ST’s MP45DT02, although this one seems pretty bad, that was the only one I found with a SNR which is not A-weighted), which should be sufficient for a slow localization.
But,  seems to argue that mobile phones speakers may be able to generate sufficient ultrasonic sound to make sub-meter localization possible. Making the devices emitters instead of receivers is the method used by AT&T Active Bat (http://www.cl.cam.ac.uk/research/dtg/attarchive/bat/) and it could be a good alternative. In their paper, the authors are currently using the 17 to 22kHz frequency range, which is stepping a little bit in the audible spectrum.
In conclusion : we need to evaluate the different frequency response of different mobile phones, determinate in which frequencies we’ll use, in order to see whether the method of sensing at mobile-phone side could be maintained.
 Filonenko, V.; Cullen, C.; Carswell, J.; , “Investigating ultrasonic positioning on mobile phones,” Indoor Positioning and Indoor Navigation (IPIN), 2010 International Conference