This paper is available through Google scholar if you search for "CCS 16 password WiFi" or click here: https://www.a51.nl/sites/default/files/pdf/p1068-li.pdf

I've been a part of a similar paper that detected exact keystrokes. This one seems to build on a similar idea. The thing to keep in mind is that these systems need user and environment specific training. That is if the user is changed or the user or something in the environment moves, the system needs to retrain.