“In the team’s experiments, three off-the-shelf WiFi transmitters send wireless waves in the area. WiFi receivers are then mounted on an unmanned vehicle that emulates a WiFi receiver grid as it moves.”
The headline says your router reads through walls. The setup needs three transmitters and a robot dragging receivers around to fake a sensor grid. The payoff is block letters of the English alphabet. The same lab lists person identification and crowd analytics among its WiFi sensing work since 2009, and that is where this goes.