From All About Circuits:  Google ATAP is bringing touchless interfaces to the market using a miniaturized radar chip no bigger than a dime. This is Project Soli.

Soli’s radar sensor is a marvel in many respects. For one thing, it solves a long-lived issue when it comes to gesture-recognition technology. Previous forays into the topic yielded almost-answers such as stereo cameras (which have difficulty understanding the overlap of fingers, e.g.) and capacitive touch sensing(which struggles to interpret motion in a 3D context).

Google ATAP’s answer is radar.

Radar is capable of interpreting objects’ position and motion even through other objects, making it perfect for developing a sensor that can be embedded in different kinds of devices like smartphones... (full article)