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WI-VI is based on the principle of RADAR and SONAR (Doppler effect). RADAR is an object detection system that uses radio waves to determine the range, altitude, direction or speed of objects. It's similar to the way radar and sonar work, but without the expensive and bulky gear and restricted frequencies that radar requires. It depends on your own transmission signal.
The underlying concept of seeing through opaque obstacles is similar to the radar and probe image. Specifically when facing a non-metallic wall a fraction of the RF signal would cross the wall, be reflected in objects and humans, and be printed with a signature of what is inside a closed room. Power signal after crossing the wall twice (inside and outside the room) is reduced by three to five orders of magnitude. Even more challenging are the reflections of the wall itself, which are much stronger than the reflections of the objects inside the room. Reflections from the wall overwhelm the analog to digital converter (ADC) of the receiver, preventing it from recording minute variations due to reflections from the objects behind the wall. This behavior is called "Flash Effect" because it is analogous to the way a mirror in front of a camera reflects the camera's flash and prevents it from capturing objects in the scene. So, how can one overcome difficulties? The radar community has been investigating these problems, and has recently introduced some ultra-wide band systems that can detect humans moving behind a wall, and show them as spots that move on a dim background.