Modern target detection devices are usually restricted to line of sight techniques. The imaging through the wall by using Wi-Fi technology is quite a new engineering topic. Researchers at the Massachusetts Institute of Technology (MIT) have developed the prototype Wi-Vi system which is basically able to detect and track moving objects through non-transparent surfaces such as walls. The following MIT video demonstrates how Wi-Vi can detect the motion of a person behind a wall.
The working principle of the device is quite simple and similar to radar and sonar devices. The Wi-Vi device is a reference point which sends radio signals through a barrier and receives their reflection by bouncing off an object on the other side. The Wi-Vi device contains two parts: one transmits the signal, and the other receives the signal. Both parts work together simultaneously. The signal processing tool is also an important part, as it compares the received and transmitted signals by determining the frequency shifts between them. The sent and received signals are correlated by a signal processing technique and the changes in the signals (peak values in the magnitude) are outputted and displayed when movement is detected.
Most of the sent signal is reflected off the wall and only a pittance of it reflects off the person on the other side. The researchers have solved this issue by transmitting two signals where one is the inverse of the other. The signal processing algorithm ensures that the inverse signal cancels out the signal that hits the wall but not the one hitting the moving object on the other side of the wall. In this case, the reflections caused by the moving person are visible even through the wall.
The next big challenge is to determine the difference between a person and an inanimate object. The solution can be monitoring the amount of power absorption. According to the amount of power, it is possible to determine if the object absorbed more or less than a typical human body (which has predefined value).
This technology is quite polarized because of concerns about privacy. Currently, it should not be concerning. This video has a very low-resolution, omitting the details. Of course, the resolution could be increased such that the device would be able to recognize faces. The MIT researchers are already developing the high-resolution system, but a real commercialization of this device would definitely not begin before defining the policies around how it may be used.
The Wi-Vi device uses inexpensive wireless technology opening the potential for widely used applications such as smartphones or devices for searching or rescuing purposes. They could be used for finding people trapped in collapsed buildings after disasters. On the other hand, they could also greatly increase safety for military troops during an operation. This technology has the additional potential to be used in medicine by detecting abnormalities in the human body such as that of breast cancer.
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