# [Not] ordinary networks: how to detect water in your bag using Wi-Fi

[Not] ordinary networks: how to detect water in a bag using Wi-Fi

A group of American researchers published a study ( PDF ), which described a method for recognizing suspicious objects in baggage using ordinary Wi-Fi. “Suspicious objects” here are metal objects and liquids.

Further on how it works.

/ PxHere / the PD

## How does the "Wi-Fi detector"

Metals and liquids interfere with the wireless signal. Bags are made of fabric or plastic - the Wi-Fi signal passes through them with ease. Therefore, it is possible to use channel state information ( CSI ) to detect suspicious objects in bags.

Researchers have developed a special system that requires only Wi-Fi devices with two or three antennas. It analyzes the CSI, depending on a set of parameters: amplitude, frequency, and phase of the wireless signal, and evaluates its changes (called fading ).

In particular, the following formula is used to calculate the complex CSI value for each subcarrier frequency:

where H (f k ) is the channel response to the subcarrier frequency with the center frequency f k , and | H (f k ) | and ∠H (f k ) denote amplitude and phase, respectively. Other formulas for calculations are described in detail in the study itself ( on pages 4–6 ).

Further work is carried out in two stages. First determine the presence of suspicious objects, and then - their type.

The bag or suitcase is placed closer to the transmitting antenna to determine if there are CSI distortions. At the second stage, both antennas are used - receiver, transmitter. They are placed nearby to intercept the signal reflected from the object. In this case, the k-closest neighbors ( k-NN ) method is used to select subcarriers .

Metal objects of different shapes and different volumes of fluid distort the signal parameters in different ways. For example, the higher the amplitude of the reflected signal, the more liquid in the conditional bottle.

The full cycle of the system looks like this:

Researchers tested the system for six months on fifteen objects that were randomly placed in six different bags. The results of the experiments showed that their installation is able to determine the presence of suspicious objects with juiciness up to 95%. The accuracy of determining the material was 90%, and the error in measuring the volume of liquid and the size of metal objects was 16 ml and 0.5 cm, respectively.

According to the authors of the study, the developed system can be useful for checking bags and luggage in museums, airports, schools, train stations, and so on. Wi-Fi-check will be faster, cheaper and will not violate the personal space of people (as opposed to "manual" inspection).

## How else do they use Wi-Fi

I think the method using information on the state of the wireless channel (CSI), used by scientists from the University of British Columbia (Canada). In their study ( PDF ), they described how, with the help of CSI, to monitor the situation of people: they are sitting, standing or moving.

Depending on the position, the human body reflects the signals differently (creates different interference). It turned out that each pose corresponds to a unique CSI. According to the researchers, the technology will be able to find application in the areas of health care (the system will notify physicians about the fall of patients) and security (Wi-Fi will become a kind of motion sensor).

/ Pexels / PD

A similar approach suggestedA group of researchers from the United States. Only for tracking the activity and movement of people in the room, they use all the connected devices: laptops, smart refrigerators, televisions, and so on.

And researchers from Norway decided using wireless technologies to count the number of passengers in buses. The device installed in the cabin captures the Wi-Fi signals of all mobile devices of people and counts their number. This will help to more accurately assess the passenger traffic and adjust the movement of public transport.

Thus, Wi-Fi can be used not only for data transfer. The technology can be useful for detecting suspicious objects, regulating traffic, caring for the elderly and children, as well as protecting the home from robbers.

The advantage of the approach is that for the operation of such systems it is not necessary to purchase additional hardware or “hang” sensors on people. Enough ordinary home Wi-Fi network - you just need to configure the device in a certain way.

Several materials from the corporate blog VAS Experts: