One of the least fun jobs when writing a scientific paper is coming up with a motivation. It should be easy and fun: look at this awesomely cool thing we did—aren’t the results interesting? Instead, we typically have to claim to reveal the secrets of the Universe, cure cancer, or protect the public. Preferably all three at the same time.

A recent paper (PDF) on using Wi-Fi as an environmental sensor has some really exciting results. But my heart shrank three sizes after reading the following: “Traditional baggage check involves either high manpower for manual examinations or expensive and specialized instruments, such as X-ray and CT. As such, many public places (i.e., museums and schools) that lack of strict security check are exposed to high risk.”

As I said, the research is totally cool. It's just not likely to ever help with security unless molesting people with hip replacements is your version of improved security.

The cool part

Wi-Fi fills our lives. Almost every public space has Wi-Fi access. But, as the dead zone in your upstairs bedroom already tells you, Wi-Fi transmission is modified by its environment. This means we can use Wi-Fi signals to learn about that environment. We’ve reported on previous work where Wi-Fi was used to create holograms of objects. This research is basically an extension of that; instead of trying to image, we are only interested in sensing the presence or absence of an object.

When it comes to detecting things with Wi-Fi, metals and liquids are the easiest to sense. Metals reflect Wi-Fi, while liquids, especially water, absorb it. The pattern of reflection and absorption is highly dependent on the Wi-Fi frequency.

A standard Wi-Fi transmitter has something like 30-odd channels, each operating at a slightly different frequency. By analyzing the phase and amplitude of each channel at a receiver, you can build up a picture of the environment. Or, more specifically, you can sense changes in the environment. At least that is the concept.

The concept is certainly true if the transmitting antenna and receiver are not too far apart, and you only have one change in the environment. This is exactly what a team of engineers has shown.

They used the phase and amplitude to detect “concealed” objects. Amplitude is essentially how loud the Wi-Fi channel is, while relative phase describes where in the oscillation cycle one signal is compared to its neighbor. So, in a sense, the phase holds information about the relative difference in distance that the signal from two different channels has travelled, while amplitude tells us how much signal was lost.

The concealed objects were things like tinfoil-wrapped boxes, knives, and bottles of water. These objects were placed in different bags and compared to bags with things like books in them. Under the test conditions, the metallic objects and bottles of water could be spotted. And, with a bit of statistical analysis, it was even possible to tell the difference between the two. The test conditions are pretty artificial, though: a fixed room with a single object placed in the intervening space (not necessarily in the line of sight) between a pair of closely spaced antennas.

The researchers then looked at the reflected signal and showed that they could estimate the cross-sectional area of foil-covered boxes.

Let’s not encourage them

I think this is all really cool. Using Wi-Fi to sense the environment is a good idea. I just don’t think it’s that relevant to security.

People are basically walking buckets of water. So any environment with a crowd is going to be revealed to be filled with dangerous liquids. There are also a lot of legitimate reasons to be carrying around chunks of metal; metal drink bottles and foil wrapping on food are two that spring to mind. Even if we exclude food, there are also implants like pacemakers, pins, and replacement joints. You can also trigger the SWAT team for external medical devices, like walkers, wheelchairs, and mobility scooters.

Which, of course, brings us to the final problem. Real environments are dynamic. There is no static signal to compare to, and at no time is there a single change that you can pull out. In short, the research is very cool, but the motivation made me very sad.

IEEE Conference on Communications and Network Security (CNS), 2018