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Using Wi-Fi To Count People Through Walls (techcrunch.com)

Posted by msmash on from the how-about-that dept.

An anonymous reader shares a report: Whether you're trying to figure out how many students are attending your lectures or how many evil aliens have taken your Space Force brethren hostage, Wi-Fi can now be used to count them all. The system, created by researchers at UC Santa Barbara, uses a single Wi-Fi router outside of the room to measure attenuation and signal drops. From the release: "The transmitter sends a wireless signal whose received signal strength (RSSI) is measured by the receiver. Using only such received signal power measurements, the receiver estimates how many people are inside the room -- an estimate that closely matches the actual number. It is noteworthy that the researchers do not do any prior measurements or calibration in the area of interest; their approach has only a very short calibration phase that need not be done in the same area." This means that you could simply walk up to a wall and press a button to count, with a high degree of accuracy, how many people are walking around. The system can measure up to 20 people in its current form.

26 comments

  1. Wifi has to much overlap to jam up channels with t by Joe_Dragon · · Score: 1

    Wifi has to much overlap to jam up channels with this

  2. Done a million times by Anonymous Coward · · Score: 0

    E.g. Dina Katabi of MIT

  3. Ethical Implications by Aighearach · · Score: 3, Insightful

    If I use this to spy on my neighbors and find out they're being periodically abducted by the Asgard, should I tell them?

    Joking aside, the accuracy is around 2 people, and it can't tell you anything about distribution. So on average, you have no idea if there are people or not. But you can tell a crowded room from an empty room, at least in rooms that if empty would have a normal attenuation pattern.

    Pretty weak sauce. Old-school DIY radar on the same frequency is generally more accurate than this.

    1. Re:Ethical Implications by EETech1 · · Score: 1

      I can do better, and recover audio with a paper cup against the wall!

  4. If they are using multiple frequencies by bobstreo · · Score: 1

    could they get better granularity? I'm thinking 2.4 and 5.0 ?

    My house has thick enough walls that it would probably be difficult to discern much, unless someone floated a drone above it.

  5. Clickbait by Obfuscant · · Score: 2
    Clickbait for sucking eyeballs to advertising.

    It doesn't matter how much power you transmit outside a room, the received signal AT THE TRANSMITTER is not going to be attenuated by the people in the room. In fact, the received signal will depend ONLY upon the transmitted signal level -- the receiver and the transmitter are connected to the same antennas.

    This is patent nonsense.

    1. Re:Clickbait by Anonymous Coward · · Score: 0

      It doesn't matter how much power you transmit outside a room, the received signal AT THE TRANSMITTER is not going to be attenuated by the people in the room. In fact, the received signal will depend ONLY upon the transmitted signal level -- the receiver and the transmitter are connected to the same antennas. This is patent nonsense.

      The receiver and transmitter are not using the same antenna. They are on opposite sides of the room. How would you even get a WiFi signal strength with just one antenna? Now, if you aren't just using WiFi, you can do all sorts of amazing stuff with just one antenna doing both transmit and receive. We call that RADAR.

    2. Re:Clickbait by Obfuscant · · Score: 2
      From an anonymous coward:

      The receiver and transmitter are not using the same antenna. They are on opposite sides of the room.

      From TFS:

      This means that you could simply walk up to a wall and press a button to count, with a high degree of accuracy, how many people are walking around.

      From TFA:

      The system, created by researchers at UC Santa Barbara, uses a single Wi-Fi router outside of the room to measure attenuation and signal drops.

      A single Wi-Fi router outside of the room cannot be on opposite sides of the room. A single Wi-Fi router outside the room likely has a pair of antennas, but they are separated by at most one foot. A receiving antenna one foot from a transmitting antenna is going to see predominately the transmitted signal direct from the transmitter antenna. The people INSIDE the room will have no effect on the signal.

      But a single WiFi router does not use one antenna for transmit and one for receive at the same time. That would be a stupid waste, since the receiver is only going to receive what the transmitter is sending. Why bother with that? No, the second antenna is for dual diversity -- the receiver can pick the best signal FOR RECEPTION by picking one of the two antennas.

      That means that the TRANSMITTER is not TRANSMITTING while the receiver is receiving. You CANNOT measure your own signal strength because you aren't receiving when you are transmitting.

      How would you even get a WiFi signal strength with just one antenna?

      By measuring the signal strength at that one antenna. You can't measure your own signal strength with one, two or a hundred antennas, because you aren't receiving your own signal, but you can measure another APs signal. That requires a SECOND WiFi router. This system unambiguously says "uses a single WiFi router". Is this a trick question or what?

      Now, if you aren't just using WiFi, you can do all sorts of amazing stuff with just one antenna doing both transmit and receive. We call that RADAR.

      And a standard Wi-Fi router has absolutely ZERO capability of doing RADAR. That's the point. You cannot have a single WiFi router that measures its own signal strength.

    3. Re:Clickbait by Namarrgon · · Score: 1

      From TFA, quoted from the original release:

      The transmitter sends a wireless signal whose received signal strength (RSSI) is measured by the receiver.

      The original release says:

      In the team’s experiments, one WiFi transmitter and one WiFi receiver are behind walls, outside a room in which a number of people are present.

      In fact, why not just read the paper itself?

      our experimental setup consists of a
      pair of WiFi nodes for transmission and reception of wireless
      signals. One of the WiFi nodes is configured as a Tx, which
      constantly transmits wireless signals. The other WiFi node,
      which acts as a Rx, measures the signals that are emitted from
      the Tx node and records the corresponding signal strength

      It would've taken much longer to write out your rant above than to spend two minutes locating the paper and bypass all the misinterpretation.

      --
      Why would anyone engrave "Elbereth"?
    4. Re:Clickbait by Obfuscant · · Score: 1

      In fact, why not just read the paper itself?

      Because the clickbait had no links to any paper. Only the claim that it was using ONE WiFi router.

      Yes, if you put TWO routers on the opposite sides of a room and measure the signal between them, you will measure how much attenuation there is, and you can assume it comes from people. This is true for almost any two radios, and isn't specific to WiFi. Use a pair of bluetooth devices, or something at another frequency. Hell, do it with ultrasonics. Bodies absorb that, too. Do'h. But then, you can't just walk up to a wall and make this measurement, you have to have a transmitter on the other side of the room, too.

      It would've taken much longer to write out your rant above than to spend two minutes locating the paper and bypass all the misinterpretation.

      Well, you know, that pretty much supports my claim that TFA is clickbait, because they deliberately misrepresent the system to make it seem more magical, and thus more likely to draw clicks from people seeking info on how it is done. If they had honestly said "path loss between two radios is dependent on the mass of water between the two antennas" I would have yawned, thought "obvious", and not clicked on the bait.

      They couldn't be arsed to include a link to the actual paper. Need even more proof about being clickbait? When I scroll down below the article looking for any link to anything relevant, the site pushes me to their main page and there is no way to get back. The back arrow doesn't go anywhere.

      Wouldn't it be nice if slashdot, a place for technically literate people, would bypass such nonsense and link to the paper directly, instead of to clickbait sites, or worse, previous stories about the same thing? No, that would be too much work for a submitter or editor. Your "two minutes" looking for the paper is better spent by ONE person submitting the article than potentially hundreds trying to find it themselves, don't you think?

      As it stands, my comments were absolutely correct. What was described cannot possibly work for the reasons I gave, and the site and /. submission are clickbait. The fact that what the paper itself describes is simple and predictable from antenna physics is a different matter. You can find path loss calculators online. All you need to add in is the attenuation from intermediate bodies. It's also, I expect, already observed in practice by people who install wireless networks when they plan for coverage. "Hey, Bob, this AP is serving a room that can hold 250 people. Is one transmitter of X milliwatts going to be enough?"

    5. Re:Clickbait by Namarrgon · · Score: 1

      I don't think you get many points for being correct about something not relevant to the subject. Yes, science reporting sucks at a lot of popular sites, we know. And given the rest of your comments, it sounds like you still haven't read the paper, which is somewhat less obvious than you appear to think.

      --
      Why would anyone engrave "Elbereth"?
    6. Re:Clickbait by ET3D · · Score: 1

      Here's how I got to the project page:

      Played on the video.

      Clicked on "YouTube" to open it on YouTube.

      Opened the description.

      Clicked on the project page.

      An error in understanding doesn't translate to clickbait, and getting to the source isn't that hard.

    7. Re:Clickbait by Obfuscant · · Score: 1

      I don't think you get many points for being correct about something not relevant to the subject.

      The summary and the news article it links to are probably relevant to the subject. If not, why do they exist?

      And given the rest of your comments, it sounds like you still haven't read the paper, which is somewhat less obvious than you appear to think.

      Path loss is established science. My points stand.

    8. Re:Clickbait by Obfuscant · · Score: 1

      An error in understanding doesn't translate to clickbait,

      Deliberate misrepresentation of the material to make it seem magical and new does translate to clickbait. Linking to a page that is 90% advertising, which automatically changes to an irrelevant page with 90% advertising when you scroll just a skootch past the clickbait article is clickbait, especially when you cannot go back to the original page using the standard "back" button.

      and getting to the source isn't that hard.

      You have to watch the video, which doesn't play well over the net to begin with and not at all when it isn't in a form that is playable, then go to YouTube, then click on something else, then click on something else to finally get to the source. I'm reminded of a filing cabinet in the basement...

      If the /. summary wasn't clickbait, it would have linked to the paper itself. If the clickbait article wasn't clickbait, it would have a direct link to the paper. Telling someone that they have to go through half a dozen steps to get from A to B is a pretty clear admission that getting from A to B isn't as easy as you claim. Especially when it takes watching a video.

      Do you dispute the point that both the summary and the article describe something that is patently absurd? If not, then stop arguing with me about it.

    9. Re:Clickbait by Anonymous Coward · · Score: 0

      Congratulations, you're not wrong, just irrelevant.

  6. Re: Brownshirt Trumpettes by Anonymous Coward · · Score: 0

    Holy shit, just die in a fire already.

  7. So old... by Anonymous Coward · · Score: 0

    I've been doing this for ages. It's quite simple, actually:

    * Set up open wifi network
    * Wait 3 minutes
    * Count number of stations connected

    That's how you know how many people is inside the room

    1. Re:So old... by Anonymous Coward · · Score: 0

      Nice one:)

  8. But not on Faraday cages by WillAffleckUW · · Score: 1

    For some reason, that doesn't work.

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