Look at the night sky on a camping trip and the stars are everywhere. Look from a city full of lights and you barely see any. The disappointment is similar when you go from a Wi-Fi network in isolation to one crowded with dozens, maybe hundreds, of other users. The problem, in both cases, is interference: signals crashing into each other. Adding more Wi-Fi access points, or APs, to extend the coverage area can cause more collisions, since they are all fighting over the same limited spectrum.
Now MIT researchers say they’ve found a fix: getting APs to anticipate how they will collide and tweak the signals to undo the damage. In today’s world of busy Wi-Fi networks, the way to avoid a crash is to take turns, like cars meeting at a four-way stop sign.
“If you’re the only person, you get to send [data] all the time,” says Hariharan Rahul, a visiting researcher at MIT’s Computer Science and Artificial Intelligence Lab. “If there are two people you get to send … about half the time. As there are more and more people, you get less and less opportunities to send.”
Rahul worked with two PhD students and professor Dina Katabi on a new solution: Instead of avoiding collisions, take advantage of them.
Now get ready for the jargon salad. The technology MIT developed is named MegaMIMO 2.0. (It’s an extension of a technology called MIMO that coordinates multiple antennas inside a single AP.) It was outlined this week in a paper called “Real-time Distributed MIMO Systems” at the Computing Machinery’s Special Interest Group on Data Communications conference in Brazil.
Wireless Pileup
To understand what happens when radio waves collide, go back to your high school lessons that show them as undulating lines with peaks and valleys. If two of these lines overlap perfectly, peak-to-peak and valley-to-valley, they boost each other. If they line up peak-to-valley, they cancel each other out. Usually, they are at some point in between the two extremes, each warping the other. The MIT team’s ah-ha moment: If they could anticipate how waves would overlap, they could tweak the signals ahead of time to counteract the warping. “Now when these modified signals come through the air, they are still going to collide with each other,” says Rahul. “But after the collision, the signal is now what you want.”
Easier said than done. In fact, the process requires continuous measurement of the wireless network as it’s disturbed by things like connected laptops and phones moving around or people walking by. It also requires keeping all the access points in communication to coordinate signal-tweaking efforts. That rapidly eats up bandwidth until none is left. “If you are coordinating 16 APs, you would essentially spend all your time exchanging information and never actually have any time to send the data,” says Rahul. That’s hardly a way to boost performance.
