I previously blogged about my concerns about unlicensed wireless. I lamented the sorry state of the ISM spectrum but hinted at a possible solution to the mess. In related news, 802.11ac is on the way. To make a long story short, 802.11ac is going to ramp up 802.11n in a couple of aspects. One of them is the natural next step of compressing more data into the channels with additional coding and wider spectrum. However, the more interesting part is the increased amounts of "beam forming".
Beam forming is the technique of combining the output of multiple antennas to create "constructive interference" in the signal. The best way to think of this is to watch what happens when two pebbles are dropped in a pond near each other. As the waves travel away from where each pebble entered the water, they begin to interfere with each other. In certain areas, troughs meet troughs resulting in deeper waves, troughs meet peaks resulting in no wave, and peaks meet peaks resulting in higher waves.
In 802.11n and 802.11ac, beam forming is used to increase the signal to noise ratio (SNR) between devices. That's practically equivalent to increasing the strength of the signal between devices - resulting in more reliable transmissions.
That's all well and good but the ISM channel is still a mess and this isn't going to make it substantially better. However, beam forming does show the way.
Breaking the Law
I gave this post a seemingly impossible title. How can you possibly create bandwidth? Isn't the Shannon–Hartley theorem the limit of how far we can go? And isn't the ISM parking lot out of spaces? Well, the inventors of DIDO would beg to differ.
DIDO stands for "distributed input distributed output" akin to 802.11n's MIMO (multiple input multiple output - basically beam forming). The difference is that while MIMO attempts to tune a single pair of devices conversation at a time DIDO aims to beam form everybody's conversation simultaneously.
One way to envision beam forming is to go back to the example of pairs of people distributed around a room and talking to each other. In that example, they were all sharing the bandwidth of the air in the room - and all interfering with each other as a result.
In the beam forming case, however, everybody is outfitted with a cheerleader style megaphone for the speakers and a parabolic microphone for the listeners. The megaphone focuses the voice energy to it's target and the parabolic microphone captures voice energy only in the direction it is aimed. In effect, there's no longer a shared medium! And that's how the seemingly unassailable Shannon-Hartley theorem gets bypassed; change the assumptions under which it applies - a shared medium.
Saving ISM From Itself
Here's where DIDO pulls a rabbit out of the hat. Rather than trying to create pairs of conversations that are isolated via beam forming, they actually are able to calculate the signal to send from a smaller set of antennas such that for every receiver, the signal that arrives at that receiver's location is as if it had been beam formed.
Going back to our room example, imagine all of the people who are talking are now speaking into microphones instead of megaphones and the microphones are connected to a computer which mashes them altogether with a lot of complex math. The resulting signal is then transmitted from say 3 speakers scattered around the room.
The magic is that if you get the math just right, each listener hears the speaker loud and clear - with no need for items such as parabolic microphones. The beam forming happens in a distributed fashion in the entire space for every listener simultaneously.
This will be downright revolutionary if the DIDO guys can pull this off. And it will be the only way I'll ever trust the performance of an ISM device in a public space.