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NRO’s domestic-policy blog, by Reihan Salam.

DIDO: The Solution to Spectrum Scarcity?



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I’ll be honest: the debt limit debate has not struck me as the most interesting story in the news. That distinction belongs to Ashlee Vance’s Bloomberg Businessweek profile of the inventor-entrepreneur Steve Perlman, who has, with his team at his business incubator Rearden, created a fascinating new technology that has the potential to offer a permanent solution to the First World problem of crappy wireless data service. It’s called DIDO, for distributed-input-distributed-output, and if it really works in the field, it will greatly accelerate the rise of immersive virtual environments, pervasive computing, and a boatload of other technologies that will knock your socks off:

DIDO, Perlman says, will right the wrongs of the wireless networks crumbling under the weight of iPhones, Android smartphones, and tablets—and create a platform for completely immersive digital experiences. He wants to build Mova facial-capture technology right into TVs and computer monitors, so people’s heads could replace those of characters in video games. “You can become Batman, and the other players in the game will see your expressions,” Perlman says. He’s also exploring virtual retinal technology. “It’s a new form of optics that allows you to see the world in 3D. It’s not just an image coming out of the TV screen. It’s viewing your entire surroundings in 3D and having them be totally virtual.” Perhaps wireless technology could be used to create standing fields, he says, so people could one day reach out and touch the virtual 3D objects. His description sounds a lot like a Holodeck, a room depicted in Star Trek where anything can appear as real. “We’re looking at creating entire virtual worlds,” Perlman says. “Eventually, we will get to the Holodeck. That’s where all these roads lead.”

We’ve seen a lot of awesome new consumer internet start-ups, which make ordering takeout, keeping track of your friends, and collaborating in the workplace much easier. This is really good and important stuff and we shouldn’t dismiss it out of hand. But Perlman seems to have hit on a real breakthrough. Right now, wireless networks are plagued by interference, which is why the WiFi network in my apartment in my dense urban neighborhood often delivers sluggish broadband to my various devices. 

Perlman had an idea. Interference happens when a device receives multiple signals at once and the wave is muddied. The physics gets very complicated here, but Perlman thought there might be a way to turn interference into a virtue—use that combining property of radio waves to “build” a signal that delivers exactly the right message to your iPad. Multiple transmitters would issue radio waves that, when they reach your tablet, combine to produce a crystal clear signal. If there’s another person in the room with an Android phone or a laptop, the system would take those devices into account so that they, too, received unique waves from the transmitters. Such a system would need to precisely analyze wireless information from the devices at all times, and constantly recalculate the complex combinations of signals from each of the transmitters on the fly. Figuring all that out in real time would of course require some extremely powerful computers.

That, in a nutshell, is DIDO.

Vance does an excellent job of describing the technology in broad detail. You can also check out a white paper from Rearden that illustrates the basic concepts quite well:

Distributed-Input-Distributed-Output (DIDO) wireless technology is a new approach to multiuser wireless that allows the number and density of users in the same area to be steadily increased without additional users reducing the data rate of others. In other words, the shared spectrum capacity is not subject to Shannon’s law: as more users in a given area share the same wireless spectrum, the data rate per user does not decline. As a result, regardless of how many users are in a given area, each user is able to use the entire Shannon Limit of the channel, despite sharing the same spectrum.

We do not know of a theoretical limitation to how many users we can add to a DIDO system without a degradation in  data rate per user. There  certainly  will  be practical limitations with each era of technology evolution, but we have not yet come close to them. So far, as we’ve increased the number of simultaneous users in the same area to 10 (limited just by the number of hand-built radios we have) we have not seen any degradation in performance. So, while our demonstrated spectral capacity  today is 10X the Shannon Limit, we expect we can get to 100X, and are optimistic that 1000X is achievable. But, until we start to see  some degradation in performance as we add more users, we will not be able to predict how far it can go.

As I understand it, the DIDO technology coordinates as many radio signal waveforms as there are users:

Normally, when you have two very  different radio signals colliding with each other, the result is one stronger signal overpowering the other (as in the radio station example given in the  Background section), or just indecipherable noise from the two signals interfering with each other.

But, not with DIDO. Instead something rather remarkable happens: the sum of the radio signals at each computer’s location results in a  clean  modulated waveform  carrying the data intended for that particular computer. So, the waveform received at User 1’s computer contains  the video data sent by website 1, and the waveform received at User 2’s computer contains the video data sent by website 2. Each computer simply demodulates the waveform and plays the video for its user.

And, here’s the really amazing part: what each user receives is what they would have received if they had the channel to themselves, without another user sharing the same spectrum. There is no interference from the other user. Each user is able to utilize the full Shannon Limit of the channel.

Is this not the coolest damn thing you’ve ever read?

DIDO reminds me of the work of Yochai Benkler, who has long argued that wireless transmission should be regulated as a public commons like our highway system:

When we speak of regulating wireless communications, we speak of managing “a resource,” “the spectrum.”  Generally, we use market-based solutions for resource management, and therefore when posed with such a problem look for something to which we can affix property rights to be traded in the market.  But there is no such “thing” as “spectrum.”  There is no ether out there, no finite physical “resource” that needs to be allocated.  There are simply people communicating with each other,transmitting and receiving messages with equipment that uses electromagnetic waves to encode meaningful communications and send them over varying distances without using a wire. “Spectrum management” means regulating how these people use their equipment.  “Spectrum allocation,” whether it be done by licensing or auctioning, is the practice whereby government solves this coordination problem by threatening most people in society that it will tear down their antennas and confiscate their transmitters if they try to communicate with each other using wireless communications equipment without permission.  This is done so that other people—broadcast licensees or spectrum “owners”—can successfully communicate.

The rhetorical effects of treating spectrum as “a resource” obscure the more important choice to be made with respect to radio communications: whether to regulate them by centralizing control of wireless communications or, alternatively, by establishing a means of allowing users to coordinate their wireless transmissions multilaterally.  Once we understand that the question is how to regulate the use of equipment, not of “a resource,” we will be able to recognize that we have alternative regulatory models in our society.  In the case of cars or networked computers, which involve similar coordination problems, our social choice has not been to give a small number of users an exclusive license or property right to control an input essential toeffective use of the equipment. Instead, in the case of automobiles, we have chosen to allow anyone to buy and use the equipment, subject to certain “rules of the road” that allow equipment users to coordinate their use and avoid interference.  In the case of networked computers, we have relied primarily on industry and professional standard setting-procedures, and on competition in the equipment and service markets. [Emphasis added]

Is it just me or does DIDO allow us to realize Benkler’s vision of open spectrum? 

What I love about DIDO is that it looks suspiciously like an entrepreneurial solution to a pressing public policy problem. While we’ve been told that the only way out of the crappiness of U.S. wireless data networks is a massive taxpayer-funded investment in national broadband infrastructure, DIDO could allow us to leapfrog shiny and expensive data networks in other countries. To be sure, this technology won’t come cheap and it will require some infrastructure — but as Ashlee Vance notes, it needs cheapo infrastructure, my favorite kind:

Since electromagnetic noise does not affect the DIDO transmitters, they can be placed anywhere. They’re small, too, which could mean no more not-in-my-backyard fights over the placement of unsightly cell towers. The multi-city tests conducted by Forenza also showed that DIDO transmitters could be tuned to bounce signals off the ionosphere, a layer of the atmosphere about 150 miles up. Using this technique, the technology could serve rural areas and even airplanes. “We can provide DIDO service down to the floor of the Grand Canyon,” Perlman says, adding that he could cover huge swaths of rural America with high-speed wireless using just dozens of DIDO access points.

Hot damn. It’ll take some time for DIDO to be ready for action. I can’t wait. If this works, Steve Perlman will be my new hero. 



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