The proliferation of mobile broadband – including smartphones, tablets, and other data-centric devices – and applications and services from Facebook to GPS navigation has transformed the wireless world on many fronts and sent ripples throughout the industry. We used to just talk on our mobile phones. Now we text, email, watch videos, stream music, download games, and browse the Web. Based on this trend, industry observers forecast continued exponential growth in data traffic over the coming years.
According to Cisco, global mobile data traffic grew 2.3-fold in 2011, more than doubling for the fourth year in a row. The mobile-data traffic growth rate was higher than anticipated in 2011 and reached a level equivalent to eight times the size of the entire global Internet in 2000. Although mobile-network connection speeds increased 66% to an average of 315 kilobits per second (kbps) in 2011, such gains can't come close to keeping up with demand.
As data usage becomes more intensive, the gap between what systems can deliver and what people need is growing. Cisco forecasts that global mobile-data traffic will increase 18-fold over the next five years, at a compound annual growth rate (CAGR) of 78%, reaching 10.8 exabytes per month by 2016. (One exabyte is one quintillion bytes: 1.0 x 1018, or 1,000,000,000,000,000,000.) In terms of what the expected demand on wireless systems will be versus the projected capacity of those systems, data from Cisco indicate that in the next two years there will be a 20-fold bandwidth gap between what you're going to want to do on your phone and what the network will be able to deliver.
Many of today's networks and products are already becoming overloaded by the explosive demand for data. And this is just the beginning.
Typically, when wireless carriers like AT&T or Verizon want to upgrade their networks to allow for more data flow and higher speeds, they have to purchase more spectrum at auction from the FCC. A recent example of this was the 700MHz auction in 2008. Both AT&T and Verizon bought large swaths of the 700MHz range to help build out their LTE 4G data networks.
The big carriers live and die by their spectrum, and they're spending billions to secure as much as possible. The problem is that we're now facing a spectrum crunch thanks to the growth of those bandwidth-gobbling smartphones and increasing demand for data across wireless networks. And it's not like we can just conjure up more spectrum.
But what if we didn't have to? What if we could boost the capacity of wireless networks by 100- or 1,000-fold without using any more spectrum? Turns out it might be theoretically possible.
The answer could lie in exploiting a relatively obscure property of light. Particles of light, known as photons, carry two kinds of angular momentum – what's known as "spin angular momentum" (SAM) and "orbital angular momentum" (OAM). SAM is associated with photon spin and is manifested as circular polarization, while OAM is linked to the spatial distribution of photons. To borrow a popular analogy, you can think about SAM and OAM like the Earth-sun system. SAM is akin to the Earth spinning on its axis, while OAM can be represented by the Earth's orbital movement around the sun.
The existence of OAM has been known for a long time, but in standard wireless communications like WiFi, we only modulate the SAM of radio waves. It was not until 2004 – when Miles Padgett and coworkers at the University of Glasgow demonstrated that classical information could be encoded in the OAM states of photons – that the wider scientific community began to ponder the use of OAM for communications networks. What's important about this is that while SAM has only two possible values, OAM can theoretically achieve an infinite number of possible states. These different states provide additional degrees of freedom within which to encode information without the need for a new frequency. Thus, since you could send any number of signals over the same frequency, OAM has the potential to tremendously increase the capacity of communications systems. This future may not be as far off as you might think.
Earlier this year, Bo Thidé of the Swedish Institute of Space Physics and while at the University of Padova in Italy, together with some Italian colleagues, confirmed the theory. They demonstrated in a real-world setting that it is possible to use two beams of incoherent radio waves, encoded in two different states of OAM, to simultaneously transmit two independent radio stations on the same frequency. The research opened the door, Thidé says, to the transmission of "an infinite number of channels in a given, fixed bandwidth." The spectrum crunch could be close to being solved.
Additional supporting evidence came just last week, when Alan Willner, an electrical engineer from the University of Southern California, and his team published a paper in Nature Photonics titled Terabit free-space data transmission employing orbital angular momentum multiplexing. Quite a title. But the gist is that through the exploitation of different OAM states, the group was able to transmit eight independent channels of data on the same signal across free space at a whopping 2.56 terabits per second (or 320 gigabytes per second). While this is below the world-record data-transmission speed of 26 terabits per second achieved last year by scientists at Germany's Karlsruhe Institute of Technology, it's the fastest wireless network so far using OAM.
We're not there yet. While the experimental results from Willner and his team are impressive, it's important to note that the transmission distance in this case was only about a meter, and the experiment took place in a vacuum. When asked about the prospects for transmission over long distances in free space, Willner replied:
This is the main goal. One of the challenges in this respect is turbulence in the atmosphere. For situations that require high capacity or spectral efficiency over relatively short distances of less than 1 km, this approach could be appealing. Of course, there are also opportunities for long-distance satellite-to-satellite communications in space, where turbulence is not an issue.
For now, however, spectrum remains a scarce and expensive resource. But we can imagine a future in which OAM technology pans out and is able to boost the capacity of wireless networks as the theory says it should. The result would be similar to how time division multiplexing (TDM) and code division multiple access (CDMA) technologies brought the price of cellphone access plummeting by allowing more information to transmit faster over existing channels. The large swaths of the electromagnetic spectrum that wireless carriers like AT&T and Verizon have paid billions for would no longer be necessary to ramp up speed and capacity; prices for unlimited data plans would plummet; and the business model for the whole industry would be rewritten overnight.
Chris Wood is the senior analyst for Casey Extraordinary Technology, which covers robotics, biotechnology, software development and every other aspect of the technology sector. He's also the manager of our analyst team, as well as a regular contributor to The Casey Report, our flagship publication, and the Casey Daily Dispatch.
If you enjoyed Chris' article today, you might also find a piece he wrote about advances in cyber warefare to be equally interesting.
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