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Almost every wireless gadget on the planet, from the lock button on your car keys to the iPhone in your pocket to the Wi-Fi in your favorite coffee shop, runs over the electromagnetic spectrum.
And those invisible signals are worth tens of billions of dollars to companies such as AT&T Inc. and Verizon Wireless.
Dallas-based AT&T has cited the need for additional spectrum as one of the main reasons for its proposed $39 billion purchase of rival T-Mobile USA Inc.
"There's just going to be a constant need for additional spectrum," AT&T Chairman and Chief Executive Randall Stephenson said during a presentation after the deal was announced.
But what is wireless spectrum? And what does it mean that we're facing a shortage of spectrum, or a "spectrum crunch," as it's been called by Federal Communications Commission Chairman Julius Genachowski?
"You throw a pebble in a pond, and it creates these waves that go out," said Dale Hatfield, former head of the FCC's office of engineering and technology. "It's the same thing. When you move electric current in a wire, it creates a wave."
And the more waves you pack into a given space, the higher the frequency.
A long way from Marconi
Using those signals to communicate and send data is a process that dates back more than 100 years to the work done by Guglielmo Marconi and other inventors. Now wireless spectrum requires massive amounts of bandwidth for millions of applications and users.
While a multitude of devices use wireless spectrum -- everything from old-school broadcast television stations to baby monitors to military missile guidance systems -- the biggest growth is in the cellphone industry.
AT&T's mobile data traffic on its network increased 8,000 percent in the past four years. Network equipment maker Cisco Systems Inc. said data traffic just on mobile networks last year was three times the amount of all data transmitted on the Internet in 2000.
For phones and other mobile devices, the most valuable wireless spectrum is in the frequency range of 300 megahertz to 3,000 megahertz. Lower than that -- in the ranges where AM and FM radio operate -- transmission towers get huge and interference becomes a problem.
Higher than 3,000 megahertz -- where radar systems and other devices operate -- cellphone signals have trouble passing through even the flimsiest physical barriers.
"As you go to higher frequencies, the radio waves get more and more like light waves and can't get through buildings," said Hatfield, who is now executive director of the Silicon Flatirons Center for Law, Technology and Entrepreneurship at the University of Colorado-Boulder. "They can't even get through a leaf on a tree if you go too high."
On every cell tower, the amount of spectrum is limited to frequencies the carrier is licensed to use in that region. Once too many users saturate the spectrum on that tower, signals fade and drop. One solution is to build more towers. Using the same slice of spectrum, you can connect more users by constructing more towers and keeping the number of users per tower low, since spectrum is reusable.
The wireless industry had 253,086 cell sites at the end of 2010, up from 104,288 a decade ago, according to CTIA-The Wireless Association, a trade industry group.
Towers take time and money, requiring sites, building permits and copper or fiber-optic cables to each tower to connect them to the main network. Wi-Fi networks or in-home mini cell towers, a technology known as femtocells, can take some of the load off cell towers.
Telecom analyst Roger Entner said that installing more towers is a major engineering headache. "Once cell sites become like 300 feet apart, it's not really physically possible to split them further unless you get pico and femtocells," he said. "It's awfully complicated. Cell site splitting is a real constraint."
Carriers can devise ways to pack data on existing spectrum, but eventually they need more of it. AT&T predicts that by 2015, its network will transmit as much wireless data every six weeks as it did in all of 2010.
"Even with more efficient radio technologies, even with some of the capabilities that we traditionally have to stretch spectrum as far as we can get it to go, we can't outrun the growth that's in front of us," said Bill Hogg, senior vice president of network planning and engineering for AT&T.
There were 78.2 million active U.S. smartphones at the end of 2010, up 57 percent from 49.8 million a year earlier, according to CTIA.
Where's the spectrum?
If the wireless industry is going to get more spectrum, it must come from someone else's spectrum piggybank.
Over-the-air TV broadcasters, for example, are sitting on some of the best spectrum for cellphones, at the lower end of that 300 to 3,000 MHz range. With the number of over-the-air television viewers shrinking, many wireless companies and industry experts think TV stations should hand over spectrum to the FCC so the agency can auction it off.
The FCC has proposed giving broadcasters a cut of the money raised at auction, which could be billions of dollars.
Jerry Brito, a senior research fellow at the Mercatus Center at George Mason University and director of its technology policy program, said there's no reason for TV stations to continue hoarding such valuable wireless real estate.
"You could auction off the spectrum completely so that it goes dark over the air, and take all these folks [who watch broadcast TV] and just write them a check so they can subscribe to cable for the next 20 years, and it would take less money than you would make to auction it off," he said.