Sep 252010
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HAVE you ever wondered, if you are of an age with your correspondent, about those missing channels on old television sets? Apart from channel two, the rest of the original VHF channels on the dial were usually just the odd numbers from three to 13. That was because, in over-the-air VHF broadcasting, the channel between two analogue stations had to be left unused so that it would not interfere with adjacent ones. When UHF broadcasting came along, empty “guard bands” were added to each channel for the same reason. In some places, this so-called “white space” of unused frequencies separating working channels amounted to as much as 70% of the total bandwidth available for television broadcasting.

Mobile phone companies and other would-be users of wireless spectrum have long lusted after television’s empty airwaves. This week, after two years of haggling and testing, the Federal Communications Commission (FCC) in Washington, DC, finally gave the go-ahead for others in America to use them.

In November 2008, the FCC voted to reallocate the various segments of white space and unused channels between 54 megahertz and 806 megahertz (channels two to 69) that would no longer be needed when the last of the country’s analogue television transmitters switched to digital broadcasting in June 2009 (see “Wireless at warp speed”, November 7th, 2008). Unlike analogue transmissions, digital signals do not “bleed” into one another and can therefore be packed closer together.

As a consequence, television broadcasters now need little more than half the spectrum they hogged before switching to digital transmission. That has not stopped them fighting tooth and claw to hang onto their unused white space. Most had grand plans for using such frequencies, after going digital, to sell information services to the public.

That is not to be. Instead, the FCC has used the switch to digital television as an opportunity to free huge swathes of bandwidth for others to use. The most valuable frequencies of all—those in the 700 megahertz band (channels 52 to 69)—have been auctioned off to mobile phone companies. Between them, Verizon, AT&T and others paid close on $20 billion to clinch this prime spectrum.

The reason these channels are so valuable—and why they were chosen for terrestrial television in the first place—is because their signals travel for kilometres, can carry a lot of information, are unaffected by weather and foliage, go through walls and penetrate all the nooks and crannies within the bowels of buildings. They will allow mobile carriers to cover, from a single tower, up to ten times the area possible from a tower using existing frequencies. Dropped calls should then become a thing of the past.

By contrast, the white space freed up below 700 megahertz is to be made available for unlicensed use by the public. Unlicensed does not mean free. Network infrastructure will still have to be built. But a new breed of wireless internet service providers using white-space frequencies will not have to pay for their spectrum. They should therefore be able to offer high-speed broadband at far lower rates than today. It also means that start-up firms lacking the deep pockets of incumbents should be able to get a foot in the door.

Indeed, by opening up television’s white space to the public, the FCC hopes to trigger another wireless revolution—one potentially bigger than the wave of innovation unleashed a decade or so ago when Wi-Fi, Bluetooth and other wireless technologies embraced the unlicensed 2.4 gigahertz band reserved previously for microwave ovens, baby alarms and remote openers for garage doors.

The difference this time is that the frequencies being released will allow larger chunks of data to be moved further and faster. The latest version of Wi-Fi (802.11n) shuttles data at 160 to 300 megabits a second. White-space devices are expected to be able to zip data along at 400 to 800 megabits a second once they start using the same tricks as the latest forms of WiFi. And while Wi-Fi signals peter out after 100 metres (330 feet) or so, their white-space equivalents could have ranges measured in kilometres.

Microsoft, an active proponent of white-space wireless, is using just two of its experimental “White-Fi” transmitters to blanket the company’s entire 200-hectare campus at Redmond, Washington, in place of the thousands of Wi-Fi routers that would otherwise have been needed. No wonder white-space is being referred to as “Wi-Fi on steroids”.

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