The English seaside town of Whitehaven in Cumbria is often at the centre of energy debates, with the Sellafield nuclear fuel dump on its doorstep. Last November, however, it was thrust into the vanguard of a very different discussion when it became the first region in the UK to switch off its four analogue terrestrial TV channels for good and replace them with up to 40 digital ones.
The resulting crisp picture quality has been well received. “When people saw the quality of digital TV pictures they realised just how ropey analogue TV had been,” says Alan Cleaver, deputy editor of The Whitehaven News. The Netherlands, Finland, Sweden and Switzerland have already made a full switch, while Germany will complete by 2008, the US by 2009, France by 2011, the UK by 2012, Australia by 2013 and Brazil by 2016.
But the pristine pictures may be short-lived. Governments in and the are planning to auction off the sections of the UHF spectrum left vacant because the new TV channels take up a far slimmer spread of frequencies than analogue services, thanks to digital compression. Companies will bid to acquire the frequency bands so they can provide high-bandwidth, always-on internet services for cellphones and other wireless devices. That prospect has engineers in both Europe and the US worried. They warn that the high-power signals from wireless devices could interfere with digital TV, potentially wrecking the beautiful images that inspired the switch to digital in the first place.
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“Interference from mobile devices could cause sudden and complete loss of pictures,” says Walid Sami, a spectrum engineering specialist at the European Broadcasting Union (EBU) in Geneva, Switzerland, for as long as someone using a cellphone remains near to the TV receiver. In the US, the National Association of Broadcasters (NAB) has accused Microsoft and Google, who have said they will bid for some of the new broadband wireless services, of “playing Russian roulette” with digital television’s service quality.
“Interference from mobile devices could cause sudden and complete loss of pictures”
How has the seemingly simple reassignment of electromagnetic spectrum resulted in this spat? In the US, the auction kicks off on 24 January when the Federal Communications Commission (FCC) begins auctioning the spare 700-megahertz band of the UHF spectrum, which will be available in 2009. The UK is planning the first of its auctions for this year.
Both governments hope to raise billions of dollars, as well as reaping the public relations dividend of expanding broadband access for their electorates. Eager to license the frequencies are a range of including 3G pioneer Qualcomm, phone giant AT&T, cellphone network Verizon and Google.
They are excited by the prospect of new gaming, internet and voice-based mobile gadgets that will be made possible by the move into UHF bands. What’s more, signals transmitted via UHF travel much further than signals used by Wi-Fi, its longer-range cousin WiMax and today’s cellphones, which operate in the microwave range and so cannot transmit at high power. This means the next generation of mobile gadgets could work in areas that today only receive patchy coverage, and will be less likely to cut out suddenly.
It sounds great, but what worries European broadcasters is that the signals emanating from cellphones are much stronger than those typically arriving at a digital TV receiver. Receivers separate out the different TV channels or stations using filters, which reject all signals except those on the frequency belonging to a given channel. But if an incoming signal is above a certain, low power limit and is of a similar frequency, filters become swamped and fail to reject those signals, even if they are not exactly the frequency the filter is designed to accept. “When the power exceeds this limit,” says Sami, “the receiver becomes saturated, causing interference.”
Tests undertaken by the EBU show that using some wireless gadgets near a digital TV set can . Sami says a next-generation broadband cellphone operating at around 250 milliwatts will easily swamp the incoming 0.5-milliwatt signal for an indoor digital TV aerial, if the two are operating at close frequencies.
Interference could also happen even if a UHF mobile device is not on an adjacent frequency to the TV channel, but is simply in the UHF range. Many mobiles use cheap oscillator circuits that produce unwanted harmonics that can cause interference as much as 12 MHz on either side of the frequency it is operating on.
One solution to both kinds of interference, says the EBU, is to keep slots between digital TV and mobile devices blank. “If the phone to base station signal is not within 16 to 48 MHz of the TV channel, things will be OK,” says Sami. The EBU is negotiating with Europe’s telecoms regulator in a bid to make such “guard bands” compulsory.
The trouble is that guard bands might not be popular with governments, wireless gadget companies, or even consumers, as they waste sections of the spectrum that could otherwise be used for new applications. “The industry objects to them because reserving them means there is far less spectrum for them to buy,” says William Webb, head of R&D at Ofcom, the UK’s telecoms regulator.
Another option, says Webb, is to split the guard band in two and sell each half to the two companies using the frequency either side. “That way the users themselves may be able to geographically coordinate things among themselves so that, for instance, they might be able to use that bandwidth in low-power, non-interfering ways.”
In the US, the FCC says it will not specify what the bands must be used for. “The uses of the bands are flexible and we are not tying any particular technology or service to each band,” says Chelsea Fallon, spokesperson for the FCC in Washington DC. She says that the FCC hopes that will foster innovation.
In fact, US broadcasters are more worried about a slightly different proposal. So that TV signals in an area covered by one antenna do not interfere with signals from an antenna in a neighbouring region that broadcasts different programmes, those antennas are not allowed to use the same frequencies. So if one antenna uses 650 MHz, its neighbours cannot – effectively, 650 MHz is a “white space” chopped out of the neighbours’ spectrum. But now, members of the Wireless Innovation Alliance, a US lobby group that includes Hewlett-Packard, Dell, Microsoft, Google and Philips, say that white space frequency in one region could be used for short-range communication within that region, perhaps allowing multiple devices to form ad-hoc mesh networks for gaming, say. As a result, the FCC is evaluating plans to allow gadget makers to use the white spaces.
Microsoft and Philips, among others, are working on “cognitive radio” technology that would make this possible. The technology senses an unused UHF frequency and uses it only if it is free. The two companies have developed a prototype device but it at the FCC’s labs. The FCC is re-evaluating the technology as Microsoft claims the tests were poorly conducted. Google also claims to have shown cognitive radio can sense and avoid critical TV frequencies.
The FCC is expected to make a decision on the use of the white spaces this year but an apopleptic NAB is rallying congressional support to oppose it. In of its own, the NAB found that cognitive radio devices at thresholds proposed by the Wireless Innovation Alliance would mistake some digital TV bands for white spaces. Lynn Claudy, senior vice-president of science and technology at the NAB, says both man-made and natural obstacles will further confuse cognitive radio. “It is unfortunate that Microsoft and Google continue to try to muscle their way through Washington in support of a technology that simply does not work,” says his colleague Dennis Wharton.
Despite these challenges Google, fast becoming the poster child for an upcoming era of wireless internet abundance, is confident that new wireless devices and digital TV can happily coexist. On Google’s official blog, Chris Sacca : “Who’s going to win the spectrum auction? Consumers.” But that will only happen if wireless newbies don’t wreck digital TV in the process.
