[AI] Cellphone signals could kill TV pictures;
ilovecold at gmail.com
Tue Jan 29 11:37:21 EST 2008
Auctioning off UHF
frequencies freed up by the switch to digital TV could
lead to poor quality pictures
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
Europe and the US 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.
"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
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 multinationals 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
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 destroy TV pictures and sound . 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 CEPT 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
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,
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 failed
tests 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
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 tests 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 writes : "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.
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