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4G interference will knock out Freeview

Screens to go blank for 30,000 households?

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760,000 households risk losing Freeview when 4G telephony comes online, and even with mitigating techniques Ofcom reckons 30,000 households face a future without terrestrial broadcasting.

The regulator reckons those 30,000 will have to switch to satellite or cable to get their TV broadcasts, as 4G telephony leaks into their television transmissions. The rest should be OK as long as they get better filters, and know enough to fit them, but Ofcom wants the mobile operators to take responsibility for that.

The regulator proposes making 4G operators pay for those filters, as well as making them fit better filters to their base stations. Operators will also be expected to pay the cost of alternatives where filtering is not enough, and Ofcom is clear that IPTV is no alternative to broadcast: so it has to be cable or satellite.

4G will, largely, exist in the old analogue TV bands which have been vacated by the switch to digital. DTT (Digital Terrestrial Transmission) is much more efficient, so can broadcast more channels in less space. The spectrum used by television has thus contracted, leaving gaps at both ends and 4G is expected to fill the top one which starts at 790MHz.

But that 790MHz band butts right up against the top DTT band, known as Channel 60*, and as radio transmissions are curved (not square) a base-station transmission at 790MHz will leak into the adjacent television broadcast.

For most people that shouldn't be a problem – our TV antennas are pretty directional and Ofcom reckons 16.3 million of us have roof-top aerials pointed in the right direction with a strong enough signal not to notice. The problem particularly hits those using amplifiers, 5.2 million attached to communal aerials, and 5.7 million users who've plugged them into their own televisions, according to Ofcom. The amplifier boosts both the DTT and the interference, enabling the latter to overwhelm the former with greater ease.

Most of those affected just need to plug in a £10 filter, which focuses the frequency received more accurately, but for communal aerials that's more complicated (and thus expensive). Mobile operators could also be obliged to fit additional filters to their base stations, to cut their own transmissions a little more squarely.

Ofcom reckons 20 per cent of us could change DTT transmitters, by pointing our aerials the other way, which should help where necessary.

Mobile operators could also restrict themselves to vertical polarisation. The vast majority of DTT (90 per cent) is horizontally polarised so that would reduce the problem significantly, but it would mean no more MIMO for the operators – so slower 4G speeds.

Most likely a combination of the above will be used, but Ofcom reckons that even if all that works around 30,000 people still won't be able to receive channel 60 and will thus have to be shifted to satellite or cable TV services – at the expense of the 4G operator.

There remain huge questions about who should run the migration service, and how proactive it should be – will customers have to complain first, or will everyone within an area be sent a filter by default?

These questions are important as they affect the value of the 4G spectrum. Ofcom still intends to auction off those bands next year, but the cost of keeping 760,000 TV viewers watching will have to be calculated before the value of the spectrum can be established.

Elsewhere in Europe, where auctions have already been held, operators have been made responsible for applying mitigation without much fuss. But, as Ofcom points out, the UK is unique in its reliance on DTT for broadcast television – in 2009 41 per cent of main household TVs in the UK were connected to DTT, as opposed to 7 per cent in Germany, and that disparity is likely to have increased since then.

The consultation (69-page PDF/596KB – well-written, but still dull) covers all those questions and is open until 11 August. ®

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