Ofcom moves to protect Freeview interference from 4G mobile devices
Ofcom has estimated it will cost £100m to deal with Freeview users who are located near to the transmitters for the next generation of mobile broadband services, which will use the frequencies (791 to 862 MHz) previously used for analogue television.
Once switchover is complete, over 10 million homes in the UK will use Freeview for their only television reception, and almost all of the remaining 17 million homes will use the terrestrial digital television service on their secondary TV sets.
The signals are provided from two types of transmitter. First there are around 80 high power transmitters located on hills that serve very large areas, such as the Crystal Palace transmitter (4.5 million homes in London), Winter Hill (2.7 million homes in the North West of England) and Sutton Coldfield (1.8 million homes in Birmingham). In addition there will be over 1,000 fill-in Freeview light transmitters, such as Boddam, which serves just 600 homes.
In contrast, the new 4G mobile services will use around 9,000 smaller transmitters located near where the services are required, which follows the current model for mobile phone networks.
4G transmitter interference locationIn places where the 4G transmitter is located close to homes receiving Freeview, it is likely that Freeview viewers will experience to forms of interference:
Signal overload - when a Freeview receiver is overloaded because the total input signal level is more than a certain level, the whole receiver will stop working and all television services will be lost.
Signal-Interference Noise Ratio degradation: this is where reception breaks down because the receiver can no longer decode the digital information in the transmission. This could affect a single multiplex or could take out all services.
The "overload zone" will occur for Freeview viewers located close to the 4G transmitter, with the "degradation zone" will affect those slightly further from the 4G transmitter:
Interference factorsNot all 4G transmitters will cause problems for Freeview reception, the other factors are:
The types of Freeview installation, with single unamplified aerials to a single set have the best resilience, with communal and systems with amplifiers more likely to suffer. The 4G transmissions are capable of overloading most types of TV amplifier.
The frequencies used for the DTT services being received, with those on the adjacent C60 being worst, C52 to C59 second worst and those on lower frequencies having the best chance of avoiding interference.
The strength of the Freeview signal received is another factor, with those with weaker signals due to being distant from the Freeview transmitter, having the most potential for 4G interference.
Not all 4G transmitters will use the same frequency, those that happen to use the lower frequency allocation having the most potential to cause Freeview interference, and those that transmit at higher power levels having more effect than low power 4G transmitters.
Homes affected by 4G interference without interventionOfcom calculate that:
Of the 16.3 million UK homes with a standard (unamplified, unshared) Freeview reception, 110,000 (0.67%) would be effected.
Of the 5.2 million homes using communal aerials systems, 550,000 (10.6%) will have problems.
Of the 5.6 million homes using amplified Freeview reception, 100,000 (1.8%) will experience problems.
Prevention and mitigationThere are several ways to deal with these 760,000 homes that will have problems.
Signal filtersUse of signal filters for the Freeview reception combined with Fitting of filters at 4G transmitters.
Ofcom's modelling finds that this is the most effective way of dealing with the 4G interference problems. Of the 110,000 standard Freeview installation homes, 87,000 will have their reception restored this way, almost 100% of the 550,000 homes with communal systems will be mitigated with filtering and 93% of the 100,000 domestic installations with amplifiers.
The total cost will be £20m for the Freeview filters and £33m for fitting of the filters in homes. Also, for the domestic filtering to be effective, the 4G providers will also have to spend around £11 fitting filters at the 4G transmitters when they are being installed.
Freeview equipment adjustmentAfter the provision of filters, there will still be 23,000 homes with unamplified and 7,000 homes with amplified Freeview reception equipment that are unable to receive their services.
Some of these homes will simply need a new Freeview box for each TV set. Whilst these boxes cost around £15, the requirement to fit these and provide for each set could come to as much as £200.
Another option, for at least 20% of homes, is to receive the Freeview services from an alternative transmitter. However, this could lead to the provision of the 'wrong' version of BBC One, BBC Two, ITV1 and Channel 4/S4C to the home. Ofcom is unsure if this will be an effective mitigation.
On Channel RepeatersThe use of On Channel Repeters (OCR) to rebroadcast the Freeview signals at higher power levels in the interference area was considered by Ofcom, but the high cost and unknown effectiveness has caused them to be distrgarded as a viable option.
Ensure 4G polarization is opposite to Freeview polarizationBecause of several factors, Ofcom does not consider that this will help prevent 4G interference of Freeview reception.
4G transmitter power reductionOfcom have concluded that causing service reception problems for the new 4G mobile services is undesirable for the services to be successful.
Provision of Freesat or free Virgin Media services for affected homesThis leaves providing a replacement Freesat installation (including multiple sets and Freesat+ boxes) for the 30,000 homes with their Freeview reception disabled, or the funding of a basic Virgin Media package. The total cost for this is £10m.
Are these filters 'no loss' devices?
Where are they to be fitted - at the antenna or at the receiver?
Anyway, surely error correction will take care of most interference? I suspect this will turn out to be similar to the 'FM-immunity' nonsense which cost many aeronautical VHF users hundreds of pounds thanks to CAA regulations - but no-one has ever noticed ANY 'FM' interference!
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nickwilcock: Yes, they are (when someone gets around to manufacuting them) lossless.
They are fitted at any point between the aerial and receiver. If you have a booster, they are fitted before the booster.
The COFDM error correction can't deal with a signal overload, as per the article.
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I don't have any interference problems with 2G & 3G from the nearby masts. I was only concerned about the new 4G stuff as per the article. I assume the 4G signal will overload some if not all of the amplified aerials in the town, most of which point towards the phone masts - that's the direction of the transmitter too! Mux 2 is on channel 59, so will be most likely to have trouble anyway. Wonder what channel the HD Mux will end up on?
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John: From the description provided by Ofcom, the filter will be required for those proximate to the transmitter, rather than having a 4G transmitter on the TV signal path.
BBCB moves to C50. Digital
Dividend - changes to the 800MHz band | ukfree.tv - independent free digital TV advice .
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