4G TV interference: not guilty or is the jury still out?

At800, the organisation in charge of assessing whether the launch of 4G services in the 800 MHz band in the UK will cause interference with DTT broadcasts, gave London the all clear last week. The trials it has been conducting in southeast and west London since April revealed “no confirmed Freeview problems”, the company said (Freeview is the UK’s free-to-air DTT platform).

So is it a case of trebles all round, don’t know what the fuss was all about and let’s get these shiny new services on the road? Not quite. At800 notes that London has distinctive characteristics that make it very different from some other parts of the UK: the DTT signal there is very strong, and is transmitted at frequencies clearly separated from the frequencies on which new 4G at 800 MHz mobile services will be carried.

This is not the case everywhere. Freeview is carried on different frequencies in different parts of the country, in a range between 470 and 790 MHz. The highest frequency at which Freeview signals are transmitted from the Crystal Palace transmitter in south London is 540 MHz, while the new 4G at 800 MHz signals begin at 791 MHz.

In addition, the high signal strength from Crystal Palace makes it less likely that DTT viewers will need aerial amplifiers. Amplifiers can cause problems by boosting 4G at 800 MHz and causing signal overload in TV equipment.

Things may be very different in other parts of the country, such as in Brighton on the south coast, where at800 has now started a new trial. In that city, Freeview is transmitted at frequencies much closer to those used for new 4G at 800 MHz services, so the next set of trial results could look very different.

Brighton is also hilly, which will allow at800 to examine the impact of more varied terrain on the coexistence of DTT and 4G at 800 MHz.

Things may also turn out to be different when the four mobile operators who won 800 MHz spectrum in the UK’s recent auction all start using the band for 4G services. Have at800’s tests been able to reproduce the conditions that will prevail when every mobile subscriber in London (population: 8 million) is using an LTE-enabled smartphone?

The organisation says it has. “The tests are conducted both in ‘fully loaded’ and ‘idle’ mode,” says communications director Ben Roome. “In fully loaded mode, it’s as if all the available spectrum is being used. In idle mode, there is no traffic, but the base station is scanning for devices. Ironically, there seem to be more problems in this mode than there are when the network is ‘fully loaded’.”

This does not mean that concerns about 4G interference are a case of much ado about nothing, he adds. “In the trial we ran in the West Midlands, we found that 15 people lost their Freeview service. That was only a very small trial, so it is clear that it will be difficult to predict what happens in all parts of the UK.”

Perhaps by the time 4G services are in widespread use, Freeview broadcasts will no longer be anywhere near the 800 MHz band anyway. If the mooted release of the 700 MHz band for mobile broadband services does end up going ahead later in the decade, then DTT could well end up being relocated to the 600 MHz band.

In which case, will at800 have to get all their equipment out of storage and use it to check that 4G (probably 5G or 6G by then) in the lower part of the 700 MHz band does not interfere with DTT broadcasts in the 600 MHz band?

If the bands change, will we have to go through all this anxiety again?•