A recent study by UK regulator Ofcom confirmed that the new LTE service could knock out some cable TV set top boxes, resulting in a poor TV picture. What is the extent of the problem and is this an ideal opportunity for femtocells to be integrated into these same boxes to resolve the problem.

Ofcom study

Ofcom, the UK regulator, commissioned this recent study by Cobham Technical Services and the report can be downloaded here. Tests show that under certain conditions, using LTE handsets indoors near to a Cable TV modem may cause significant interference with the picture and sound quality. Talking (or surfing the web) from your latest LTE handset in the living room might upset the rest of your family and affect your own enjoyment of the TV.

The underlying problem is that the frequencies allocated for LTE are those released from earlier analogue TV – the so called digital dividend. However, these same frequencies continue to be used within the cable networks.

Excentis Study

This follows on from an earlier study by Belgian test house Excentis, who had identified similar issues in the 790-862 MHz band. The press release from the Cable Europe suggested that most LTE handsets would be operating at high power  (i.e. >20dBm) when indoors and could cause a problem if within 6 metres of the TV, with most likely issues if within 1 metre.

As reported by TheRegister, the problem is specific to cases where the handset is close to the TV, operating at high power, and at the same frequency. Not all cable set top boxes suffer from this problem, some are built with better RF protection than others.

Since the cable TV box isn’t radiating RF at this frequency, the handset may not suffer interference itself, and so would be less likely to switch to an alternative frequency where available.

Ofcom study showed less of a problem

Compared to the earlier Excentis highlighted figure of 6 metres, the Ofcom commissioned study indicates that handsets need to be closer than 1 metre to cause a problem. The issue seems to be constrained to certain (older) designs and can be mitigated in many cases by swapping the connecting cable for a better quality/screened variety.

BroadbandTV News had a good analysis of the situation at the middle of last year.

Other service providers keen to distance themselves

TelstraClear, the New Zealand service provider, was quick to clarify that this problem won’t exist with their set top boxes.

Clearing new frequencies isn’t just a problem for Cable TV

This particular issue isn’t new and is not unique to Cable TV interference. Spectrum reassignment often identifies equipment that is affected by new use of the same frequencies, whether indoors or outdoors. The cost of upgrading and protecting the equipment is often born by the new users – sometimes in addition to payment of expensive spectrum licence fees.

What’s the solution?

I can think of three quick solutions to the Cable TV problem

1)    Buy a large TV. Manufacturers will be happy about this – if you have to watch from a distance, then the larger the screen the better. Won’t work for those with small living rooms.

2)    Swapout the set top box for a model that has better interference characteristics. Costly for the Cable TV company, since there are no immediate additional benefits.

3)    Deploy a femtocell. The latest 3G femtocells can whizz along at 21Mbit/s, which will be comparable (or better than) the likely LTE indoor rates in busy networks. Could be integrated within the replacement set top box for a complete cure.

More generally, the lesson of targetting LTE for outdoor/non-residential purposes seems to me to make more sense. Deploying small cells, whether 3G or 4G, which operate at much lower power levels and range mean the chances of interference are much reduced. This frees up much more capacity for outdoor cellsites to reach users in places where they don’t have other choices.