4G TV interference: Up to a million homes 'need filters'

[Ben]

An interesting development courtesy of BBC News: http://www.bbc.co.uk/news/technology-17125468

Basically, it looks like some people will need filters for their digital televisions to prevent interference when LTE starts hitting the analogue TV airwaves.

All in the name of progress I know. Still, give it a few more years and surely broadcast television will be falling by the wayside...

[Wilt]

Hmm, interesting. Although I don't understand how picture quality would be affected when tuned to a channel that exists. If the TV/Box is tuned to a MUX that is at a frequency which is different to the LTE broadcast, then surely it should have no effect.

Otherwise we would be seeing issues now with analogue broadcasts taking up these frequencies, if it is a case of pure interference. I would have thought the issue would be buggy software assuming that any digital broadcast within its frequency range is a TV MUX while scanning for channels and then gets confused when it tries to read the data.

[DBMandrake]

Hmm, interesting. Although I don't understand how picture quality would be affected when tuned to a channel that exists. If the TV/Box is tuned to a MUX that is at a frequency which is different to the LTE broadcast, then surely it should have no effect.

Otherwise we would be seeing issues now with analogue broadcasts taking up these frequencies, if it is a case of pure interference.

Unfortunately its not that simple in real receivers. Yes in theory two different services on adjacent but non overlapping frequencies should not interfere with each other.

However all practical receivers suffer from a number of limitations including "de-sensing", (loss of sensitivity due to strong signals on nearby frequencies) cross modulation (two or more different unrelated frequencies mixing together giving a resulting frequency which is on top of the frequency you're trying to receive)

If you have two different TV stations on nearby frequencies they are still going to be miles away from you and relatively low in signal level. But if one of your signals is a distant TV station station and the adjacent signal is an LTE device that's only a couple of metres from the TV it doesn't matter that it's no exactly on the same frequency - it will interfere. The only solution is adding a filter which filters out the frequencies used by the LTE device.

A similar issue is happening with a company called Lightspeed in the US who are trying to roll out LTE on frequencies that are adjacent to those used by GPS - practical GPS receivers will be significantly interfered with even though the frequencies are different, so there are attempts to block Lightspeed from using the frequencies they legally own on the grounds that they will just cause too much interference to critical GPS services:

http://freegeographytools.com/2011/how-the-fcc-plans-to-destroy-gps-a-simple-explanation