When we first read the headline that LTE signals could interfere with cable TV signals, we openly ridiculed it as the ravings of some Technorati ignoramus. However once we learned that the UK regulator Ofcom was behind the study and that the interference was only on semi-protected set tops, not at the cable level, and that it was on a handset under a meter away from said set top, it started to make some sense.
For years the cellular industry has bemoaned the use of virtually every known spectrum and transmission technology in order to block them or control them (such as ultra wide band) and now the shoe is most firmly on the other foot.
The tests were carried out at Virgin Media in the UK, and were also on cable modems, and the gist of the problem is that when a handset uplink signal is using exactly the same frequency as the TV channel you are viewing (which it can if you build LTE in 800 MHz or lower), then the radio waves can get into the set top or cable modem and block the TV signal.
A company called Cobham Technical Services has produced a spectacularly detailed 83 page report on the subject that you can download from the Ofcom web site, and it’s clear that it is a potential problem. What seems to be the conclusion is that newer set tops and cable modem designs will have to have better shielding and where there are gaps in the metal surround for wires to get in and out, these cannot be in-filled with plastic fillers, or will at least need some kind of metal backing to block interference signals.
Cable TV usually uses up to 860 MHz of signals and has two ways of sending ‘ the conventional Hybrid Fiber Coax system allocates a 6MHz channel to each TV channel with some channels left over for VoD and Broadband. In a Switched Digital Video (SDV) version of the system where the set top sends a channel request to a distribution hub and then it picks one or more channels to use. If such a system is re-tuned, it may well pick a different channel from the one with LTE in, so switching off and on again might fix that problem.
Cable systems could be configured to miss the 20 MHz or so which is used by an LTE system, but LTE is such a flexible system that it could potentially use bits of bandwidth up and down the spectrum. On the other hand the handset might also take a peek at spectrum being used and try to avoid it. Detect and avoid systems are becoming better understood and more common place, but add a considerable cost to any system.
SDV systems might be built to sense the occurrence of interfering signals, but whatever the outcome, wherever LTE is built in the same spectrum of 860 MHz or under, this is going to be a problem and potentially add to costs. We can’t see that it’s fair that this cost is taken on by a cellco which has just paid $billions for exclusive use of this spectrum, and anyway it’s the cable TV system which suffers from the interference, not the other way around. So this looks like an inbuilt future set top cost and another sticking point between cable and cellular operators everywhere.
In the US this would mean companies like Comcast taking on more cost because of Cellcos like Verizon, and potentially companies like Verizon having occasional problems internally between its future LTE systems when they use it in 700 MHz, and its FiOS pay TV service.
This is an unpleasant hurdle just as the mass rollout of LTE is being prepared globally, and we would expect it to become an irritant passed on to the ingenuity of the set top and cable modem makers, along with any cost implications but with some rewards, in the way of increased business, for anyone that comes up with the best solution.