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Power line tech could crash aircraft and shut down the Archers

Ofcom admits Anti-interference legislation needed

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An extensive study commissioned by Ofcom concludes that power line networking could crash aeroplanes and block Radio 4, but that technology will probably solve the problem before that happens.

Power-line networking has been increasing in popularity, partly because BT bundles it with Vision, but pushing networking signals over mains wiring creates a broadcasting circuit. Today that only annoys fans of short-wave radio and hams, but tomorrow it could block Radio 4 and stop aeroplanes landing unless legislation forces manufacturers to make a change.

According to the study (pdf - 145 pages, a little dry) the problem isn't just that power line networking (PLN) kit is getting more popular, but also that the frequency range used is expanding in order to drive up the data rate. That pushes the interference into the realm of FM radio and aeroplane navigation systems. This new threat prompted Ofcom to fund the study, carried out by PA Consulting.

Existing PLN equipment operates between 2MHz and 30MHz - right at the bottom of the dial. The only things down there are the amateurs, who've been complaining for a while now, and a few vertical users who don't operate near the homes where PLN is being deployed. But the next generation of kit will run from there right up to 300MHz, which includes the FM radio band (87.5-108MHz) and various safety and communication systems – whose owners will probably kick up more of a stink than the bearded hobbyists in the ham-radio community.

PLN equipment, unlike Wi-Fi, doesn't restrict itself to a single frequency but splatters its message across the whole wavelength. This gives great capacity at the cost of widespread interference.

Equipment that spreads that interference up to 300MHz is already on the UK shelves. This video demonstrates Belkin's Gigabit Powerline HD Starter Kit completely knocking out FM reception while files are transferred:

The report also mentions possible interference with ADSL provision. Right now ADSL slips under the range used by PLN, but ADSL 2+ runs up to 2.2MHz, so it could suffer interference - especially as the Belkin kit demonstrated above has been shown interfering right down to 0.1MHz. VDSL could also have problems, running as it does up to 12MHz, while VDSL2 stretches that to 30MHz - well within the PLN interference pattern. Of course, xDSL runs over phone lines, while PLN runs over electricity cables, so as long as they're not lying side by side then there shouldn't be any problem.

The manufacturers and PA Consulting reckon that technology will solve all these problems with a combination of smart-notching and power management, though PA Consulting concludes that legislation will be necessary to ensure that happens.

"Notching" is when the device keeps specific frequencies free, based on the requirements when it was built. "Smart notching" is where the same thing is done dynamically by detecting frequencies in use and avoiding them – the same detect-and-avoid technique that white space technologies have so comprehensively failed to do.

Smart power is supposed to knock down the transmission power when possible (based on the connection quality), and keep the idle transmissions (where PLN boxes ping each other) to a minimum.

But those both cost money and capacity. PA Consulting admits that Sony owns the patents on smart notching, so it won't come cheap. Consumer electronics are very price-sensitive. Punters won't pay more for the privilege of not interfering with their neighbour's Radio 4 reception or even to help the local airport get planes on the ground, which is why the report concludes that more regulation will be needed.

Ofcom likes technology to take away its problems, so it won't take the recommendation of more legislation well. But with the kit already generating interference Ofcom will need to move fast.

Perhaps the regulator is hoping that FM interference will shift more users onto DAB (which suffers less), though it will still have to make a few mandatory notches around the air-safety industry, otherwise it will have to face a lot more flak than a few radio geeks can manage. ®

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