Bill Pechey

There's been a lot of talk recently about a broadband access system called Power Line Communications (PLC). Trials are being carried out on both sides of the Atlantic, and the IEEE is working on standardising the technology.

With PLC, broadband data signals are injected into the local power distribution network and are carried along the cables to all properties connected to the same transformer. Anyone who wants the service just plugs a unit into a convenient power socket and, bingo, there's the broadband connection.

This all sounds like good news but let's take a closer look at the way it works. It's pretty difficult to transmit high-speed data signals over power cables because of the high levels of interference present, so engineers have had to work hard to devise systems that can run at broadband rates.

Acceptable performance has been achieved by transmitting at high power levels and by using large bandwidth with frequencies up to about 30MHz to carry the signals. Unfortunately power cables were not designed to carry frequencies much above 50Hz and they are not well balanced at high frequencies, which means that the signal leaks out of the cable as a radio transmission.

Tests have shown that these emissions can be quite intense and can cause serious problems for licensed users of the radio spectrum such as short wave broadcasters, aviation transmitters and radio amateurs. In fact a technical argument has been running for several years between the radio users and the proponents of PLC. In the latter camp is the European Commission, which thinks that PLC should be deployed quickly because it is another competitor to ADSL and that means better deals for broadband users.

The EC now seems fed up with the arguments and recently issued a recommendation to encourage trials to go ahead but with no control on the levels of interference. This decision has enraged the radio people.

The government seems to be backing PLC. One might expect the MoD or the Civil Aviation Authority, as heavy users of radio communications, to be against PLC but they have been strangely silent.

I think that the radio people have a point. It is a bad idea to wreck the performance of radio systems so that we can have yet another broadband system to compete with ADSL and wireless.

Users will soon want speeds higher than can be provided by PLC and I doubt that it makes economic sense to invest in a technology that can only have a few years of life. Nevertheless, the power people will roll out their systems and the radio people will continue to fight. Time, politics and the market will decide the result.

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