Data over power lines causes RF worries

Current communications regulations might underestimate the radio interference from systems that push broadband data services over electrical power lines, a Commerce Department study has found.

The report, released this week by the National Telecommunications and Information Administration, did not recommend any changes in Federal Communications Commission regulations but suggested some techniques for minimizing the interference.

The emerging technology, known as e-broadband or broadband over power lines, transmits data at radio frequencies over electrical lines, using electrical outlets as data ports. In other words, users' existing indoor wiring does the networking.

NTIA conducted its study of interference following a 2003 request for information from the FCC.

The study focused on the possible impact of e-broadband on radios operating between 1.7 MHz and 80 MHz, the spectrum where federal agencies have been assigned more than 59,000 frequencies. NTIA used computer modeling to predict the amount of RF radiation from the broadband devices that inject the signal into the power lines as well as from the power lines themselves.

NTIA found that mobile land radios receiving moderate to strong signals could experience interference up to 30 meters from a broadband device or power lines carrying the broadband signal. Receivers on boats could receive interference up to 55 meters away, and base stations up to 230 meters. Those distances would increase if the radio signals were weaker.

Aircraft flying below 20,000 feet could also experience interference within 8 miles of a deployment area.

'NTIA recommends that the FCC not relax field strength limits for broadband over power line systems and that measurement procedures be refined and clarified,' the report concluded.

It made these recommendations to mitigate interference:

  • Service providers should use the minimum power level necessary to transmit data.

  • Providers should carefully select a frequency in each area and avoid locally used frequencies; Web access to radio license data could help carriers make such decisions.

  • Transmitting on the same frequency in different directions on parallel power lines could cancel out signals and reduce interference.

  • Filters and signal termination equipment should be used to cut off signals where not needed.

  • A limited number of broadband devices should be permitted within a geographic area.

  • Installation plans and equipment registration could help radio users identify and mitigate sources of interference.


About the Author

William Jackson is a Maryland-based freelance writer.

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