What is “Harmful Interference” and Why is it Important?
What is “Harmful Interference” and Why is it Important?
What is “Harmful Interference” and Why is it Important?

    Get Involved Today

    “Harmful Interference”/HI is the test FCC uses to see if a new spectrum use is acceptable with repect to incumbent uses.  All radio systems impact other systems, or at least have since Marconi built his 2nd radio.  Thus it is important to limit this impact and FCC does so through the HI criterion. In effect, HI is also vital in determining whether spectrum can be reallocated.  As demand for spectrum to support mobile and fixed broadband increased, HI is a key factor in allocation decisions.  It will probably be crucial in the followon to the spectrum inventory in determining whether the vacant spectrum that is discovered can actually be used.

    HI is defined in Section 2.1 of the FCC Rules in a 30 word definition borrowed from the ITU Radio Regulations and probably written more than 50 years ago.  It is also incorporaterd as a criterion, without addional definition, in 5 different sections of Title III: 47 U.S.C. §§301(a)(1), 303(g), 309(j)(16)(C), 337, 922.

    HI determinations have been often been lengthly and expensive to participate in. For example, the Commission has been deliberating more than 4 years on whether TDD use of the AWS-3 band would cause HI to the lower adjacent 3G downlink band.  The Northpoint/MVDDS rulemaking was also drawn out for years over the same basic issue.

    On May 6,  IEEE-USA sent a letter to both FCC and NTIA urging them to work together in clarifying the present definition of HI. IEEE-USA is part of the IEEE, the world’s largest technical professional society with over 395,000 members in over 160 countries and advances the public good and promotes the careers and public policy interests of more than 210,000 engineers, scientists and allied professionals who are U.S. members of the IEEE.

    I have written previously about both the difficultly of defining HI and its importance after the Spectrum Inventory for identifying new spectrum that can be used without causing HI to incumbents. While an unambiguous definition of HI is impractical, any clarification and simplification for the process for making an HI determination will greatly speed up the availability of spectrum for new services and thus speed technical innovation.

    Some quotes from the IEEE-USA letter:

    Uncertainties like this discourage capital formation for new radio technologies. Such uncertainties also affect capital formation for incumbents in determining how much protection they are entitled to from later entrants


    ITU-R M.1635 states that “Parts of the assessment procedures need to be based on a statistical methodology, well known as the Monte Carlo technique.” The UK’s national spectrum manager, Ofcom, has also advocated and frequently used statistical technique in spectrum policy decisions.2 Statistical techniques have been used by both FCC and NTIA in authorizing point-to-point fixed links where weather-related propagation losses are time variable3, and by FCC in authorizing broadcast stations because the Grade B contours include a statistical factor.

    FCC and NTIA do not have a general policy on the use of statistical techniques in interference prediction even though many of the most contentious issues in recent years have focused on such issues. But no general guidance exists on how to extrapolate to cases involving other propagation phenomenon or mobile stations. Your agencies are in a position to begin to create such guidance.