The concept of “harmful interference” is central to FCC spectrum policy. The FCC has never said just what the term means. Oddly, though, that might be a good thing.

Nearly every band of the radio spectrum is shared among two or more categories of users. If we think of the spectrum as being spread out horizontally, the users of each band are stacked vertically. To see how this looks, click here.

Each band has a predetermined pecking order among its users: primary, secondary, and unlicensed. The relationships among all of these turn on harmful interference. Specifically:

  • “Primary” users are protected against harmful interference from all other users.
  • “Co-primary” users – services in the same band jointly designated as primary – may not cause harmful interference to each other.
  • "Secondary” users may not cause harmful interference to primary users, and must accept harmful interference from primary users.
  • Unlicensed users may not cause harmful interference to primary or secondary users, and must accept harmful interference from everybody.

The notion of harmful interference being key to the whole enterprise, we might expect to find a crisp and objective definition in the FCC rules. But when we look, we find something else. It comes in two parts:

In the case of a radio-navigation service (like GPS) or a safety service (police, fire, distress beacons, etc.), harmful interference is anything that “endangers” its functioning.

In the case of any other licensed service, harmful interference is whatever “seriously degrades, obstructs, or repeatedly interrupts” the service.

The two criteria are very different, giving safety services a lot more protection. But it is not always obvious which criterion should apply in a given case. Cell phone service, amateur radio, and satellite service all belong to the second, less-well protected group. Nowadays, though, a third of 911 calls come from cell phones. Should these receive the more stringent, “safety service” level of protection?   How should we treat amateur radio operators coordinating rescue efforts during a flood? Or a satellite service carrying an OnStar subscriber’s request for an ambulance? The FCC has never ruled on the level of protection for these kinds of communications. (And of course it may be difficult in practice to single out some calls for more protection than others.)

Other terms in the definitions are just as opaque. Does “endangering” a safety service necessarily entail an actual impact to the service, or is a mere threat of interference enough to qualify? What does it take to “degrade” or “obstruct,” or “repeatedly interrupt” a service? We don’t know.  Common sense suggests that an occasional minor blip would probably not qualify as harmful interference, while long-term blanketing interference probably would. But common sense is a risky guide to FCC rule interpretation. And anyway, most real-world interference occurs in the vast grey area between these extremes, where the FCC has provided almost no guidance.

Periodically one or another spectrum management body calls for the FCC to promulgate a more objective definition of harmful interference. But that would be ill-advised. We will do much better to continue with a vague formulation that leaves the FCC the flexibility it needs to reach the right result in case-by-case decisions.

Accommodating New Technologies

The question of harmful interference emerges in its sharpest form whenever the FCC tries to squeeze a new radio technology into the already-crowded spectrum. Today there is no empty spectrum below about 50 GHz (and higher frequencies are suitable only for limited purposes). That leaves only two ways to find room for most new technologies: have the newcomer share a band with incumbents, or clear a band by moving its current users somewhere else – where they will have to share with incumbents. Either way, someone has to move in to an already-occupied band.

The existing occupant nearly always objects. Some genuinely fear interference. Some reflexively protect their spectrum, even if the actual threat of interference is very low. Some, particularly associations, object in hopes of looking important to their members. And some, sad to say, object in order to impede new competition.

Typically the proponents of an incoming technology insist it will not cause harmful interference, while the incumbents claim that it will. In reality, both are usually wrong. Harmful interference is rarely a yes/no proposition; it tends instead to be a matter of probabilities. The newcomer might impact X % of an incumbent’s service Y % of the time. Those numbers are rarely either zero or 100%. The question then becomes: What values of X and Y indicate a level of harmful interference that should justify the FCC’s denying access to the newcomer?

In a better-run world, the FCC would disregard the oppositions of all except incumbents who are realistically threatened. In the world we live in, however, the FCC must consider all oppositions. Making its job more difficult is the near-universal tendency of both opponents and proponents of the new technology to overstate their claims. This costs both sides credibility, and ultimately damages everyone’s interests.

Incumbents often take positions along these lines:

  • My service is vital to the public interest and therefore is entitled to maximum protection.
  • I paid for my spectrum, so I don’t have to share it with anyone. (Wrong, as a legal matter.)
  • I can show incoming interference by stringing together worst-case possibilities; using implausible transmitter-receiver geometries; calculating with no other sources of radio noise (the “Jurassic assumption”); and burying unstated safety margins in my calculations.
  • Any small risk of any interference to my service is harmful interference.

Newcomers likewise have their own typical positions:

  • My proposed service is vital to public safety (or to  broadband competition, or rural broadband, etc.)
  • The high public interest in my service justifies making other people accept some interference.
  • I can show my application won’t cause interference by downplaying the likelihood of interfering geometries; overstating the effects of ambient radio noise; underestimating victims’ receiver characteristics; and overstating victims’ tolerance of brief interference.
  • My innovation must have the benefit of the doubt (citing 47 U.S.C. § 157, which indeed puts the burden of proof on an opponent of new technology).

The advantages of a new technology are often very real.  Public safety and homeland security are benefiting from new devices that help to protect both officers and the public.  Consumers are enjoying a host of go-anywhere communications and entertainment options.  The FCC’s assessment of harmful interference amounts to finding the right balance among the competing claims of the innovators and the incumbents.

Making the Call

When it comes down to making actual decisions, though, harmful interference is not just a matter of numbers, or even of probabilities. It depends heavily on context. These are some of the issues the FCC will properly take into account:

Public interest in the interfering service vs. the victim service. If an innovation promises to save the lives of first responders (for example), and the opposition is a hobby service, the threshold for harmful interference will be set very high. But the opposite is also true: a proposed video-game device in a band used for aeronautical communications would be held to very exacting standards.

Whether the incoming application can work in less critical spectrum. Some bands (such as the Wi-Fi bands) that are already congested with non-critical applications can accept more usage with little or no incremental harm. Others, like satellite uplink bands, are inherently tolerant of other signals, if their power is low. The proponent of a new technology that seeks to operate in sensitive frequencies (such as a satellite downlink band) may be asked why it cannot use a more robust band instead.

Whether the incoming application can be made less interfering.Can the new technology get by with lower power? Less bandwidth? Shorter transmission times? Is it amendable to frequency coordination (which helps individual users keep out of each other’s way)? Can it be set up to reliably detect and avoid victim devices? (TV “white space” devices and those for the expanded unlicensed U-NII band must have this capability.)

Whether the incumbent’s equipment is unduly sensitive to interference.An incumbent that is running a badly-designed system – one unable to tolerate reasonably expected levels of interference – will generally get less sympathy from the FCC, and may get less protection as well.

Whether the victim service can tolerate occasional interruptions.Interference that causes brief shut-downs – say, a tenth of second – may cause severe harm to a data system that needs several minutes to re-synchronize, but would barely be noticeable to an analog voice service.

Whether an interfering device can be turned off if needed.The FCC moves with the greatest caution when the new service entails large numbers of transmitters which, once shipped, are beyond the manufacturer’s control. It is more tolerant of units at known, fixed locations that can be disabled if they cause trouble. It may also accept a higher risk of interference from a system that can be remotely shut down in case of interference (as is effectively required for TV white space devices).

Given the wide range of valid considerations in play, no fixed definition of “harmful interference” can give the right answer in every instance. Any such definition would run a very high risk either of letting in new services that in fact are likely to disrupt existing applications, or of unnecessarily locking out useful technologies that could be accommodated without harm.

Besides, innovation is hard to predict. (If predictable, it wouldn’t be innovation.) Even if we could somehow formulate a definition that appropriately covers all of today’s cases, chances are that some new technology would soon raise questions that the definition cannot properly answer.

The present definition, vague though it is, allows the FCC to reach the right result in difficult cases. And nowadays, most of the cases are difficult.