Monday, March 19, 2012

Interference protection for unlicensed devices

It’s generally accepted that an unlicensed device must accept interference from any source, and may not cause harmful interference to any licensed service; for example, Weiser and Hatfield explain that Part 15 “safeguards only the rights of licensed spectrum users and provides no protection to commons access users”  in their 2005 paper Policing the Spectrum Commons.

I don’t read it that way. Unlicensed devices shouldn’t cause harmful interference to other unlicensed devices, either.

Part 15.5 (b) states:
Operation of an intentional, unintentional, or incidental radiator is subject to the conditions that no harmful interference is caused and interference must be accepted that may be caused by the operation of an authorized radio station, by another intentional or unintentional radiator, by industrial, scientific and medical (ISM) equipment, or by an incidental radiator.
It’s not clear that the injunction against causing harmful interference is limited to licensees-as-victims. There’s no doubt that Part 15 devices may not cause interference to licensees; the question is whether the rules extend that protection to other Part 15 devices.

The rule stipulates that “no harmful interference is caused and interference must be accepted that may be caused by [a long list of possible interferers].”  There is no limitation that the “no harmful interference” condition only covers licensed interferers. In other words, the rule requires that “no harmful interference is caused [to anyone, including other Part 15 operators] and interference must be accepted [from anyone].”

CSMA protocols such as LBT in 802.11 are designed to prevent one unlicensed device interfering with another (in some cases), so one could reasonably say that some engineers are already attempting to implement such a rule. Not all of them, though; as I understand it, Bluetooth frequency hopping is designed to avoid suffering interference, not avoid causing it.

Part 15.5 (b) therefore gives the FCC the authority to require that unlicensed devices do not interfere with other unlicensed devices.

And as Mitch Lazarus recently related in FCC Seeks Comment on Interference into Unlicensed Devices, the FCC has sought comments on a requirement that licensed Location and Monitoring Service (LMS) devices at 902-928 MHz should not cause “unacceptable levels of interference” to unlicensed devices in that band!

A sufficiently large and valuable installed base of unlicensed devices therefore seems to earn them a degree of protection from interference – even from licensed devices.

However, the question arises: how could one assign, and how could operators enforce, such a right of non-interference to unlicensed services?

The ambiguity of the current definition of harmful interference is a long-standing problem. I think receiver protection limits (aka interference limits; I’m still groping for the best term) offer a solution; see e.g. the thread of posts starting here, or the two page summary at The FCC could require that the aggregate field strength delivered co-channel and adjacent to an operating unlicensed device may not exceed some limit, with some statistical confidence.

As for enforcement, the information and enforcement burden of individual unlicensed device owners trying to prevent interference from other individual unlicensed devices seems very high. In the USA, class action lawsuits are always a possibility, but that seems a stretch. However, WISPs and operators of campus Wi-Fi systems would have the means and motive to seek protection. Receiver limits gives them a basis for establishing whether they have a basis for a claim, and finding the counter-party may be relatively simple when there are only a couple of WISPs in play; see e.g. Matt Montgomery’s discussion of the Neptuno interference cases in the draft Petition for Notice of Inquiry (PDF; note, though, that the FCC declined to referee this argument).

And in any case, since there’s no argument that unlicensed devices must accept interference, do they even have standing to complain? This gets into questions of rights and duties, and IANAL – though I suspect a Hohfeldian analysis of rights and duties might be one place to start. The FCC’s proposed limitation on LMS systems mentioned above suggests that unlicensed devices, at least in some cases, may have a right to non-interference, and licensed systems a corresponding duty not to cause interference.

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