Monday, August 31, 2015

FCC approves robotic lawn mower, rejects worst case analysis

On August 12, 2015 the FCC granted a waiver (pdf) of some Part 15 rules to allow iRobot to market a robotic lawn mower transmitting in the 6240-6740 MHz range (Order in Proceeding 15-30).

The National Radio Astronomy Observatory (NRAO) had expressed concern that the waiver could lead to interference to radio astronomy operations.

The Office of Engineering and Technology noted in its waiver grant that "because the NRAO analysis looked at line-of-sight separation distances, it has greatly overestimated the interference potential of transmitters that are located less than two feet above the ground."

It added, "We find that when taking into account the variability in propagation characteristics due to terrain, low antenna heights and other propagation factors, grant of this waiver is very unlikely to increase the potential for harmful interference."

The glass-half-full reading is that the FCC rejected a worst-case analysis; the glass-half-empty view is that it missed an opportunity to do a probabilistic risk analysis that quantified expressions like "greatly overestimated the interference potential" and "very unlikely to increase the potential for harmful interference."

The cynic's take is that this is to be expected; providing hard numbers would expose the Commission to having its reasoning questioned during subsequent litigation.

Worst case in interference analysis for medical interference

In its second order on reconsideration regarding the incentive auction released on June 19, 2015 (docket 12-268, pdf) the FCC noted that its analysis interference into wireless medical telemetry systems its work "is a worst case analysis and in most installations one or more of the parameters we assumed here will provide additional protection" (recon order at para 119).

Even this wasn't good enough for GE Healthcare, who filed a petition on July 28, 2015 asking the FCC to reconsider its reconsideration, saying "Due to the severe and wide-ranging negative consequences of interference to Channel 37 WMTS, the Commission's expressed intent to use a worst-case (i.e. minimum coupling loss) analysis in evaluating separation between Channel 37 WMTS and 600 MHz band mobile base stations is appropriate, but its adopted separation rules are not, in fact, based on a worst-case analysis, as the Commission appears to believe."

The trouble with worst case is that there is no worst case. That is: one can always imagine something worse. It’s not a sufficiently stable concept to be usable. This leads to oxymorons like the “realistic worst-case” GE HealthCare refers to in its petition. It leads to oxymorons like the “realistic worst-case” GE HealthCare refers to.

There’s even a term of art: RWCS, the Reasonable Worst Case Scenario, that even has a definition in the UK: a scenario "designed to exclude theoretically possible scenarios which have so little probability of occurring that planning for them would be likely to lead to disproportionate use of resources” (House of Commons Select Committee Report). (There's also the term "reasonably foreseeable worst case use scenarios" that's used in passing in IEC 60601.)

It’s related to the unbounded character of the maximum of a distribution.  It’s well known among statisticians, but apparently not that many spectrum folk, that the longer you sample a parameter with an unbounded distribution (e.g. a propagation path loss with a log-normal fading distribution), the larger the maximum you find will be.

Friday, July 31, 2015

Q&A: Risk-assessment, harm claim thresholds and adjudication

In my testimony before the Senate Commerce Committee on Wednesday July 29, 2015 I recommended three spectrum management reforms. A summary and links to the written testimony and video are in an earlier blog post. This post offers some Q&A.

The three reforms were: (1) moving away from worst case interference analysis and using risk-informed methods that consider not only the consequences but also the likelihood of harmful interference; (2) providing more clarity about operators’ interference rights and obligations by specifying harm claim thresholds; and (3) giving any spectrum user the option of taking action directly against any other, either in front of an FCC judge or in a federal Court of Spectrum Claims.

Wednesday, July 29, 2015

Senate Testimony: Risk-assessment, harm claim thresholds and adjudication

I testified today before the Senate Commerce Committee hearing on “Wireless Broadband and the Future of Spectrum Policy.” My written testimony is here; this is the summary I presented during the hearing. I’ve posted some Q&A in a subsequent post. My remarks are recorded in the archived webcast, starting at 58:02; see also a question from Chairman Thune and my reply starting at 2:05:43.

Saturday, June 13, 2015

Principles for interference assessment and receiver protection in FCC rulemakings

A key consideration in spectrum policy, particularly the allocation of new services, is the protection of existing services against harmful interference. However, regulators take an ad hoc approach to defining harmful interference. We would be better served if everyone knew, up-front, the principles a regulator was going to use when making decisions about a new allocation.

Julie Knapp’s comment at a recent FCC TAC meeting have inspired me to sketch out some principles/guidelines/framework for the FCC when looking at protecting services during rulemakings. Julie pointed out that the TAC Spectrum and Receivers Working Group now has an opportunity to synthesize and make actionable our work of the last few years.

(Update: The FCC TAC adopted "Basic Principles for Assessing Compatibility of New Spectrum Allocations" [pdf] at its meeting on December 9, 2015. I participated in drafting this document, and I'm delighted that it aligns well with the principles I outlined here.)

Thursday, February 12, 2015

Risk-informed interference assessment

I've spent the last year or so thinking about ways to complement worst-case interference assessment with a more comprehensive approach that considers many potential interference hazards, not just a possibly implausible nightmare scenario. I have concluded that quantitative risk analysis, used in many regulated industries, is a suitable tool.