Note on receivers in the EU Radio Equipment Directive and ETSI standards

The European Union has focused new attention on radio receiver standards through terms in the Radio Equipment Directive (RED) which came into force last year. The RED’s requirements that any “receiver [must have] a level of performance that allows it to operate as intended and protects it against the risk of harmful interference, in particular from shared or adjacent channels” have been reflected in new ETSI standards.

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.)

Harm claim thresholds for satellite earth stations

I'm reasonably confident at this point about deriving harm claim thresholds for cellular neighbors and TV receivers (see e.g. the TPRC 2012 paper http://ssrn.com/abstract=2018080). Here's a first cut (a few months old, but I'm behind on blogging...) at thresholds for satellite earth stations.

800 MHz receiver criteria as harm claim thresholds

Bob Pavlak at the FCC prompted me to read the 800 MHz “unacceptable interference” rules in the context of an interference limits approach (root post).

The 2004 Report & Order (pdf) introduced the concept of unacceptable interference, “a term of art adopted for the limited purposes of this proceeding … that defines a bright-line test for interference protection that takes into account, among other factors, the strength of the desired signal and the characteristics of the receiver being employed” (Report & Order, footnote 8). While this sounds like a receiver performance requirement, I think it actually amounts to an interference limit or harm claim threshold. In fact, if it is reformulated as a harm claim threshold, it becomes more powerful because it does not enshrine a particular set of receiver performance parameters in the rules, leaving manufacturers and system operator with more flexibility.

Receiver Interference Tolerance: The Tent Analogy

A postcript to my testimony (see previous post) at the House sub-committee on Communications and Technology hearing on receivers: It sounded like Rep. Walden, who chairs the sub-committee, hoped my oral presentation would've mentioned the tent analogy I included in the written testimony. So since at least one person liked it, here it is:

Testimony: Harm Claim Thresholds

I was privileged to testify yesterday at the House Committee on Energy and Commerce’s sub-committee on Communications and Technology hearing on the topic “The Role of Receivers in a Spectrum Scarce World.”

My testimony (pdf; my oral testimony at time code 0:17:23 in the YouTube video on the hearing web page; the members' questions start at 0:27:00; some press here and here) tried to make four points:

1. We need to improve the ability of radio systems in one frequency band to tolerate reasonable signals in adjacent bands. 
2. Receiving system operators must bear some of the responsibility, but need to know what those responsibilities are.
3. Regulators can bring receiving systems into the mix by setting harm claim thresholds (aka interference limits or receiver protection limits), i.e. the interference levels that a service needs to tolerate without being able to bring a harmful interference claim.
4. Congress can play a role by keeping up the pressure, allowing the FCC to move ahead, and funding FCC technical investigations.

Receiver regulation: Why no progress?

(Written with Madelaine Maior, Silicon Flatirons research fellow)

There’s an emerging consensus that the role receivers play in interference should be recognized in wireless regulation. But why has it taken so long to come to a conclusion about the performance of receivers?

PCAST Report endorses receiver interference limits

The President's Council of Advisors on Science and Technology (PCAST) released its report Realizing the Full Potential of Government-Held Spectrum to Spur Economic Growth today (PDF, Administration’s blog post, webcast).

The main thrust of the report is the need for a shift from clearing and reallocating federal spectrum to dynamic sharing. As part of implementation, the report recommends that interference limits are used to include receiver considerations in spectrum management. (I was an advisor to the PCAST committee that wrote this report.)

Recommendation 3.1: The Secretary of Commerce working through the National Telecommunications and Information Administration (NTIA), in cooperation with the Federal Communications Commission (FCC), should establish methodologies for spectrum management that consider both transmitter and receiver characteristics to enable flexible sharing of spectrum. To safeguard primary Federal users, FCC should require that future non-Federal devices will be permitted to share government spectrum as Secondary Access users only if they are certified to operate within the stated interference limits for the band of interest. Initial specification of protection should be reviewed such that they safeguard new FCC assignments against harmful interference while grandfathering in existing devices and operations.

The report recommends that “[i]n order to facilitate more intensive and efficient sharing among Federal users, the NTIA should set and publish receiver interference limits using a transparent process for government assignments” It also recommends that “in the immediate timeframe, the FCC should begin the Notice and Comment cycle on implementing receiver interference limits as part of license terms for new allocations, updating old licenses to include receiver interference limits, and ex ante enforcement mechanism for non-Federal devices sharing with Federal users.” (Section 7.3, p. 77-78)

A detailed discussion of interference limits is given in Appendix D (p. 107 ff.).

From Kwerel & Williams to Interference Limits

Evan Kwerel & John Williams have proposed that future allocations should self-protect against projected adjacent band interference by assuming that they will receive only the “protections provided between flexible use bands” (Kwerel & Williams 2011, references at the end). The slide deck in Kwerel & Williams (2012) provides more detail: when a new allocation is being established next to a band likely to be repurposed for flexible use, the new allocation must (1) protect existing systems and future flexible use systems in that adjacent band, and (2) self-protect against interference from those systems, where flexible use systems is defined as “a dense deployment of base, mobile and fixed transmitters operating at fully functional power levels typical of a modern wireless cellular architecture.”

 Requirement (2) bears on the receivers of the new allocation. It resembles a qualitative interference limit based on the resulting energy from a “modern wireless cellular architecture.” A key selling point of this approach is that it doesn’t go beyond familiar parameters already used in regulation, like transmitter EIRP, compared to over interference limits that introduce probability distributions of resulting signal strength.

Thinking about a cellular deployment in the adjacent band is a very useful starting point. However, I do not believe it is precise enough to be useful in regulation, and particularly in enforcement. If one removes the studied ambiguity of the Kwerel & Williams proposal, the apparent familiarity and resemblance to existing rules evaporates, and one ends up with interference limits.

Vendor representations: a solution to the decoupled receiver problem

Requiring receiver vendors to represent to buyers that their equipment is fit for purpose is a way to avoid cheap receivers from reducing the performance of coexisting systems in the “decoupled receiver” case, i.e. when there isn't a license holder to negotiate with.

Protection Limits are not "Interference Temperature Redux"

My post Receiver Protection Limits may have left the impression that reception protection limits are similar to the dreaded and ill-fated interference temperature notion introduced in 2002 by the FCC’s Spectrum Policy Task Force.

Receiver protections are part of the "Three Ps" approach (Probabilistic reception Protections and transmission Permissions - see e.g. the earlier post How I Learned to Stop Worrying and Love Interference, or the full paper on SSRN). While both the Three P and Interference Temperatur approaches share a desire to “shift the current method for assessing interference which is based on transmitter operations, to an approach that is based on the actual radiofrequency (RF) environment,” to quote from the first paragraph of the Interference Temperature NOI and NPRM (ET Docket No. 03-237), the Three Ps approach differs from Interference Temperature in four important ways:

1. The Three Ps focus on solving out-of-band, cross-channel interference, whereas Interference Temperature is concerned with in-band, co-channel operation

2. The Three Ps are used to define new operating rights, whereas Interference Temperature tried to open up opportunities for additional operations in frequencies allocated to existing licensees

3. The Three Ps do not grant second party rights, whereas Interference Temperature permits second party operation.

4. Three Ps rights are probabilistic, whereas Interference Temperature definitions are deterministic.

Receiver protection limits: Two Analogies

I argued in Receiver protection limits that there are better ways to manage poor receivers causing cross-channel interference problems than specifying receiver standards. Here are two analogies to sharpen one’s intuition for the most appropriate way to handle such situations.

Cities increase the salinity of rivers running through them, affecting downstream agriculture. However, the choices that farmers make determine the degree of harm; some crops are much more salt-tolerant than others. In order to ensure that farms bear their part of the burden, regulators have a choice: they can either regulate which crops may be grown downstream, or they can specify a ceiling on the salinity of the water leaving the city limits, leaving it up to farmers to decide whether to plant salt-tolerant crops, perform desalination, or move their business elsewhere. Limits on salinity protection are a less interventionist solution, and don’t require regulators to have a deep understanding of the interaction between salinity, crops and local geography.

Sound pollution is another analogy to radio operation. Let’s imagine that the state has an interest in the noise levels inside houses near a freeway. It can either provide detailed regulations prescribing building set-backs and comprehensive specifications on how houses should be sound-proofed, or it could ensure that the noise level at the freeway-residential boundary won’t exceed a certain limit, leaving it up to home-owners to decide where and how to build. Again, noise ceilings are a simple and generic regulatory approach that does not limit the freedom of citizens to live as they choose, and that does not require the regulator to keep pace with ever-evolving technologies to sound-proof buildings.

Receiver protection limits: a better way to manage interference than receiver standards

Radio interference cannot simply be blamed on a transmitter; a service can also break down because a receiver should be able to, but does not, reject a signal transmitted on an adjacent channel.

More on this topic in subsequent posts:

Receiver protection limits: Two Analogies (June 2011)
Protection Limits are not "Interference Temperature Redux" (June 2011)
The LightSquared Mess Shouldn’t Count Against Coase (June 2011)
Licensing radio receivers as a way to facilitate negotiation about interference (August 2011)
Incremental management of reception: When protection limits are not sufficient (February 2012)
Four Concerns about Interference Limits (May 2012)
Transmitter versus receiver specifications: measuring loudness versus understanding (July 2012)
Testimony: Harm Claim Thresholds (November 2012)
Receiver Interference Tolerance: The Tent Analogy (November 2012)

I have also written a two-page summary document, see http://sdrv.ms/ReceiverLimits.

The LightSquared vs. GPS bun fight is a good example of this “two to tango” situation. GPS receivers – some more so than others – are designed to receive energy way outside the allocated GPS bands which means that operation in the adjacent band due to a new service like LightSquared can cause satellite location services to fail. Without the LightSquared transmissions, there wouldn’t be a problem; but likewise, if GPS receivers were designed with the appropriate filters, they could reject the adjacent LightSquared transmissions while continuing to receive the satellite location signal and function normally. [1]

While the responsibility for interference is, in theory, shared between transmitters and receivers, radio regulation has traditionally placed the onus on a new transmitter to fix any problems that may arise. [2] As I will argue, receiver standards are an impractical response; limits on reception protection, formulated in terms of the RF environment rather than equipment performance, are preferable.