Resulting field strength rules: New reasons for an old idea

Current transmit power limits don’t provide sufficient constraints on interference, particularly when applied to modern systems (such as in the millimeter-wave bands) that deliver signal levels that change dramatically and rapidly from moment to moment, and place to place. I believe that limits on resulting field strength, rather than transmitted power, will be necessary in new allocations, particularly in the millimeter-wave bands.

Managing next-generation satellite interference

Big-time spectrum sharing is coming to space operations. There may soon be thousands of satellites, from half a dozen or more operators, operating in the same bands, at the same time, flashing in and out of interfering alignments as they crisscross the sky. These planned non-GSO (non-geostationary orbit) deployments bring unprecedented complexity to the space business.

Is harmful radio interference decreasing?

Spectrum analysts (including me) often proclaim that harmful interference is a growing problem, or at the very least a growing risk. That sounds plausible, given the growing profusion of radios, packed more and more densely together. But what if the opposite is true?

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.

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).

TV/cellular guard bands - second thoughts

In a recent post ("The FCC's TV/cellular guard bands don't compute") I wondered whether the guard bands between TV and cellular service that the FCC proposed in its Incentive Auction NPRM (pdf) had been designed to make room for more unlicensed in the TV bands. Having spoken to some experts, I’ve concluded that I was probably wrong about that: my new best guess is that they’re a benefit for the cellular industry.

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.

The FCC's TV/cellular guard bands don't compute

The FCC incentive auction NPRM [1] proposes 6 MHz guard bands between cellular and TV services (actually 6-11 MHz, depending on how the auction works out). The number is arbitrary, and could well have been chosen on political grounds to make room for more unlicensed in the TV bands.

The impact on interference from cellular systems into TV receivers is much the same whether the guard band is 1 or 20 MHz: most receivers will be unaffected, and for the small but significant number that suffer harm (0.5-5%?), only receiver filters will really help. The real question is: who's responsible for buying and installing those filters - the consumer or the cellular companies?

TV whitespace vs. cellular power limit anomalies

In the previous post, I considered interference between cellular base stations and TV receivers. What about interference between cellular handsets and TV? Considering this case highlights striking contradictions between the low power allowed for TV whitespace devices and the high power the FCC proposes for cellular operation: 20 dBm for whitespace personal devices but 37 dBm for cellphones, in both cases with a 6MHz guard band.

Post-auction cellular interference into TVs?

How many TV receivers will be affected by interference from cellular services as a result of the FCC’s "incentive auction" plan?  The FCC’s proposal doesn’t venture an answer; I don't think it even asks the question. Ofcom’s technical analysis in the UK’s rearrangement of the TV bands to accommodate more cellular service suggests that the number will be small, but not negligible. Ofcom therefore decided to require cellular operators to install filters on TV sets where there is a problem; the FCC has not raised this possibility.

In summary, the UK modeling suggests that TV reception will be affected in about 5% of  homes if there's a 11 MHz guard band between TV and cellular channels; the FCC's proposed guard band will be 6 - 11 MHz, depending on auction outcomes. By far the most effective way to mitigate this interference is by installing a TV receiver filter in affected homes.

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.

Transmitter versus receiver specifications: measuring loudness versus determining understanding

In arguing that regulators should attend to receivers as well as transmitters, I’ve may have mistakenly left the impression that they’re symmetrical, e.g. that one is a reflection of the other. For example, in “Four Concerns about Interference Limits” I observed that a communication system can be engineered to operate successfully either by improving receivers, or by delivering more transmitted power.

However, this framing is potentially misleading. It could be taken to mean that transmission and reception are two sides of a coin, that they are similar in kind, but that one is the reflection or complement of the other. They’re actually more like the outside (transmission) and inside (reception) of a black box - a black box like the human head, say.

Interference Limits for AWS-4

I have filed a comment (PDF) on the FCC's proposed AWS-4 rulemaking.

I argued that receiver performance affects the allocation of spectrum and opportunities for the creation of new services; that receiver management does not necessarily mean imposing receiver standards; that the Commission should define interference limits, an explicit statement of the radio interference level that a licensee’s system needs to tolerate, for AWS-4 licenses; but that receiver performance standards are not needed in this allocation.

Is 2.4GHz Wi-Fi the next GPS/LightSquared?

No, unlicensed devices in the 2.4 GHz band (2400 – 2483.5 MHz, operation under Part 15.247) probably won’t be the next GPS/LightSquared, where a large installed base of unlicensed devices with significant susceptibility to out-of-band interference was threatened by the deployment of a cellular service in an adjacent band. However, some similar characteristics raise concerns: tens of millions of devices, poor adjacent channel rejection, and a quiet band next door. What would happen if there were a large cellular deployment next door to 2.4 GHz?

There are significant differences to temper concerns: Wi-Fi devices don’t depend on such exquisitely low signal levels as GPS receivers; we’re not talking about safety of life applications; the RF front-ends of Wi-Fi devices are not open many tens of MHz away from the allocated band; and there already is some cellular operation nearby, at least in the US (Clearwire/Sprint’s 4G service in the 2.5 GHz band).

Still, as I’ll argue, the fact that interference has been observed between 4G service in 2.5 GHz and unlicensed devices in 2.4 GHz even with at least 10 MHz of guard band between them suggests that we’ll see interference problems to and/or from unlicensed devices if a cellular service were allocated in the fallow 2360 – 2400 band. That in turn suggests that it could make sense for the FCC to start encouraging or mandating better filtering for unlicensed devices over the 2.4 GHz band now, well before the 2.3 GHz band starts being populated with a potentially interfering service.

Update 4/23/2012: Monisha Ghosh kindly let me know that  2360-2400 MHz has been requested by healthcare device manufacturers (GE, Philips etc.) for Medical Body Area Networks on a secondary basis to Aeronautical Telemetry (OET proceeding 08-59). The June 2009 Notice of Proposed Rulemaking (PDF) provides good background information on the current uses of the 2360-2400 band.

The LightSquared Mess Shouldn’t Count Against Coase

It seems there’s a new meme floating around DC: I’ve been asked from both sides of the spectrum rights polemic whether the Lightsquared/GPS situation proves that Coasian make-spectrum-property advocates are crazy because the rights seem to be pretty well defined in this case, and yet the argument drags on at the FCC rather than being resolved through market deals. I suspect the source is Harold Feld’s blog My Insanely Long Field Guide to Lightsquared v. The GPS Guys where he says:

For a spectrum wonk such as myself, it simply does not get better than this. I also get one more real world example where I say to all the “property is the answer to everything” guys: “Ha! You think property is so hot? The rights are clearly defined here. Where’s your precious Coasian solution now, smart guys?”

The “Coasian” position does have its problems (see below), but this isn’t an example of one of them. I think Harold’s premise is incorrect: the rights are NOT well-defined. While LightSquared’s transmission rights are clear, GPS’s right to protection – or equivalently, LightSquared’s obligation to protect GPS receivers from its transmissions – is entirely unclear. There’s no objective, predictable definition of the protection that’s required, just a vague generalities, built into statute (see e.g. Mike Marcus’s Harmful Interference: The Definitional Challenge).

LightSquared’s transmission permissions are in some sense meaningless, since “avoiding harmful interference” will always trump whatever transmit right they have, and there’s no way to know in advance what will constitute harmful interference. I believe that’s a fundamental problem with almost all radio rights definitions to date, and why I’ve proposed the Three Ps.

The “Coasian” position’s real important problems are on view elsewhere:

(1) While negotiation between cellular operators to shift cell boundaries show that transactions can succeed in special cases, there is no evidence yet that transaction costs for disputes between different kinds of service will be low, and thus that negotiations will succeed in the general case. Even if one can ensure that rights are well defined, it may prove politically impossible to reduce the number of negotiating parties to manageable levels since radio licenses are a cheap way for the government to distribute largesse to interest groups. This is most obvious in the case of unlicensed operation, but many licensed services such as public safety and rural communications also result in a myriad of licensees.

(2) The FCC’s ability and proclivity to jump in and change operating rules (i.e. licensees rights) in the middle of the game makes regulatory lobbying more efficient than market negotiation. This may be unavoidable given law and precedent. There is no way for today’s Commission to bind tomorrow’s Commission to a path of action; legislation is the only way to do that, and even statute is subject to change.

(3) A significant chunk of radio services aren’t amenable to market forces since they’re operated by government agencies that can’t put a monetary value on their operations, and/or can’t take money in exchange for adjusted rights. Nobody is willing to quantify the cost of a slightly increased risk that an emergency responder won’t be able complete a call, or that a radar system won’t see a missile, even if those systems have a non-zero failure rate to begin with. And even if the Defense Department were willing to do a deal with a cellular company to enable cellular service somewhere, it can’t take the Cellco’s money; the dollars would flow to the Treasury, so there’s absolutely no incentive for the DoD (let alone the people who work for it) to come to some arrangement.

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.