Four Concerns about Interference Limits

Dennis Roberson has flagged some concerns about the interference limit approach. (The approach is summarized in http://sdrv.ms/ReceiverLimits; to trace the evolution of the idea, see the root post Receiver protection limits, and subsequent posts listed there).

His questions have helped me understand the importance of characterizing interference limits as a way to define harmful interference more precisely, rather than as a description of the radio interference environment. I probably over-played the interference environment angle in some earlier accounts in an attempt to distinguish interference limits from receiver performance standards using the slogan, “It’s not about the device, it’s about the environment.” That’s still true – the environment is indeed more important than the device – but using interference limits doesn’t require the regulator to measure and monitor the RF environment.

Dennis’s concerns relate to:

1. The regulator’s ability to adequately define the current stable environment, much less a dynamic future state environment
2. A designer’s ability to figure out what environmental limits mean from a receiver design perspective
3. The ability to test a device to ensure that it really works in the specified environment and to convey these test results to the FCC to demonstrate the adequacy of the design
4. The regulator’s ability to field an economical approach to resolving issues when a receiver does not work and the manufacturer says the environment is the issue

Markets for adjusting interference rights

One of the problems I tried to solve with the proposal “Stamps and Stewards: A third way to regulate radio operation” was negotiating adjustments to boundaries (e.g. power levels) between unlicensed bands and their neighboring bands given of the collective action challenges faced by unlicensed operators. There are other possibilities; this post explores using auction mechanisms.

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.

Drones, the RF noise floor and encouraging directional antennas

According to a story in the WSJ, Silicon Valley techies are building drones for $150 to $1,000 each, and some of them provide live video feeds. The story doesn’t specify the wireless channels that are being used, but I’d guess amateur bands and 2.4 GHz or 5 GHz unlicensed.

 

The impact of broadband unlicensed transmitters up to 400 feet (the current limit for amateur operation) in the air on the RF noise environment is likely to be significant once a significant number start flying. As Carl Martin mentions in the comments to the WSJ article, applications include real estate videos, movie shoots, police monitoring and local small parcel delivery (can you say pizza?). The commercial/professional impact will be even larger once the FAA develops rules for the licensing of commercial drones by 2015 as it has been required to do by Congress. Even now, the there is a surprising range of uses; according to data released by the FAA to the EFF following a Freedom of Information Act request, drones are operated not only by the Customs and Border Protection, DARPA and branches of the military, but also by universities, police departments, and small towns and counties across the United States.

If these live video feeds are transmitted from omni-directional antennas, the signal will cover huge footprint, potentially interfering with many other devices. I wonder whether more should be done to encourage the use of adaptive antennas by unlicensed devices so that transmitters only deliver their signal where it’s needed. As I understand it, EIRP transmit power limits effectively put a ceiling on power delivered in any direction; the rules don’t contemplate allowing transmitters to concentrate their power using antenna gain. However, there could be a benefit to allowing transmitters to deliver a higher amount of power in a small area; yes, the interference there will be greater, but fewer other players will be affected.

There are constraints, of course. The directionality is a function of the antenna array size as well as processing, and even a spot beam could be quite wide at a range of a few hundred meters. There’s also cost; how does one encourage drone wireless links to spend the extra money?

Coda on Privacy.  The EFF FOIA request to the FAA mentioned above seems to have been prompted by concerns about privacy and surveillance. The obvious application is video surveillance, but intercepts of wireless networks are of course possible, too. If Google Street View could amass a trove of passwords, Internet usage history and other highly sensitive personal data by driving around, imagine how easily such an operation could scale with an airborne platform. I wonder, though, if it doesn't get harder from a drone: how well could a receiver disentangle the myriad signals from all the base stations it could see? Not too hard, perhaps, since they're not all operating simultaneously.

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.

Receiver interference limits presented at FCC workshop

I made a presentation and participated in a panel at the FCC Workshop on Spectrum Efficiency and Receivers on March 12-13, 2012 (Public Notice PDF). My presentation is at http://sdrv.ms/AwoAZj. My presentation is on the day 2 video starting at time code 36:00 (there is also video for day 1); my opening comments on the subsequent panel are at 86:00, and various times subsequently.

A two page summary of the proposal is available at http://sdrv.ms/ReceiverLimits 

Placebo Legislation: Doing Good by Doing Nothing

I recently sat in on a conversation at the Silicon Flatirons Center among a group of cybersecurity experts about the need and nature of government regulation, given the vulnerability of critical infrastructure to cyber-attack, and indications that the market alone may not provide the desired degree of communal action needed for defense. I concluded that the answer was “placebo legislation”: Doing Good by Doing Nothing.

Apparently the U.S. federal government has recognized the need for action, and the Administration and leaders in Congress have proposed a number of legislative solutions. However, I ended up feeling that we’d be better off if the government pretty much did nothing given that:

A number of practitioners described how regulations too easily lead to a “compliance culture,” where statutory requirements incentivize companies to check the right boxes, rather than implement better security

White hat hackers explained that vulnerabilities and exploit strategies are evolving much, much more rapidly than any conceivable government framework

Cybersecurity is a complex and messy socio-technical system, and the unintended negative consequences of any regulation could easily outweigh the benefits

A leading computer scientist observed that the incentives for appropriate behavior were already about right – and that a process-oriented standards framework was guaranteed to drive the technical experts out of the room.

The government therefore needs to be seen to be doing something – but the less it does, the better. This is not unlike the placebo effect, where a patient given a dummy treatment experiences a true improvement in their condition – without harmful side effects.

Incremental management of reception: When protection limits are not sufficient

As the growing demand for wireless services squeezes radio operations ever closer together, we can no longer afford to ignore the costs that poor receivers and ambiguous interference standards impose on society. There’s a growing consensus that radio regulation needs to attend to reception issues as much as to transmission, which has led to a clamor for receiver standards.  As I’ve argued in the June 2011 post Receiver protection limits: a better way to manage interference than receiver standards, however, the best way to manage receivers is to specify the radio environment in which they have to operate (i.e. receiver protection limits) rather than government getting into the minutiae of setting performance requirements (i.e. receiver standards).

However, while protection limits are necessary, there may be cases where they’re not sufficient. In this post I outline a progression of increasingly interventionist steps in managing reception, starting with protection limits and adding more and more requirements until one reaches full-strength government-imposed receiver standards.

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.

From spectrum efficiency metrics to parameter spaces

In my post FCC white paper shows that “spectrum efficiency” is meaningless I argued that spectrum efficiency metrics are not very helpful.

They won’t go away, though, because engineers and economists instinctively characterize systems numerically. Both tribes strive to separate a problem into smaller independent parts, each described quantitatively; metrics are just a symptom.  The goal is to convert a complex mess into a problem amenable to objective analysis, yielding an incontrovertible answer. No more messy politics! 

Since politicians always look for cover behind engineers and economists, simplistic metrics will always be with us – not least in radio regulation.  Given that reality, I’m going to dig into spectrum metrics a little more. I conclude that it could be more productive to define a series of axes in a parameter space than a single metric.

Stamps and Stewards: A third way to regulate radio operation

Radio operation to date has largely been regulated in two ways. The dominant approach has been licensing station operators, whether they’re amateurs, TV broadcasters, or companies operating cellular systems. In the last twenty years or so, device licensing (aka unlicensed in the US, and license exemption in Europe) has also become widely used: if a device has been certified to meet regulatory requirements, anyone can operate a “station” using it without needing a license. [1] In these two approaches, the regulation controls either the system operator (for licensed), or the device manufacturer (for unlicensed).

I’m exploring another way, where the regulator accredits a limited number of “stampholders” who can each an issue an unlimited number of “stamps.” One can see these stampholders as the designated stewards of a "spectrum commons," and the stamps as the mechanism they use for controlling access to a common pool resource. A device may only be sold if it bears the requisite stamp or seal, in addition to any other statutory requirements such as Part 15 certification. Control is exercised at the point of sale through labeling or marks.

This notes builds on the previous posts Licensing radio receivers (Aug 2011) and Licensed Unlicensed (Sep 2011). I learned long ago that if I can think of something, someone’s already done it. However, I haven’t found good precedents yet, and I’m still looking for canonical examples or ringing metaphors. Stamps (in the sense of signet rings and seals) and Stewards is the best analogy I’ve found so far. [2]

Follow-up: In Markets for adjusting interference rights (May 2012) I explore another way of negotiating adjustments to boundaries (e.g. power levels) between unlicensed bands and their neighboring bands given of the collective action challenges faced by unlicensed operators.

Spectrum utilization and a Buddhist perspective on space

The “Spectrum as Space” metaphor implies that spectrum is a neutral container that can be filled with radio signals, leading to naïve notions of utilization such as empty and full spectrum bands. “Spectrum” is imagined a collection of axes which mark out an abstract space, such as frequency, geography, and time (e.g. Robert Matheson’s “electrospace” concept, cf. Matheson & Morris 2011, The Technical Basis for Spectrum Rights: Policies to Enhance Market Efficiency).

However, that’s not the only way to look at it. Non-spatial models such “Wireless as Trademark” work just as well (see my 2008 paper De-Situating Spectrum: Rethinking Radio Policy Using Non-Spatial Metaphors): by analogy, a trademark stands for both a part of the wireless resource (customarily, frequency band x geographic region x time slot), and signals. The wireless resource is all possible radio operations. In such an approach, one is much less likely to ignore the importance of receivers in this approach than spectrum-as-space, where only transmitters can “fill the space” with signals. Any radio operation includes the use of a receiver, and that receiver-transmitter pair influences what transmissions are possible by third parties.

Curiously, I found a relevant perspective on this problem in a book on ethics – Stephen Batchelor’s Living with the Devil. He writes:

One tends to think of space in terms of physical extension and location. A body “occupies” or “fills” a space. For there to be “no more space” means that nothing more can be fitted into a room or a vehicle or a document. Outer space is that virtually infinite expanse speckled with galaxies and stars separated by inconceivable distances. “Inner space” suggests a formless expanse of mind in which thoughts, mental images, memories, and fantasies rise and pass away. Space seems to be the relatively permanent place where temporal events happen.  

Buddhist philosophers see space differently. They define it as the “absence of resistance.” The space in a room is under stood as the absence of anything that would prevent one moving around in it. To cross from one side of the room to the other is possible because nothing gets in your way. Rather than being the place where things happen, space is the absence of what prevents things from happening. The space in the room is nothing in itself; it is just the absence of chairs or tables, glass walls or hidden tripwires that would obstruct movement within it. In encountering no such resistance, we are able to move about freely. [In the footnotes, Batchelor ascribes this approach to the Geluk school of Tibetan Buddhism.] 

The customary view that Batchelor outlines is “space as a set of dimensions” that informs the Spectrum as Space metaphor. One can transpose his summary to spectrum as “the relatively permanent place where [radio operations] happen.” The “Buddhist” view, on the other hand, would see spectrum as the absence of factors that would obstruct radio operations. Existing radio operations, including receivers, would provide resistance to new operations, even in quite distant frequency bands. And there is an interaction between the agent that wants to move about and the nature of obstructions: neither a mouse nor a monkey would have no trouble scurrying around in a restaurant, while a person would be obstructed by all the tables and chairs. Likewise, one has to first define the new operation one has in mind before deciding that spectrum is “occupied”; calculating utilization is not a straightforward matter of marking spectrum as “empty” or “full.”

FCC white paper shows that “spectrum efficiency” is meaningless

The FCC Technical Advisory Council’s (TAC) draft white paper on spectrum efficiency metrics (25 September 2011) is an excellent piece of work. It is authoritative, instructive, and demonstrates decisively that spectrum [1] efficiency metrics are a meaningless concept.

While they don’t say this in so many words, members of the Sharing Working Group perhaps intended this conclusion to be drawn; “spectrum efficiency” is a DC catchphrase that is hard to avoid, and it would probably be unwise to refute it overtly…

The following elements of the paper imply that the “spectrum efficiency” concept is useless:

1. There is no metric that can be applied across the myriad of different wireless services.
2. The metrics are incomplete, even within a service.
3. While the paper suggests metrics for specific services, the taxonomy of services is arbitrary.

Consequently

• There is no way to compare the “efficiency” of one radio service (aka one “spectrum use”) to another, denying politicians the pseudo-scientific rationale they dream of for converting a frequency band allocation from one use to another.

• Even within a given service type, there is no defensible way to rate one deployment’s performance over another; even if one scored much lower using the relevant efficiency metric, its defenders could invoke any of the long list of “additional efficiency considerations” to deny that the comparison was valid.

The paper also misses an opportunity: It hints at the importance of cost effectiveness rather than mere efficiency, but doesn’t address this broader context.

Follow-up posts:

• From spectrum efficiency metrics to parameter spaces (December 2011)
•  Three meanings of “spectrum efficiency” (October 2012)

The extent of FCC/NTIA frequency sharing

Take a guess: What percentage of US frequencies are controlled by the Federal government (represented by the NTIA), and what percentage is shared with non-Federal [1] users, who are under FCC jurisdiction? And what’s the remainder, devoted solely to non-Federal users?

My intuition, for what it's worth, was completely wrong. I thought the Fed/non-Fed split was roughly 50/50, with a bit (say 10%) being shared. As I pointed out in my recent post about partitioning Fed and non-Fed allocations, the amount of sharing should be easy enough to establish. It turns out that Peter Tenhula of Shared Spectrum Company has done a lot of work on this [2], and he pointed me to the FCC’s spectrum dashboard where one can download an XML snapshot of the allocation database (currently the API only covers the range 225-3700 MHz).

The answer? It depends on the frequency range and how one counts [3], but very roughly 10% is Federal, 40% shared, and 50% non-Federal (i.e. FCC) only.

Here's a picture (click it to enlarge); an Excel file with my analysis is here.

Update, 21 October 2011: Perhaps the flaw didn't lie with my intuition, but with my interpretation of the data. A senior FCC person has pointed out to me that many supposedly "shared" allocations are, to all intents and purposes, controlled by Federal agencies, and non-Federal (i.e. FCC-managed) services are present only on sufferance, if at all (e.g. 220-2290 MHz); or "sharing" only occurs both Federal and non-Federal entities used the same service (e.g. air traffic or maritime radar). So the question still stands, pending further digging: how much sharing (for various values of "sharing") is really going on?

Partition, not sharing: An alternative approach to the Fed/non-Fed spectrum divide

South Sudan. Serbia/Kosovo. India/Pakistan. Britney Spears and Kevin Federline. Sometimes a clean break is best for everyone, particularly when there are fundamental differences in mindset. Enforced coexistence is not, for many couples, the best way to live.

Sharing between Federal and non-Federal wireless users (aka Fed and non-Fed) is a favored way to realize the FCC’s dream of finding 500 MHz for commercial mobile broadband services; as reported in TheHill.com, “It is unclear where the 500 megahertz of spectrum will come from, but a large portion will likely come from government agencies that do not use the frequencies efficiently.”

Fed/non-Fed sharing can be made to work, and worthy efforts are being made. However, I doubt it’s worth the effort, given the insane difficulty of negotiating band re-allocations, let alone sharing agreements; questions over whether 500 MHz is, in fact, either needed or would make a dent on cellular companies’ problems; and fundamental concerns about jurisdiction (see my August 2011 post No Common Authority: Why spectrum sharing across the Fed/non-Fed boundary is a bad idea).

It would be a better use of time and effort to go in the opposite direction: make the partition between Federal and non-Federal as clean as possible, and let each group of figure out sharing among its own constituents.