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.

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.”