Regulating Receivers, a Regulatory Framework transition whose time has now come.

I want to begin by congratulating Federal Communications Commission (FCC) Chairwoman Jessica Rosenworcel and ?Commissioner Nathan Simington for their efforts to issue a Notice of Inquiry on receiver standards at the FCC’s April Open Commission Meeting.??In so doing, the FCC will be seeking to address one of the most critical inhibitors to the release of desperately needed spectrum, the life blood of our wireless world. ?That need not only includes helping to fill the insatiable demands that 5G and soon 6G cellular provide, but also supporting the wide variety of new and exciting wireless applications that are emerging from labs around the world.?Though there have been various FCC efforts in the past on this topic, none have ultimately been seriously pursued.?Given some of the recent spectrum conflicts, now is the time for concrete action.

For those unfamiliar with the issue, the FCC, and most telecommunications regulators around the world, have regulated the ever-increasing variety of transmitters for over a hundred years.?In the process they have established strict specifications on the power that can legally be radiated both in the assigned spectrum for an application, and beyond the assigned spectrum as well.?The authorized power levels are described by what is called a spectrum mask. ?As an example, the opening Figure (see above) illustrates the spectrum mask for the basic Wi-Fi signal which has a channel bandwidth (a single Wi-Fi channels spectrum) of 20 MHz, but there is an ever-diminishing signal that extends to 30 MHz from the center of the channel.

This mask is used as a fundamental specification for the transmitter design.?At the same time, while there are a few standards that specify the characteristics of receivers, to my knowledge, these specifications have not been regulated.?Since receivers can’t interfere with other signals, it has been the prevailing view that they need not be regulated.?Further, it has been argued that any form of regulation on receivers would inhibit innovation and/or impact the quality and cost of receiver designs.?This has been the status quo since the dawn of wireless systems well over a century ago.

So, what is the problem and why can’t we maintain the status quo for the next hundred years??The difficulty is that good engineers have found innovative ways to design receivers to have better signal reception at the lowest possible cost for the environment that currently exists, both in and around the spectrum band assigned for their application.?If the “neighborhood” around their assigned spectrum band is quiet, e.g., used by very low power satellites or for scientific applications (earth observation or radio telescopes), the designer will skip the step of filtering the input to the receiver which saves cost but also enables the receiver to look “outside” their assigned band with the hope of securing a little better reception ?In addition, the filter on the input has some loss, so skipping the filter not only saves cost, also slightly increases the received signal strength.?Even though this enable the receiver to sense signals outside the assigned band, this is perceived to be good engineering since it is less costly than designing the receiver with filters to ensure that the receiver only see energy in its assigned spectrum.

The problem with this approach often occurs years or even decades after the receiver is designed when the adjacent, near-by, or in some cases even distant spectrum, is re-allocated to an application with greater economic utility that is allowed to transmit at a higher power level than the previous application.?This is precisely the case with the high profile 5G – C-Band allocation when cellular systems replace satellites in the band.?Because of the design approach described in the preceding paragraph, older radar altimeters that did not anticipate anything but low power satellites in their spectrum neighborhood suddenly had a potential problem.?This situation is also largely the case for the contentious use of Ligado’s spectrum for cellular applications in the general vicinity of GPS.?

In both of these cases, ?receivers were designed for manufacturing cost and performance optimization.?The companies that sold these devices assumed that the neighborhood would not change during the useful lifetime of their product(s).?This was a good decision for decades in both cases.?The difficulty only came when, after a long process, the FCC re-allocated the neighborhood spectrum for a higher value use.?Since the receivers had been working well prior to the change and working well for a very long time for both GPS, and especially high-performance GPS receivers, and for old radar altimeters, ?the problem was framed as the FCC’s decision and/or the ??companies that were now trying to use the spectrum for a new purpose.?In either case, it was a problem caused by others and not the companies that designed and sold the receivers, or the companies that were currently using them.

What was not appreciated by ?receiver company management (which was in most cases not the management overseeing the original design of the receivers) was that ?the companies were taking a risk by optimizing receivers for the existing environment and not for the spectrum assigned by the FCC.?While the receiver designs were perfectly legal since the FCC doesn’t regulate receivers (nor do other regulators), they also weren’t protected for this out-of-band use by the FCC if a new and more valuable application was placed in their spectrum neighborhood.?Nonetheless, the argument has been strongly advanced that the incumbent use of the spectrum should be protected. ?Failing that, if they couldn’t be protected for whatever reason, the companies now owning and/or depending on the use of the devices should be compensated for the cost of replacing their receivers along with any related financial damages by either the new entrant companies or by the government itself.??

In both the GPS and the Altimeter cases, the incumbents have sought relief first from the FCC itself, requesting that they reverse their decision to authorize the use of nearby (or not so nearby) spectrum.?When this effort failed, the courts were involved and in parallel Congress was engaged including hearings on the FCC’s decision interestingly by the Senate and House Committees that are responsible for the GPS Community (Senate Armed Services Committee) and for the Aviation Industry (Aviation Subcommittee of the House Transportation and Infrastructure Committee) and not the Communications Oversight Subcommittees. Congressional Leaders provided their strong support for the incumbents in letters to the FCC, press briefings, speeches and op-eds.?In the case of the Ligado decision, the issue was even referred to the National Academies of Science, Engineering, and Medicine to make a determination on whether or not the FCC’s decision was appropriate from a technology perspective. ?????

While the GPS and altimeter receiver issues are the ones that are currently being debated, there are numerous historic examples of similar issues, and surely there will be many more in the future if some form of receiver regulation is not enacted.

So, what is to be done to resolve these challenges.?Interestingly, there have been significant advocates for receiver regulatory action for at least the last 40 or 50 years with perhaps the leader among them being Dale Hatfield, a legendary wireless figure and former senior official at both the FCC and the NTIA (National Telecommunication and Information Administration within the U.S. Dept of Commerce).?More recently Pierre de Vries, has provided considerable thought leadership on how to attack the problem from a technical perspective.?Both individuals are currently affiliated with the Silicon Flatirons which is in turn affiliated with the University of Colorado in Boulder.?Both were heavily involved with the FCC’s Technological Advisory Council’s work in 2012 on the receiver-transmitter issue creating a White Paper on the topic entitled “Interference Limits Policy.?The use of harm claim thresholds to improve the interference tolerance of wireless systems”.?In full disclosure, I was a co-author for this paper.?This work and its follow-on efforts identified the significance of the receiver problem and posed a potential approach to assigning responsibility for resolving harmful interference issues as they arise.?

While this policy approach has considerable merit, an even simpler approach would be to require companies to certify that their receiver products are designed and tested to ensure that they only operate in their assigned bands and that they are tolerant to a defined out-of-band transmission power level.?Though the definition of a “reasonable out-of-band signal” does require effort on the part of the FCC’s technical team, and there is an administrative burden associated with the certification of receivers, this would appear to be a very reasonable cost given the demonstrated value that spectrum possesses as seen through the recent FCC $81B C-Band auction, and the enormous benefit that the use of the spectrum has provided and has the potential to provide in the future.

Hopefully this short paper is helpful to the reader in understanding the impact that the lack of receiver regulations has had and will continue to have on the use of our increasingly scarce and valuable spectrum resource.?I look forward to comments and questions that readers may have about this critical topic.