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

1. The regulator’s ability to adequately define the current stable environment, much less a dynamic future state environment

An interference limit is not an attempt by the regulator to define or describe the interference environment, now or in the future, but rather a criterion for making a harmful interference claim; it’s an attempt to make operating rights more objective so that interference disputes can be resolved more easily between operators, rather than always ending up in front of the regulator.

If there is already an incumbent in an adjacent band over which the interference limit is to be defined, the limit will an upper bound to the environment that neighbor creates – most likely a very generous one for reasons of political and engineering prudence.

2. A designer’s ability to figure out what environmental limits mean from a receiver design perspective

Even though interference limits do not define the RF environment, they do give radio designers more guidance on the environment than they currently have, which is at most the allowed transmit power of allowed operation in neighboring bands, with no information on the density/deployment of those transmitters. The interference limit gives resulting field strength at the receiver, which is what really matters in terms of performance, not power-at-the-tower as in EIRP transmitter rules. Given this information, the designer can then do the trade-offs between minimum desired signal strength (sensitivity), out-of-band filter performance and carrier-to-noise ratios required to deliver the specified quality of service for their system as a whole, that is, the combination of transmitters and receivers.

In fact, interference limits allow designers to consider the overall system design; a communication system, say, can be engineered to operate successfully either by improving the adjacent channel rejection of receivers, or by increasing the strength of desired signal relative to the interference by reducing the distance between the receiver and transmitter (e.g. by increasing the number of transmitter locations) or through higher transmit power.

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

There is no requirement for the FCC to test all receivers. In cases where only receiver interference limits are given, assignments would not include any requirements on receiver performance, and no FCC equipment authorization would be required.

Where self-certification is a warranty-of-fitness, i.e. the vendor says the device works to its intended purpose given the interference limit, non-compliance with this warranty would be via a false advertising claim. Where self-certification requires that the manufacturer submits a testing protocol to the appropriate regulator, the vendor would specify the quality of service metric that would have to be met in the presence of given desired signal and out-of-band interference at the interference limit. Assuming that the FCC approved the proposed criteria and testing regime, devices would be authorized following approval by the relevant testing lab.

4. The regulator’s to field an economical approach to resolving issues when a receiver does not work and the manufacturer says the environment is the issue

The case may arise where a receiver does not work satisfactorily and the manufacturer/operator blames the interference environment; that is, they make a claim of harmful interference against neighboring transmitters in adjacent bands. The plaintiff would bear the burden of proving that interference exceeds their protection limit, and that this is due to the transmitters. This could be done via field measurement or propagation modeling.

Neither method provides absolute certainty: the number of possible field measurements is finite, and thus always merely a sample of reality as a whole; and the results of modeling depend on assumptions about propagation and terrain that cannot perfectly represent reality. However, perfection is not required (let alone feasible or desirable) in this or any other rights enforcement regime, just a well-defined arbitration mechanism that leads to a tolerably prompt, fair and certain outcome. It is advisable that the regulator specify the arbitration mechanism up-front for a particular allocation, e.g. a field testing protocol that specifies the spacing and time resolution of measurement locations, or a propagation model and a terrain data set.