Defining Harmful Interference Efficiently

Abstract

This paper describes the role of the current definition of harmful interference in spectrum policy and offers an alternative method for defining interference limits that would promote efficiency and fairness. Radio interference exists, and mechanisms for preventing or limiting it are necessary. Several recent papers suggest that the current mechanisms are overprotective and that modified mechanisms would allow spectrum use to deliver greater value. This paper begins with a review of the concept of harmful interference and its use to inform spectrum policy decisions and a review of the recent literature on harmful interference and related topics with the goal identifying and assessing key concepts and suggestions for improvement. The next section sets forth a framework for analyzing definitions of interference and mechanisms to avoid interference — identifying (1) goals, such as efficiency and fairness, that any definition of interference should serve; (2) constraints, such as the difficulty of identifying sources of interference, that may limit the applicability of many approaches; and (3) the tension between transmitter regulation and receiver performance. Important aspects of efficiency are avoiding uncertainty regarding future interference and avoiding overprotection — goals that conflict.The final section describes current practices that appear to be suboptimal and proposes a new way to define interference limits. The general case is illustrated by an example. Consider a future rulemaking in which the FCC allocates a band for wireless service. The FCC anticipates that licensees in the band will use LTE systems. As part of the rulemaking, the FCC would create a model of an LTE system deployed in the new band in a medium-sized city. (The wireless industry has developed several system planning tools that permit calculation of the coverage and capacity of hypothetical wireless system designs.) The FCC would use such a tool to estimate the capacity of the hypothetical system. When the FCC auctioned licenses in the band, it would accompany those licenses with a guarantee that future FCC rulemakings would not degrade the capacity of the model system by more than X%. The X% constraint would apply to the combined effect of all rulemakings; not to the effect of any single rulemaking. A reasonable value for X% might be 3% or 5%. Such a guarantee would reduce the uncertainty associated with the impact of future rulemakings on the value of the license. At the same time, it would leave the FCC with flexibility regarding future rulemakings involving adjacent bands, systems operating in aircraft or space, and underlay licenses. Moreover, it might speed up the rulemaking process by (1) reducing the incentives for a licensee to protest future rule changes that would create small increases in interference and (2) weakening the persuasiveness of such protests.Challenges for such a system include (1) properly modeling the effects of interfering systems, (2) predicting the market acceptance of systems authorized after future rulemakings, (3) incorporating new technologies for interference mitigation into the model, and (4) defining a mechanism for ensuring that the future FCC keeps the promise.