Data on interference trends are hard to come by. Anecdotes abound, and published information usually covers just narrow slivers of time, space and frequency. The FCC doesn’t publish comprehensive data on interference incidents, in spite of repeated calls to do so. (It does publish a non-comprehensive list of Amateur Radio Service enforcement actions, but at the time of writing it covers only 2009–2016. FWIW, the number of RFI incidents reported by amateurs is flat over this period.) When the FCC’s panel of technical advisors (the TAC) launched a formal inquiry in 2016 to find out whether there was an increase in background radio noise, many commenters pointed at data, but nobody offered a thorough analysis. The TAC didn’t come to any firm conclusions about whether radio noise was increasing or not.
The perception of increasing harmful interference is skewed by cognitive biases. There is certainly selection bias: for example, the people who file comments about a rise in radio noise are predominantly the ones that believe there’s a rise in radio noise. Since we tend to over-estimate the likelihood of events which are recent, unusual or emotionally charged, availability bias also plays a role: striking stories about exceptional interference sway our judgment. And last but not least, there’s confirmation bias: once we’ve taken a position – that harmful interference is getting worse, for example – we’re likely to search out, focus on, remember and interpret data in a way that confirms our preconceptions.
The obvious null hypothesis is that the incidence of harmful interference isn’t changing in either direction. However, experience with other technologies suggests that the most plausible hypothesis is that it’s probably declining – in spite of lamentations to the contrary.
For example, consider car accidents in the United States (Wikipedia). The US population more than doubled from 127 million in 1935 to 321 million in 2015, and the number of vehicle miles traveled (VMT) increased more than tenfold from 229 billion to 3,095 billion per year. However, the total number of road deaths stayed flat (34,494 in 1935 and 35,485 in 2015), and the number of fatalities per mile driven decreased tenfold from 15.09 to 1.15 per 100 million VMT.
There are many reasons for this. Improved vehicle safety, i.e. technology like crumple zones and anti-lock brakes, played a major role in reducing fatal collisions – that is, in reducing “harmful interference” between vehicles.
Wireless technology has also improved dramatically over the last few decades, and those improvements have made radios less vulnerable to interference. Examples include transmitters leaking less interference outside their transmit bands, receivers that are more resistant to interference, and more-directional antennas that only accept signals from desired directions. Hence, it’s reasonable to expect that there is less harmful interference today than there was in the past.
Regulation – such as requiring seat belts, and better enforcement of drunk-driving laws – also played a role in reducing automobile fatalities. By comparison, there have been few new regulatory mandates to improve interference-rejection by radios. (The EU Radio Equipment Directive’s receiver requirements are the exception that proves the rule.) Limits on transmitter leakage have hardly changed, and there are effectively no requirements on receiver performance. I don’t believe government-mandated receiver standards are the solution, though industry standards encouraged by government, such the new ETSI receiver standards, may improve matters. I prefer technology neutral ways to incentivize receiver performance, like harm claim thresholds; one automotive comparison is the U.S. Corporate Average Fuel Economy (CAFE) standards, where the government sets targets for average fleet fuel economy without telling manufacturers how to meet them.
So what if harmful interference is going down?
First, we might worry less about predictions of spectrum crisis. Technology improvements seem to have allowed enormous radio densification without coexistence crises. Regulators could probably be more aggressive about cheek-by-jowl sharing arrangements, and more skeptical about incumbents’ claims that new arrangements will cause catastrophe.
Second, the success of the wireless community in avoiding a spectrum meltdown without stringent harm-avoidance rules and enforcement on both device performance and user behavior – such as in the automotive sector – suggests that light-touch regulation may be sufficient for wireless. (Whether this is indeed so, and why, is an interesting research problem.)
Third, even if the radio noise floor is going up, the risk of harmful interference may be constant or going down because systems are more interference tolerant. The glass-half-full implication is that we needn’t worry about the noise floor. The glass-half-empty interpretation is that our technologies are hiding a growing problem that may suddenly emerge with irreversible force, like market crashes, the eutrophication of lakes, and other tipping points in complex systems.
What's "interference"?
Some readers might be wondering: if the number of transmitters is going up, doesn’t interference necessarily increase? No – not if one uses the regulatory definition of interference as the effect of unwanted energy, rather than the engineering notion of interference as unwanted energy per se.
Here are the regulatory definitions of interference given in the FCC rules, 47 CFR 2.1, which echo the ITU Radio Regulations (RR) and the annex to the ITU Constitution (CS):
Interference. The effect of unwanted energy due to one or a combination of emissions, radiations, or inductions upon reception in a radiocommunication system, manifested by any performance degradation, misinterpretation, or loss of information which could be extracted in the absence of such unwanted energy. (RR)
Harmful Interference. Interference which endangers the functioning of a radionavigation service or of other safety services or seriously degrades, obstructs, or repeatedly interrupts a radiocommunication service operating in accordance with [the ITU] Radio Regulations. (CS)
Other technology precedents
One can multiply examples of other technologies where rates of harm have declined even as usage has increased. Take aircraft safety, as reported in The Economist:
For air pollution, this image from a recent statistical release by by UK Department for Environment, Food & Rural Affairs is instructive:
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