FAA Shows ‘Sample NOTAMs’ for Possible 5G Restrictions | Hacker News Reader

FAA Shows ‘Sample NOTAMs’ for Possible 5G Restrictions

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61 pointsStevvo114 comments

Can somebody explain what 5G is actually good for?

With 4G you get fast enough coverage at long distances for basically everything. Maps for sure, but even streaming video as long as not everyone on the tower is trying to do it at once.

With 5G you get more bandwidth, but you're not going to get it in the middle of the woods because it also requires more towers closer together. So it's for high population density areas. But those areas are the places where it's extremely cost effective to install fiber, and cellular will never ever be as fast as fiber, and installing a million towers everywhere is just about as expensive.

If 4G is about all you're going to get in the middle of the woods and fiber (with the last hundred meters over Wi-Fi for mobiles) is the thing you actually want in the city, why is anybody even wasting money installing 5G towers anywhere?

5G covers what 4G does as well IIRC. The really high frequency stuff is only what makes the news.

Also the advantage of 5G as far as I'm concerned is being able to connect things (possibly in a mesh?) At very high throughput without cables. If you have an oscilloscope and a signal generator, you should be able to synthesize a (say) network analyzer out of the two of them by linking them via 5G.

> 5G covers what 4G does as well IIRC. The really high frequency stuff is only what makes the news.

Right, right, the really high frequency stuff is what we're talking about. It's confusing that they stick so many things under the same label. The parts of "5G" that are just 4G with better radios and algorithms are useful but also not so contentious. But for that you wouldn't need more towers at closer intervals.

> Also the advantage of 5G as far as I'm concerned is being able to connect things (possibly in a mesh?)

And now we're talking about a third thing.

It could actually be quite useful to open up more unlicensed spectrum for things like this -- if nothing else so we could get better Wi-Fi to hook up to the fiber.

But that's a different thing than having Verizon install a cellular tower on the end of every street corner for high frequency cellular 5G and then not having them come the rest of the way with the fiber into your house, which still seems like some kind of a bad joke.

I think they aren’t wasting money installing towers. You’ll pay for those increased speeds as they build many towers.

Although I agree, what we will end up with is going to be broad area WiFi effectively. Seems like the goal is to replace fiber and then you can move around the city always connected at high speeds.

Comcast is already doing this, their routers will let other Comcast users connect over WiFi.

Personally, I believe that is the end objective. I personally am just getting satellite via starlink and probably will get fiber if it’s available.

5G is under 3GPP release 17 now [1] which includes many new features than 4G as release 8 [2]. 5G is much more than just changes in speed.

[1] https://www.3gpp.org/release-17

[2] https://www.3gpp.org/technologies/keywords-acronyms/98-lte

Not to mention vastly higher power consumption.

In an age of increased awareness around climate change, it's fairly mind-boggling that 5G is being pushed so hard, given the questionable value of the benefits it brings.

How does it increase power consumption?

Source: https://www.ericsson.com/495d5c/assets/local/about-ericsson/...

This is a brochure for Ericsson's lower power innovations, but look at the baseline without their system. Graph on page 2.

>In an age of increased awareness around climate change, it's fairly mind-boggling that 5G is being pushed so hard, given the questionable value of the benefits it brings.

How much % of global carbon emissions is due to cell equipment? Is it a big enough portion to worry about?

5G is kind of three things.

5G on existing frequencies can do more data with the same spectrum. Something something MIMO. This is broadly useful, if you have an 5G capable device and your network updated their base station equipment. 5G and 4G are supposed to be better at sharing spectrum on demand vs 4G and 2/3G.

5G on new, nearby frequency, in the US, there was a second round of taking historically TV spectrum and reallocating it for communications. These bands will most likely need new antennas on towers as well as on handsets. These tend to be lower frequencies, which could be useful for distance and structure penetration, but may not always be built out.

Then there's the millimeter wave stuff. Very high frequency, very fast, great for density, like maybe transit stations and arenas. This is the stuff that needs a tower on every light pole or whatever. I don't think this is going to get a lot of deployment, but it gets a lot of press. I think this is the part the FAA is objecting to for now.

It’ll be a boon for fixed wireless where last mile issues prevent running cable

> Then there's the millimeter wave stuff. [...] I think this is the part the FAA is objecting to for now.

mmWave 5G signals are in the 24-54 GHz range (f = c / 0.001 m ≈ 30 GHz)

This makes me nervous. The O2 peak at 60GHz. What are your opinions on it? If we get that peak of attenuation at 60GHz, what's happening to that energy?

The common wisdom would say it heats up but that's not guaranteed.

https://i.imgur.com/KUR4XmX.png

https://i.imgur.com/iUD2v4D.png

https://www.5gamericas.org/wp-content/uploads/2020/12/InDesi...

What else would happen to the energy? It’ll turn into heat + some re-emissions.

Not to feed the trolls but here is one example:

Tumor Treating Fields are an FDA-approved mechanism for treating certain cancers (mostly gliomas). The exact mechanism isn’t understood, but it seems that prolonged exposure to low power alternating electrical fields in the 100-500kHz region interferes with cell mitosis [1]. I haven’t read about it recently but at the time the speculation was that it was basically disrupting microtubule formation by physically agitating polar molecules involved with the process.

The application from Optune and others uses contact electrodes to drive a small potential gradient though the skull and into the brain.

We know radio frequency also affects polar molecules, mostly to heat up meals. Obviously at multi-GHz carrier frequencies we wouldn’t expect to see this behaivor emerge. However, most digital protocols don’t operate with continuous carriers..the break comms into frames and do things like frequency hopping at much lower periodicity than the carrier. If there was something dumping energy into the cytoplasm on a 2-10 microsecond interval, it could have similar effects.

Do i think 5g is going to cure brain cancer? Probably not, but the body is ridiculously complex and can be surprising. I think we just have to find a way to be able to talk about these concerns fruitfully rather than just try to squash them, because that’s generally where the trouble seems to start.

1 - https://pubmed.ncbi.nlm.nih.gov/33080774/

Thank you. Pretty much the answer I wanted to give, but much better than I could.

And I am not just trying to latch onto the first unknown that's brought up under my comment, these are very similar to the concerns that I had in mind and wanted to bring up.

At the wavelengths we would now be dealing with and exposing ourselves to, there may be - and with some evidence already pointing there probably being surprising and not immediately intuitive (mostly) biological interactions or disruptions to interactions our biology relies on that we are yet unaware or haven't understood leading to phenomenons we haven't considered such as the example you've given.

I tend to be a bit of a fence rider when it comes to these types of things.

Re: 5g we have so much accumulated knowledge and experience dealing with RF that the likelihood that we're going to be caught completely offguard and have no reasonable mitigation seems pretty low. At the same time, it's pretty obvious that our biology didn't have an opportunity to adapt to the types of energy we're putting into the environment and we'd be well served to keep an open mind about what the possible effects could be and how we would test and/or monitor for them.

There are other contemporary topics that suffer this same pathology. Reasonable questions raised in good faith get slapped down regularly and I think we pay a collective price for it. Of course some questions aren't reasonable and certainly not all are raised in good faith, so it's not exactly an easy problem to solve.

I keep wondering if any insects are going to discombobulated by thinking along those lines - but am a complete layman

Affected, absolutely. Unknown to what degree or if it would be any worse then the billions smashed on windshields or fogged out of existence every year.

Interesting paper on using EM simulations on finely detailed insect models to determine net energy absorption and associated properties: https://www.nature.com/articles/s41598-018-22271-3

Keep in mind that the EM field described in that study is quite strong and the transmitter is very close (actually in contact with the patient's skin). They talk about field intensities of 0.6V/cm. That means that if you put the two probes of a multimeter in the air separated by 1cm, it would read 0.6V. That's quite a lot. I'm not sure how to translate that to RF power, but it is many orders of magnitude more intense than what you could ever expect to receive from a mobile phone tower. Radio receivers are basically very fast and very sensitive voltmeters that work with field strengths on the order of microvolts per meter.

Totally agree there's extremely limited risk here from towers. The broader issue to me is that all of the conspiracy talk and bizarre science fiction around the topic of 5G (or vaccines or flat earth) has created a sort of conversational countermeasure where its acceptable to clap back and shut the topic down any time people appear to be wandering in those woods. As a result, some folks with legitimate questions and concerns end up frustrated and looking for answers elsewhere.

Endless patience for off the wall ideas isn't a sustainable option either, so I'm not claiming to have all the answers, I just wish we were maybe a step or two in that direction.

I asked for more details on the implication as the parent quote

> what's happening to that energy? The common wisdom would say it heats up but that's not guaranteed.

As the first and second laws of thermodynamics are well understood and tested theoretical principals that have been experimentally and theoretically verified for ~170 years, there is far more than "the common wisdom" backing the expectation that the system would heat up.

A common conversational tactic to lead the conversation into conspiracy territory is to cast unsubstantiated doubt on a settled topic - then frame the orthodox view as something pushed by an "establishment". A far more interesting discussion emerges when data is presented, or absent data a plausible mechanism for harm.

There is nothing wrong with studying the biological effects of EM fields. We certainly know that small quantities of Benzene, or Arsenic have large impacts on biological organisms - and that biological organisms react to certain portions of the EM spectrum in complex manners e.g. Visible light, and infrared. However we do have some good bounds on how much impact there can be and where it is likely to emerge based on ~120 years of radio spectrum deployments.

"The common wisdom would say it heats up but that's not guaranteed."

If it does something other than turn into heat the laws of thermodynamics are wrong which is somewhat unlikely.

Sure, 60 GHz may be a resonant frequency of O₂ molecules and signals at that frequency may transfer energy to them and thus heat them up.

However, remember that radio signals lose energy to free space proportional to the (distance and frequency) _squared_ [1]. A 60GHz signal is attenuated by 88dB at 10 meters. That means that if it is transmitted with 100 W (20dBW), the RF power that arrives at your position is -68dBW, or 0.000158 milliwatts. That isn't going to heat up much.

[1] https://en.wikipedia.org/wiki/Free-space_path_loss

Note that this math is misleading for calculating how much things will heat up, because effective antenna aperture of the thing that you'd heat up increases with the square of frequency. So the square terms of frequency cancel each other out.

It's still small, but the best simple approximation is just to take the 100W and divide by the proportion of the antenna pattern your target occupies.

A 100W source on 2GHz or 60GHz at 10 meters distance is going to dump about the same amount of energy into you. If the transmit antenna has 3dB of gain, that's going to be about 50mW.

That's interesting. I'm not quite sure how that works but I'll look into it. If you have some resources, I'd appreciate it!

See the derivation section in your link. It's assuming an isotropic antenna at the receiver, whose effective area gets smaller as the frequency increases.

But people do not get smaller as the frequency increases: they occupy the same fraction of the 10m radius sphere around the antenna.

The power levels are low to begin with, so heating up the air is hard to even measure, let alone notice.

It's why 60GHz is license free and used for WiGig, wireless HDMI and short haul microwave links like the ones from Mikrotik.

The heavy oxygen attenuation makes it commercially worthless.

Same reason why there's a license free band on 24GHz too. 24GHz suffers from water vapor absorption and has heavier attenuation than adjecent bands thanks to that. For example Ubiquiti AirFiber uses license free 24GHz.

Stop spreading FUD. ~60ghz radar has been used by cars for for cruise control and parking for many years now at much higher power levels than what phones can transmit. Some phones have mmWave radar for gestures too. We’d use even higher frequencies if it wasn’t for the terahertz gap.

Spreading FUD isn't my intention. If anything I am hoping to alleviate the FUD that open-mindedly reading these documents and plans induce in my mind.

I appreciate and read the honest and detailed technical takes on the topic here.

Especially acknowledging uncertainty and a healthy amount of doubt are pretty essential to the epistemological pursuit. And fear may arise when the uncertainty and doubt are not dispelled with reason.

There's also the issue that with 5G phased array antennae, tx power limits are not restricted as tightly. With a standard single tx antenna, it's easy to calculate the amount of EM energy it can create in the 3D space around the antenna. With phased array beamforming, you're using using multiple small antennae and amplifying the signal beyond the sum of the individual outputs. The safety ratings for EM radiation around phased arrays tend not to be calculated on the theoretical maximum TX power, but on the typical usage. The result is that with low enough level access to the firmware, someone could theoretically weaponize a 5G antenna to produce unsafe levels of RF energy, as well as aim the antenna without physically moving the hardware.

This sounds far-fetched, and it is, but I wouldn't be surprised if we see it done in a lab, like some of the other practically infeasible but bleeding edge cool hacks.

> With phased array beamforming, you're using using multiple small antennae and amplifying the signal beyond the sum of the individual outputs.

No, that's impossible. The RF power imparted onto the EM field by any antenna array is equal to or less than the sum of the output power of each antennae.

I think the argument is that while the total power over the full 4π is equal to the sum of the input power, the constructive/destructive interference cause there to be higher power output within some solid angle. But my understanding is that the limit are for avoiding interference from other devices, not for human safety, and that any reasonable amount of non-ionizing radiation is incredibly safe for humans. Congestion wouldn't be impacted by the beamforming, because on average any increase in intensity at one angle would be countered by a decrease at some other angle.

Higher power density, not higher power. You aren't going to get more watts anywhere than went in, but you will potentially get more watts per cm^2 than at the antenna.

The FAA is objecting to the use of the 3.5GHz band, as it's supposedly too close to the 4400-4700MHz radar altimeter band. This band is the primary one for 5G deployment world wide and interestingly no planes have been dropping or crashing due to it's use. Strange how the same planes landing in European capitals with widespread 3.5GHz 5G deployments seem to do fine there, but will experience dangerous problems on USA soil.

3.5GHz is also exlusively 5G and not used for anything cellular before. Outside of some old wimax deployments here and there, but those ware usually for fixed users not mobiles.

3.5GHz is also firmly in the centimeter bands. :)

Europe is only using 3.4-3.8 GHz. The US is going up to 4.2 GHz.

Ah! That certainly explains some of it. I even checked and sure enough, the highest allocated block here is 3670 - 3800 MHz.

AIUI the European usage of this band only goes up to 3.8GHz (see https://www.everythingrf.com/community/5g-frequency-bands-in... for example), but the FCC auctioned off band space up to 3.98GHz. Aircraft altimeters use the 4.2-4.4GHz band, so the new US allocation nearly halves the band gap. That is what the FAA is worried about.

>halves the band gap. That is what the FAA is worried about.

It's worth noting that a band gap of 200mhz is ridiculously wide, at least according to the top comment from the last thread https://news.ycombinator.com/item?id=29642777

The UK makes available shared access licences in the 3.8 to 4.2 GHz band. These are power limited and location constrained, to manage interference with satellite earth stations.

You can get a license in this band and run your own private 5G network in it. There doesn't appear to be any concern from the UK aviation regulator (the CAA).

Perhaps they're able to achieve sufficient spatial separation from airports and their final approach paths, or perhaps they've found there aren't issues with the reduced transmit power available on these shared access licences? Nonetheless, it will be interesting to see if this 200 MHz guard really is insufficient in the US - at a certain point you need to consider if existing equipment needs to be replaced (like with the swapping of 800 and 700 MHz from digital TV to mobile, and the need for some people to put a band-stop filter onto their TV antenna feed to avoid interference from mobile networks).

Using this band is only an aviation problem if you're in the middle of an approach (approximately 20 miles from a runway, but not a big circle). Hence the location and power limitations.

3.5 is fine, that's CBRS. This is about C-Band, which is 3.7ghz to 4.2ghz in the US.

NTT Docomo operates n79 (4.5GHz) for 5G in Japan. Maybe it's the only network use n79. (It's hard to find supported phones in open market rather than docomo model, except iPhone) https://en.wikipedia.org/wiki/List_of_5G_NR_networks

According to assignment map, it uses 4500-4600MHz. https://www.tele.soumu.go.jp/resource/search/myuse/use/10000...

I want to think it also handled congestion better. I wanna think I've experienced aread (airports? Downtown) where apparent thruput is better than I was used to when places were full of people.

Semi conspiracy alert: 5G is mainly motivated by surveillance through IoT. 4G is still mostly ok as a regular "cellphone" network but isn't cheap enough to be embedded in every device. Many consumer goods now have a wifi option (like my new dishwasher) but it still need to be configured after installation, which many people won't care to do. 5G will make it cheap enough for manufacturers to bundle always-on cellular modems in every device. The cost of networking plan will be assumed by the manufacturer and be offset by the value of telemetry data collected and subscription services made possible by the always-on network. Special dishwasher cycles for an extra 3$ and other shenanigans. Obsolence no longer has to be preprogrammed but can be declared unilaterally at the time most convenient by and for marketing departments.

I'm not seeing it.

I mean yes, corporations are definitely going to attempt that and they can all die in a fire, but telemetry like that requires minimal bandwidth. It doesn't need "5G" for anything.

And beamforming is going to be useful for having precise location as a part of the protocol itself.

Doesn’t the number of antennas make location more precise without beamforming? Doesn’t seem like it has anything to do with location precision.

> 5G will make it cheap enough for manufacturers to bundle always-on cellular modems in every device

How do you figure that? Is there something special about the 5G electronics and chipset manufacturing that make would make it much cheaper than 4G electronics? I don't think this is true.

Maybe scale? Not that long ago shields for Arduino with WiFi were crazy expensive, now esp8622 devices cost so little I don't know how many I have lying around.

Why would 5G be cheaper to implement in IoT devices vs 4G?

Lots of stuff covered. Here’s a few more.

In theory you should be able to take your private network anywhere there’s 5G.

Industrial Ethernet. Latency, jitter, and guaranteed delivery are required in many factories. Meaning they’re still relying on cable, wiring, and proprietary PLC hardware close to the sensors.

Private network slices.

> but even streaming video as long as not everyone on the tower is trying to do it at once

5G Broadcast solves that. Radio, live TV, popular Netflix TV shows etc. could be broadcast and save a lot of bandwidth.

How is that supposed to work when everything is on demand and Netflix alone has thousands of shows?

5G means so many things to people it gets muddy.

If you are Verizon or ATT, you have lots of mmwave spectrum, so your interest is in dense urban environments and multi-dwelling units where your speed let's you compete against wifi and cable providers. You spent a bazillion on spectrum and you need to give your customers reasons to pay extra for their service.

If you are t-mobile, you have a ton of mid range spectrum. After all, that was the only reason you bought sprint. You will want to go after consumers with a coverage play, since that is better than mmwave from T and VZ. You might also dip your toe in business services like IoT for farmers and factories.

If you are a hyperscaler, you need CBRS (3.5GHz) to take off. You'll be pitching IoT and edge compute like crazy because the radio coverage is better than wifi, the speeds are good and best of all the spectrum is available at essentially nominal cost.

What you won't get just yet, but will hear about a lot, are the toys like vr robots. The ugly truth is that no one really knows what the killer app for 5g is yet, but every operator is terrified that just like 4G theyll miss the value chain and leave apple or whoever to ride their network and slurp up the profit.

Yet, those legacy operators haven’t invested a bit in an engineering and software organization capable of competing with the apple’s that ride their network.

Id argue that the telco industry tried, and failed, as opposed to not trying at all. ATT for example sent a boatload of people on training from Georgia tech, invested massively in automation... and couldn't pull it off.

I also blame the industry for missing the point on the whole pivot to virtualization. For those of you not in telco, telcos got together in 2012 and said, "how can we be more nimble like the cool kids?" and concluded that virtualizing everything, was the way to go, as if all it took was a vm and a k8s pod. Really it was about culture and supply chain, but that's a whole other story.

They like to outsource everything. Vendor, vendor, vendor. No rich culture and own fundamental IP generation.

> Can somebody explain what 5G is actually good for?

Surveilling your irl location 24/7 with centimeter-precision

https://www.fastcompany.com/90314058/5g-means-youll-have-to-...

https://venturebeat.com/2019/04/29/sk-telecom-will-use-5g-to...

https://www.ericsson.com/en/blog/2020/12/5g-positioning--wha...

> but you're not going to get it in the middle of the woods because it also requires more towers closer together

This is a misconception. You're only thinking about high-band (aka mmWave) 5G. The media has largely focused on this because of the higher speeds and its low range which makes it easy to dunk on with clickbaity articles. But this isn't particularly common outside of sports stadiums or music venues, large train stations, airports (but note that the interference in the article is not in the mmWave spectrum), university campuses, etc.

5G has much better range than 4G at the low-band, and at the mid-band they're much faster at a similar distance. If you're in the US, the only carrier that's been rolling out low-band 5G at scale thus far is T-Mobile.

Since there's a lot of FUD going on here, I'm hoping I can clarify. This has nothing to do with 5g specifically, but rather a handful of channels in a specific band (of many possible 5g bands) that was newly auctioned. And the problem isn't technical, but rather political. This problem has been solved using existing techniques and regulations and tools in various countries.

The political problem was that the FCC auctioned off the spectrum under the assumption that only half the licenses (the lower frequencies in the band), would have restrictions, and they would be lifted after a 3-year clearing period for satellite base stations to move to different bands. Now the FCC has realized that Landing instrument interference is going to be a problem, so they have to draft new regulatory restrictions. But this regulatory change is going to affect the market value of these licenses, and so the FCC is basically both preparing the new regulations (they are being modeled after similar regulations in Canada), as well as preparing to be sued by Verizon, and potentially AT&T, to try to recover some of the money they spent in the auction.

The regulations are coming, but due to the politics and legal challenges, they're taking longer than expected. So the FAA is publishing their drafts as a PR strategy to force the FCC to hurry up and finalize the regulations.

And it's working... I work in spectrum interference regulatory compliance, and we've been preparing for months for this rollout. Our software is already basically ready to deploy our compliance mechanisms within a days notice. All we need to do is a little bit of geospatial data entry on exclusion zones where transmission is not allowed. So we know that this nightmare scenario isn't gonna actually happen, but the threats have to happen in order to get the FCC to hurry the fuck up and finalize the regulations.

I recommend checking out Juan Browne’s video [1] that contrasts studies from the aviation world and one from the telecoms. Spoiler: the former brings more technical analysis than the latter.

I’d love to know why the FAA is ‘rolling over’ to the telecoms / FCC on this matter. The spectrum was in use for RADALTs before 5G and safety systems like autoland, used when visibility starts coming down to minimums, rely on them.

Seems to me that, like Canada, the US should just not allow 5G cell sites near critical areas around an airport.

[1]: https://youtu.be/942KXXmMJdY

I’ll second that recommendation. For any who don’t know him, Juan Browne’s videos are the best resource I’ve found for sober, no-bullshit reporting and analysis on everything related to aviation.

Juan (a.k.a. Blancolirio on YouTube) is an airline pilot and former military aviator who doesn’t sensationalize and really knows his stuff.

Not to mention all his videos on the Oroville dam spillway failure and reconstruction process. Top notch reporting on the subject from an individual.

The spectrum in question was never allocated for use by RADALTs. Prior to being allocated for 5G use it was used for C-band satellite transmission. If there is an issue of interference with RADALTs the real question should be why did the FAA allow deployment of RADALTs in aircraft which could not operate in their allocated frequency band without risk of dangerous interference from already assigned frequencies. The second question should be what is the FAA, aircraft and airline industry going to do to quickly correct the issue they created.

I think there's a lot of pressure to get the spectrum in the hands of wireless carriers. They spent nearly $100B on that spectrum and Verizon and AT&T are anxious to put it into play. T-Mobile already has mid-band spectrum for 5G at 2.5GHz and they're seeing customers on their mid-band spectrum get speeds of around 300Mbps. As their mid-band coverage expands, Verizon and AT&T won't want to be left behind. Likewise, the administration would like for the US to be seen as a 5G leader. We see articles with headlines like "US carriers advertise 5G, but their speeds lag other countries." A huge part of that is that Verizon and AT&T haven't been able to deploy new spectrum.

I don't know enough about how far around airports they'd want to block C-Band from being used, but in a city like San Diego, it would probably block off a decent amount of the city given where its airport is. In a place like New York, how far would you want to block off? Clearly not all of bravo airspace, but how much is needed (maybe you know how much Canada had to block off)?

I'm not sure that the FAA is rolling over. In some ways, saying "we're going to make air-travel suffer huge problems if they start activating C-Band spectrum" is saying "if you activate C-Band spectrum, Congress is going to start having hearings as to why the government is letting wireless carriers prevent people from traveling." I think the FAA is noting exactly how bad the situation might be if they go forward with C-Band plans.

I think it's also a situation where people might be unsure who has authority. Can the FAA ban wireless signals that the FCC says are ok? Can one government agency take another to court despite being part of the same administration? At some point the administration controls both and might need to make a decision.

But I think there's a lot of pressure to let C-Band go forward. Wireless carriers are looking to 10x their wireless networks and it seems likely that wireless home internet will become a big way that more rural areas get high-speed internet (even C-Band signals can get decent range with good antennas and using low-frequency spectrum for uploads).

I was thinking the exact same thing re: San Diego since I live here. Whenever I walk or jog northbound on Harbor Drive past the airport I run by a cell tower disguised as a tree that's six lanes of traffic away from the airport fence. My apartment is a 1.5 mile walk from the terminal and I hear planes on a regular basis. It didn't really occur to me until I moved here how truly close the airport is to downtown.

Its also worth remembering that the FAA insisted for years and years and years that any sort of cell phone / iPad / laptop use in flight would cause a accident - with the same sort of scare tactics as used here. They eventually backed off of that and insisted it was only during takeoff and landing..

This playbook has been run before.

How could the 200MHz spacing between the frequency allocations be insufficient? I can't imagine the oscillators in either system are specified for deviations as high as +-100MHz over their lifetime.

You're right: they're perfectly fine. This whole thing is FUD from someone... just not sure who yet. My understanding is this band has been used for Europe for a few years already.

With effectively 400 MHz guard band, though.

Yeah, and all this drama vs actually you know, testing things in the 10 years they've had is rediculous by the FAA.

Seriously, just test it.

You have thousands of flights a day. On a day with visual flight rules put a transmitter at 4.0ghz, max power, worst case radiation pattern and land a 737, 787, A320 and A321 etc as a test.

There is something going on here that we don't know about.

I was also curious about this, so I looked at the Airworthiness Directive[1] referenced in the example NOTAM. Basically, the argument is that the radio altimeter receivers "must detect faint signals reflected off the ground to measure altitude" and [C-band 5G signals] "could significantly degrade RA functions [...] if the altimeter is unable to sufficiently reject those signals".

So it seems it is basically a question of how wide is the RF input filter bandwidth of RAs on commercial aircraft.

[1] https://rgl.faa.gov/Regulatory_and_Guidance_Library/rgad.nsf...

I'm not good with analog circuits, but someone mentioned in previous thread that designing a 200 MHz wide filter that would have "brick wall" response to frequencies outside it is non-trivial, and technically the 200MHz is just the transmit range, not receive range...

That's why virtually every RF frontend oversamples sufficiently to do the sharp filtering in DSP.

Although when receiving a weak signal with strong out of band interference, you may still need a lot of dynamic range to successfully filter it out digitally.

3rd order IMD performance can become an issue.

3rd ord IMD for non-EE could be described as low power mixers like mixing low power signals and usually the low power signal comes from your intentionally designed radio oscillator, but, whack it with enough incoming RF power and it will go non-linear as its designed to do and start mixing with that incoming signal, mostly making massive levels of interference. You can always improve 3rd order IMD by simply running the oscillator and mixer at higher power levels (which all turns to heat), usually not the best strategy for battery powered gear.

This is all ridiculous, there's still a 200 MHz band guard between the FAA band and the 5G proposed band.

Here is what a $1 ESP wifi dongle has to follow:

https://en.wikipedia.org/wiki/IEEE_802.11

"The mask requires the signal to be attenuated a minimum of 20 dB from its peak amplitude at ±11 MHz from the center frequency"

So 2 dB/MHz filter.

I let you do the math.

FAA is just ridiculous here if they let old junk radio hardware handle safety landings for airplanes, but well after 737 max what do you expect...

The super wide bandwidth is on purpose - not accidental. It's also not "old junk", it was serving perfectly fine till some people decided they wanted to change the rules, and even then, work on avoiding interference been ongoing for decades due to different sources of interference.

n.b. Wifi (and BT and others) got to use 2.4GHz band because of interference from aircraft kitchens causing a push to make 2.4GHz into junk spectrum with less regulation.

That's a 200MHz spacing at a 4400MHz working frequency. For some sort of physical filter this stuff tends to work in terms of percentages. That is only something like 5%. So pretty close.

Normally you have lots of space to waste at higher frequencies for guard bands and you can allocate compatible uses for next door bands. Still, when you get a powerful transmitter right next door there are often problems. The previous tenants of that spectrum (C band satellite downlinks) used to have a problem with interference from radar altimeters. So I suppose there is some sort of karmic process here where the service that used to cause interference is now the service that is getting hit with interference.

Phase noise is obviously the problem. Plenty of microwave broadband gear isn't all that narrowband to begin with...

Note there's a pretty big difference between "it should be good enough for 99.99% of planes" and "we can guarantee there won't be even one crash even under unlikely conditions".

Aerospace is already VERY comfy doing ADS-B at 1090 and DME/TACAN from 960-1215 and the old GPS L1 allocation at 1575.42 and the military L2 allocation at 1227.60 (admittedly not many civies using that LOL) and someday probably L4 and remember ASR-11 airport radars blast at 25 KW power over 2700-2900 MHz. But, you know, the straw that broke the camels back and all that.

Most things will be fine. A small amount of things will operate degraded, which is the point of the NOTAM. Some things (like very few) will all fail the same time 5G is deployed, which will be a headache. Going forward every installation of every device on the ground or in the plane will be tested against 5G and it'll be just fine. Most likely for a week or a month everyone will have to treat their radar altimeter as potentially broken until proven otherwise, then it'll be fine and they'll drop the notam. Some equipment will indeed fail and connectors will be tightened or even devices replaced, and it'll be fine.

I’m not sure I follow how phase noise is related to an interfering channel emitting meaningful radiation 100MHz out of band. If all of these transmitters are compliant then it’s a non-issue. I’ve also never seen a transmitter where phase noise was more of a factor than IF in the context to out of band emissions.

Could you elaborate on how phase noise would cause an issue in this scenario?

N.B. Most of my experience is in optical systems, my wireless knowledge is limited.

A document from pilot unions that expands a bit on the issue: https://www.icao.int/safety/FSMP/MeetingDocs/FSMP%20WG11/IP/...

That has helpful technical info on the technical problem. Radar altimeters work in 4.2-4.4GHz. Some 5G systems use 3.7-3.98GHz. The document describes mostly theoretical problems; the only actual example they site was part of a deliberate radar jamming military test.

Yep. There has been a lot of talk about intererence. But no measurements or proof of any real world harm has not been presented. It has been going for years.

Is there a path to disprove the potential for interference here? This seems like an nearly impossible “prove a negative” task that likely becomes so common people ignore it as noise which seems counterproductive.

Side note: NOTAM is a notice to pilots [1] the article doesn’t expand on.

[1]: https://en.m.wikipedia.org/wiki/NOTAM

Presumably once this is broadly deployed, flight checks can be performed to check for interference.

Really get annoyed when publications introduce an acronym yet never define it. That’s 101 of writing. Thanks for doing their work for them :)

>> There are concerns that 5G signals, which use 3700-3998 MHz, can overpower radar altimeter signals, which operate in the 4200-4400 MHz band.

RADALTs are tricky things. They are finicky. Sometimes they get good reflections and sometimes they are confused by irregular terrain, particularly over urban areas. Imho 5G would, at most, cause radalt blackout/failure (ie no displayed altitude) rather than the far more dangerous false readings. And any interference should be intermittent. Unless you are hovering over a cell tower the strength of the 5g signal should rise and fall pretty quickly as the aircraft moves between cells.

That's why the areas most considered problematic are use of radar altimeter in terminal approach stages and automatic landing combined with high C-band 5G transmitters close to airport.

In my opinion this is required reading: https://wetmachine.com/tales-of-the-sausage-factory/what-the...

What's different in the US from the rest of the world that's been running 5G for a while?

5g has been running for a couple of years in the US. The FAA’s problem is new spectrum that carriers want to deploy.

They have deployed it, it's just not turned on yet.

Different countries use different frequency bands for both 5G and radar alitmeters.

The FAA's concern here is that the bands will be neighbouring in the USA, which means if either the 5G equipment accidentally transmits a bit of signal outside its allocated band, or the altimeters accidentally interpret out-of-band signals they ought to ignore, then problems could happen.

Since radar altimeters are pretty old tech, and very analogue, their concern isn't totally unwarranted.

Thanks; so then the question is hth did the US manage to do this 5G allocation without thinking about the consequence?

FAA is actually quite good at ignoring theoretical non-problems.

This is a sample notam for what the anti-interference alert will look like.

The bands seem to be 200mhz apart, which is a massive gap. If it was my decision, I wouldn’t think of any consequences. I still think this will only affect the crappiest of hardware with the crappiest of filters.

Most people don't realize that the vast majority of GA aircraft are positively ancient though. When the average hardware among hobbyists is >30 years old the definition of "crappiest of hardware" takes on a different meaning.

There are no ancient GA aircraft with RADALTS or autoland systems.

Considering that Garmin sells a full kit that integrates wit their displays (https://www.garmin.com/en-US/p/135561), and autoland systems are just becoming available, I think you're incorrect in your assumptions.

Many GA airframes are old, but have been refitted with "glass cockpits" from Garmin or other avionics companies.

a single radar altimeter's operating bandwidth can be as high as 200MHz, though (In fact, higher bandwidth increases resolution and quality of return)

That has nothing to do with the frequency allocation. You can’t just transmit outside your band hoping it will increase resolution. And receiving 200mhz outside your band is not a thing we do in <current century>. DSP exists.

I am pretty sure that the concern is with the results of front end overload, which happens well before DSP can be applied.

To add detail... A major part of FCC testing of hardware is that it will work correctly even when other people are transmitting stuff on other frequency bands.

So, provided all hardware out there went through the proper FCC tests, and nobody cheated, then it's guaranteed to work.

The issue is that what the FCC rules say, and how devices in the field actually behave, can differ...

> The issue is that what the FCC rules say, and how devices in the field actually behave, can differ...

That's the point of the situation. You replace the radar alt on aircraft #2153 and you do not trust it until its got a few hours and thats "ok". There are published minimums and a safe pilot will not push those with something that was just bolted together.

The exciting part of 5G is its like every radar alt in every aircraft flying in the entire country just got replaced and can't be trusted until proven otherwise for a week or two. No biggie. I suspect some will indeed fail and be replaced and that'll be OK because nobody will be trusting their radar alt for the first week 5G is turned on. And after that it'll be "fine".

Its kind of like the effect of EMP on cars. Disaster doomers like the mythical story of one "boom" and all cars stop, however comically unrealistic that is. Reality is you could still cause chaos logistically by hitting just 0.1% of all cars because our supply chain, even without covid, is absolute trash. And that's how this 5G/radar situation will turn out. Most likely the supply chain for avionics literally can't handle replacing, say, 0.1% of all radar alts currently installed in aircraft today. Can aircraft spruce and specialty literally ship 500 radar alts on monday if someone snapped their fingers and turned 5G on? Are there enough bored A+P mechanics to install all of them or will aircraft sit waiting for parts to arrive or get installed? Hilarious to consider a fixed base operator trying to order ten radar alts shipped by air priority and the air priority is down, so we're waiting on trucks. Then just like toilet paper or microcontrollers during covid, you'll have idiots panic buying two year supplies "because there's a shortage" resulting in plenty of people needing parts and having zero access. So if your average airline needs ten replacement radar alts per year and the supply chain might be F-d for two years they need to panic order twenty radar alts and stick them in closets (and order that RIGHT NOW like today...); they may only need three when 5G is turned up and then seventeen other aircraft will have to sit on the ground waiting for parts because "their" radar alt is sitting in someones hoard/warehouse.

Is this a legado (neé lightsquared) issue, or a whole nother problem?

This is different from the Lightsquared L-band LTE proposal.

This is about the C-band 3.5GHz 5G spectrum.

It is similar in that both situations resulted from the reallocation of a satellite downlink band to terrestrial use. Suddenly the the band that only had transmitters in space barely detectable on the ground has actual transmitters on the ground. It's like having noisy people move in next door when the previous occupants were very quiet.

Layman’s explanation:

When modern planes land in bad weather, especially fog, the height off the ground is determined by bouncing radio waves off the ground. Essentially radar pointing down off the bottom of the plane. This provides added safety over the normal altimeter measurement using air pressure. 5G frequencies are too close to the frequencies used for this radar and so the theory is that this will cause dangerous interference that could cause issues during a critical phase of landing. Without these systems aircraft would not be able to use certain instrument approaches and thus couldn’t land during certain weather conditions when they can today. If the FAA pulls the plug on these it would cause big headaches during times when today planes can land without issue.

tldr 5G might stop you from getting to your destination on time or at all on a future flight.

That explanation is correct but somewhat incomplete. Precision approaches with low minimums (the decision point where a missed approach must be initiated) also have an alternate source of vertical guidance (in the typical case of an ILS, it’s a radio beam bounced off the ground and up along the approach path and the pilot has a very tight indication of whether they’re below, on, or above that beam).

The radio altimeter is a secondary or tertiary source of height above terrain information, not a primary.

Without a functioning radio altimeter, some aircraft on approach to some runways would have to use higher minimums than would be available if they have a functioning unit. This will contribute to some missed approaches or diversions in low weather. (Where a missed approach almost never ends in a crash in airline operations but rather a second attempt or a diversion.)

Will 5g also affect radar-based cruise control in cars?

So 5G may end up protecting us from chemtrails? What's a conspiracy nut to do?

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