The US C-Band Spectrum Saga, Explained | Hacker News Reader

The US C-Band Spectrum Saga, Explained

payloadspace.com

40 pointsaml18313 comments

https://rickcaylor.websitetoolbox.com/post/the-beginning-of-...

Here’s a discussion from the side of the nerds who still consume these feeds.

The article wears its editorial bias rather strongly, and to me seems to imply that the public auction was a de facto giveaway to telcos. It fits my preconception of Ajit Pai's tenure.

Is there another perspective here? Were other interests served by reallocating this spectrum? I'd love to learn that this was sound technical policy or something.

Isn't it the opposite? The public auction went to the highest bidder.

It turns out this spectrum is far more valuable for terrestrial use ($81billion paid for it) than it was for satellite use. So the satellite users are being kicked off it since they couldn't pay that amount.

To me, this reads as "the telcos wanted the spectrum for themselves, and muscled out the satellite providers with the help of their inside man".

If you consider the spectrum's value to be fairly represented by what telcos paid, and that the societal value of the spectrum is best realized by those who can pay the most, then sure, this represents the optimal outcome. I don't necessarily believe either of those two points - but that's why I'm asking here, because I don't really have a good grounding in the facts.

The initial pitch pre Ajit's decision was a private auction by the CBA. Ie, the corps behind these sats would themselves auction and keep the $81B.

Supposedly this would be "better" for the taxpayer, but I always thought that was bogus. In the US the taxpayer has rights already to this spectrum, we don't have to buy it back from these folks.

Anyways, thankfully that all fell apart, public auction generated $81B (a staggering sum showing the potential value of this spectrum for this use). Folks clearing spectrum still getting a massive windfall.

Technically, a 400Mhz guard band is ridiculous. That's guard in initial roll-out. If planes have issues with stuff 400Mhz away that is ridiculous.

Also ridiculous would be that they can't figure out their height using other means. Airports should report baro pressure for standard baro altimeters. They should look into GPS and RNAV style navigation used internationally - all this is common overseas. Idea that planes can't figure out there height seems bogus.

My understanding is even later in roll-out there will be 20Mhz guard band around the airplane frequencies - do these airplane systems have no clue about filtering? This is plenty in most applications.

This smells political or interagency related. This auction and use has been years and years in the making, and last minute we get this stuff.

> ridiculous

I don’t think that word means what you think it means.

Baro altimeters don’t tell you your height above the ground (or other obstruction). A digital terrain model is only as good as your navigation source, and GPS has well known failure modes.

> do these airplane systems have no clue about filtering?

Do you have no clue how much it costs to retrofit a filter on a commercial airplane?

It’s not that adding a filter is technically challenging. It’s that it wasn’t previously a requirement, and leaving it out saved weight (and every gram adds up when you’re building an airplane). If you added every ‘nice to have’, the airplane couldn’t lift its own weight.

Now that radar altimeters exist without high Q filters, retrofitting them costs money. You’ll need to have the filter approved by the manufacturer (because you are normally required to follow their installation guidance) and the FAA. Neither of them are going to help you for free.

It's actually even tougher than just certification issues. High-Q filters tend to impact timing. Timing is what is being used to calculate distance above ground. It's not even just as simple as adding the filter, you have to find a way to do so that won't impact the signal timing.

What I'm not understanding is that by most commercial radio standards, these guard bands are huge. 400Mhz? 20Mhz?

Can they not just do a test with 50 planes and get some hard data?

There really is no excuse for the FAA holdup happening currently. The same spectrum has been in use in European nations for a while now as their primary 5G band. They have not run into any interference issues that would delay their rollout like this.

It also further helps T-Mobile as the spectrum they've been allocating for 5G is in the 600 MHz and 2.5 GHz range. T-Mobile owns some C-band but it's not set to be available until 2023 regardless. AT&T and Verizon, however, will be relying on C-band spectrum for the brunt of their 5G capacity. Without it, their networks are a tiny sliver of 850 MHz and/or mmWave with very limited coverage.

Here's the reason that the FAA is currently attempting to block this. I'd love to see more real-world testing, and a commitment from the telcos that _if_ interference arises, they'll vacate the spectrum. Flight safety trumps greater mobile bandwidth IMO.

https://www.rtca.org/wp-content/uploads/2020/10/SC-239-5G-In...

If you read into more detail, it's also easily seen that outside of Japan only few countries tentatively allowed business verticals in the 3.8GHz-4.0GHz range, it's just european "mid-band" overlaps 3.7GHz-3.98GHz that is proposed as a block in USA - and where FAA complains about frequencies closer to 4GHz (4.0 GHz to 4.2 GHz being radar altimeter range)

To add a data point, the UK makes available 3.8 to 4.2 GHz spectrum on a local basis (with some transmit power limits), nominally for "verticals" and private networks.

Not aware of any issues with radio altimeters, but the power limits might help in this context too.

Might be also a case of locations being considered far away enough from established effective flight paths

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