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Comment by teeray | Hacker News Reader

This is interesting, for sure, but the steps here are the easy part. The challenge is doing this legally, since I’m pretty sure srsRAN is going to use bands you almost certainly don’t have a license for.

The only viable options for the hobbyist is a network using unlicensed bands (like 2.4 GHz / 5GHz, or the 33cm ISM band) along with CBRS[0]. As I understand it, most UEs (handsets) that support one of those bands will not use it exclusively—only in tandem with another licensed band.

The other alternative is Amateur Radio bands, which some hams have apparently experimented with: https://github.com/mmtorni/HamLTE.

[0] https://en.wikipedia.org/wiki/Citizens_Broadband_Radio_Servi...

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Unlicensed bands typically have the requirement that you implement some kind of backoff or collision detection mechanism to ensure fairness between users of the band and guarantee you are not sending more than X% of the time. I don't think LTE is prepared to do that at all.

Plenty of other unlicensed band users don't do this. Even bluetooth and wifi typically don't avoid eachother on 2.4G, with the exception of transmissions from the same device.

This paper is a good summary: https://arxiv.org/pdf/1910.11002.pdf

And of course all other unlicensed band users do this. Again, this is a regulatory requirement. And ultimately, it is not just that you need to ensure others have access to the band, you also need to be robust in the face of persistent high interference given the number of users in the unlicensed bands. It's not a good fit for default LTE.

ISM is the only that doesn't do avoidance (where it has primary status, like the 27MHz band). If course it doesn't blast excessively, but it's also not mandated to shield to death.

Sadly srsRAN does not seem to implement TDMA. Ham radio operators have a slice at 2.3 GHz which overlaps Band 40, supported by a good number of phones.

There are other stacks that implement TDD-LTE. Could be possible with OpenAirInterface though I'm not sure if it's ever been successfully tried.

Also band 40 is the only widely supported TDMA in range of the cheaper SDRs. Even a limesdr mini can speak that, though one may want to get an LTE quartz to keep phase noise in spec (the fraction from the normal quartz has a really big denominator, that makes fractional PLL problematic if you don't sense fractional phase of the reference quartz) with the PLLs on those SDRs.

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