[femto]

Complex: yes

Proprietary, Closed Standards: no

Here are all the 3GPP 4G/5G standards that govern cellular phone networks:

https://www.3gpp.org/specifications-technologies

Also look under the "Specifications" drop down menu at the top of the page. Every release is listed.

These standards being freely available is why SRS were able to write srsRAN. There is nothing stopping others from doing so either, apart from the amount of information that has to be understood and digested.

Edit: Apart from complexity the other potential barrier is patents, but that's not a technical barrier.

[advael]

While it's technically true that the specs are public, the implementation not being public is most of the issue being solved

[kurokawad]

The 3GPP spec is “available”, but the implementation in practice is closed. Stuff like the scheduler is what actually brings value to the company delivering a good service. Being closed, the interoperability of network components from different vendors is just impossible. That’s the point of O-RAN Alliance.

[miki123211]

Apart from complexity and patents, the other other barrier is licensing.

Those technologies aren't terribly useful to use in your house like WiFi is, and even if they were, you need more government paperwork to do it. That means less hobbyist interest.

[blacklion]

Patents equal proprietary.