There are a couple projects of differing scope:

- srsLTE (https://www.srslte.com/)

- OpenAirInterface (https://www.openairinterface.org/)

I've successfully used srsENB as a base station, largely to generate "model" LTE waveforms so that I could then decode them and verify my understanding. srsLTE is quite well written code-wise and is actively improving/growing. Seems to have a supportive single company and community behind it.

OpenAirInterface is more geared towards 5G research and is a lot more ad-hoc in terms of documentation and capabilities.

Please note: OpenAirInterface is not open source software. Large parts of it (particularly everything related to the RAN) are under a license that does not comply with either DFSG, nor OSI Open Source Definition, nor the FSF Free Software Definiton.

A couple more links to such projects:

https://open5gs.org/ https://github.com/nextepc/nextepc

indeed, if you want a 100% open source 4G network, you can use srsLTE for the eNodeB, and open5gs for the EPC. No proprietary bits needed, and no non-free licenses needed either.