On: Software-Defined Radio for Engineers (2018) [pdf]

[asdefghyk]

If always wonder if their was a 4g to 3g converter to keep old devices working when 3g network shut down.

[genewitch → asdefghyk]

Yes, but very expensive.

https://www.ettus.com/products/

Also the limeSDR are supposed to do this. I had one, it arrived inoperable, I sent it back, never heard from them again. Still a little bitter, because I paid in to the crowdfunding.