I mean RF is a solved problem, there are no secrets in that part of modem design anymore. You can pick an off the shelf cable modem RF frontend (AD9866 etc) and implement 4G baseband. What you will need is experienced software people and IP rights.
On a tangential note Intel totally screwed DOCSIS modem chipsets, after taking over TI's Puma product line, by stuffing it with x86 cores and assigning work experience students to write firmware, after all everyone can code x86, right? :) Puma 6/7 can be DOSed by as little as 10KB/s stream. Supposedly there are firmware fixes, but from what I gather they just move critical processing tasks from integrated x86 cores to external ARM SoC - prevents DOSing, but latency issues persist.
On a tangential note Intel totally screwed DOCSIS modem chipsets, after taking over TI's Puma product line, by stuffing it with x86 cores and assigning work experience students to write firmware, after all everyone can code x86, right? :) Puma 6/7 can be DOSed by as little as 10KB/s stream. Supposedly there are firmware fixes, but from what I gather they just move critical processing tasks from integrated x86 cores to external ARM SoC - prevents DOSing, but latency issues persist.