The radio frontend is typically a downmixer and then straight into digital.
Some of the typically "software" bits like FFT's, various encodings, checksums, clock recovery etc. are frequently done in digital hardware acceleration blocks for performance, and saving power. If you were writing the firmware of the device, you needn't use them though.
With enough human years of effort, you could take almost any radio hardware for sale today and repurpose it to speak nearly any other radio protocol in similar frequency bands. Performance will probably be terrible though!
It's rare people do this though - all the chips don't have their firmware documented (again mostly to avoid publishing documentation that proves they are violating someone elses patents), and many have various cryptographic elements that makes reverse engineering hard.
The one exception to this is WiFi chips used in the Nexus 5 by Broadcom, which has had a reasonable amount of reverse engineering because Broadcom accidentally published the source code because the firmware code was in part shared with published Linux kernel driver source code.
:O
Enlightenment moment :(
Companies already in the industry typically have cross-licensing agreements - ie. I can use your patents if you can use mine. Either that or they just violate each others patents knowing that a patent war would be mutual destruction and in neither companies interests.
But a newcomer has nothing to offer - the minute they release any product, every incumbent company will go through their patent portfolio and sue them out of the water.
I thought only the hardware could be patented, not the software, and so SDR would level the playing field, but that's perhaps too naive?