Extending OpenBTS to handle GPRS might be the easiest way to get
that done. WiMax would be a big project.
I can tell you from experience that it’s bigger than you think. I
know someone who is trying to monitor just a single channel, and even
the process for the OFDM pilot tones was super complicated (if you
made a flow chart I’m pretty sure it’d be about 10 pages long).
My experience is with the USRP1, and I think that if you are only
interested in monitoring one channel (ie, not having an actively
communicating receiver), then you can get by using the 8 bit sampling
mode, otherwise you will have to try to work something out within the
FPGA. From what I can tell, most of the space in the FPGA image is
taken up by the code that lets you change the decimation/interpolation
rates. If you fixed those you would free up a lot of space for you to
work with (maybe do some sort of intelligent decimation by handling
the OFDM on chip? You’d have to be a super HDL coder I think)
Anyway, that’s my (limited) wimax-on-usrp experience.