here is an application that requires significant FPGA
try to design the first stages of a spread-spectrum receiver,
ie, the part of the receiver that does pn code acquisition
(and tracking). This is a demanding task that needs to be performed on
hardware. Once code acquisition/tracking is performed, the input data
can be despread (this is also done in hardware) and then only the
symbol-spaced samples are passed to the Gnu-radio software for
further processing (ie, decoding, etc).
Some receivers require 2 or 3 such blocks (fingers) running in parallel.
Check out how IS-95 mobile standard works for more info.