[Discuss-gnuradio] New digital modulation implementations

[Tom Rondeau]

I've been working on redoing the digital modulations in the background 
for a little while now, and I just merged in a branch with newer 
features. So that I didn't interrupt the flow of anything, I just added 
a version 2 for everything in the digital modulation path instead of 
changing what was inside them. Hopefully, we will be able to switch over 
to the new stuff and replace the old ones so we don't have all of this 
duplication lying around.

Here's what's new:
dbpsk2 and dqpsk2 - these run a couple of new algorithms for the 
synchronization, including a frequency lock loop in the front 
(gr_fll_band_edge), a filterbank clock synchronizer (gr_pfb_clock_sync) 
for timing, and the standard Costas loop that is mainly used for phase 
alignment. I also replaced the interpolation filter in the transmitter 
with a resampling filter that can handle any real number for the number 
of samples per symbol. This along with the new clock synchronizer allows 
us to use (almost) any value for the bitrate we want instead of being 
constrained to a set of discrete values like before. I say almost any 
value because you can't go too low as this would require a huge number 
of samples per symbol, which has other problems.

To use these new modulations, you have to run benchmark_*2.py. In the 
same examples digital folder you'll find a version 2 of everything. You 
can only access these modulations through them.

The reason for these changes is to make the digital modulations more 
robust and tolerant of channel conditions. The old way was too dependent 
on the receiver parameters. The new synchronization methods are much 
more tolerant based on their parameters. The new frequency lock loop 
will also make initial acquisition a lot better and the USRPs can be 
farther off frequency than they could be in the old scheme. You still 
have to be careful about this, because I have two USRPS with RFX2400s 
that are so off frequency that the signal is getting hit by the filter 
in the USRP before the FLL can get it. Since we don't retune the USRP, 
there's not much we can do here except decrease the decimation rate and 
alter the receiver samples per symbol. Just something to watch out for.

I'm interested to hear any input about how these new blocks work for 
people. My biggest concern is the amount of lock-in time for the FLL, 
which may have to be rethought to make this system practical for bursty 
signals. Once we work out any major issues and if people find this works 
better, then we can remove the old stuff.

Thanks,
Tom