Re: [Discuss-gnuradio] channel coding questions

[Achilleas Anastasopoulos]

Ben,


I have a simple example of turbo coding/decoding (i think it is a sccc) 
in the examples section of gr-trellis. It is SLOW!
I don't think there is any particular reason other than you are 
essentially have to run 2 siso blocks per iteration, ie, roughly
4 VA's per iteration (recall forward/backward pass)...
The comment i made about buffering is from my experience:
You can always get a better performance if you combine the functionality 
of multiple gnuradio blocks into one gnuradio block...


Regarding suboptimal receivers, I don't have any intention of writing 
code for them, but if anyone is willing to work seriously on that i can 
help (maybe an undergrad student project for the summer...)
Similarly, for a turbo decoding hierarchical block: it is trivial to put 
together siso blocks and do it (the  code is essentially there: see the 
python code in the examples). I can think of an sccc and a pccc block 
whereby you define the 2 constituent FSMs (or even more), an interleaver 
structure, and the number of iterations; that's all there is to it.

Regarding integration with the "constellations" class i think it is a 
wonderful idea. When I was writing gr-trellis I thought of 
constellations as arbitrary one-dimensional look-up tables with n-dim 
entries. Repackaging the "metrics" code to use constellations would be 
great.
YES, I believe that the "constellations" class has to be general enough 
to include n-dimensional constellations. What if you want to implement 
4-FSK, or even arbitrary CPM (I am currently working on that).
Also, there are trellis-codes that output 2 QPSK (or 2 8PSK) symbols at 
every step and having these constellations as 1 2D complex constellation 
makes integration with gr-trellis very easy...

Achilleas