BATC Forum

Nice to see the first results of the Rhode and Schwarz Engineering challenge
https://www.gnuradio.org/blog/rohde-sch ... gnu-radio/

Bit of history.

The GNU-Radio T2 code was derived from the GNU-Radio S2 code which was derived from
the DATV-Express S2 code hence my interest in the competition. The original GNU-Radio
code was written by Ron W6RZ.

So congratulations to the winners!

- Charles G4GUO

I saw presentation at SDR Academy about how they improve DVB-T2 modulator. Good team and interesting tweaks. Hope they will maybe pass their ham licence.

Some of the mods they have done are also applicable to DVB-S2 like the scrambler, BCH encoder and LDPC speed ups.
They have not done anything I would call really radical but it was a very good exercise as they would have learned a lot just
by understanding the code in the first place.

- Charles

That's really interesting. I can see DVB-S2 as becoming a standard for nanosatellite downlinks if the encoding can be done quickly enough within the power budget. Its about time we moved on from Convolutional/RS coding of BPSK. Be nice to see some work on software decoding of DVB-S2. The argument has been it needs too much CPU, but CPUs are getting faster at the same time we are using lower symbol rates so maybe that's not the issue it used to be.

As you say, T2 and S2 have things in common. I don't think T2 is suitable for satellite but it would make a big difference for 2m DATV.

Mike

I started work on a DVB-S2 decoder and I do have a working LDPC decoder using belief propagation
aka sum-product message passing working but it is currently too slow to be practical but that is
just my poor coding as I know what I am trying to achieve can be done. I say it is working because
I am getting text book BER vs EbNo performance with it. The same applies to the BCH decoder
it works but it is just too slow, the slowest part of the BCH decoder is the syndrome calculations.
The Berlekamp Massey and Chien search parts of the decoder seem fast enough though.
So I can add it to my list of failed projects.

- Charles

I would rather you added it to your list of successful to be improved projects... If we are looking at data rates below 100ks/s how important is the speed? We are already doing DATV at 125ks and if we can get that down further it opens up some interesting options for higher power 2m operation.

Mike

Too many projects on the go sadly. They are proposing to use DVB-S2X for the planned
cubesat mission to the moon (if it ever gets launched) so that might be an interesting
motivation.

-Charles

Charles,
Could you explain in a few words what DVB-S2X brings in addition to DVB-S2 (in general and for us for DATV)
73
Christian

Hello Christian,

Basically DVB-S2X adds larger signal constellations (more bits per symbol), more FEC rates (to match the channel better),
tighter roll-off filters and a new mode called VL-SNR (Very low Signal to Noise Ratio).
VL-SNR is what is being proposed for Amateur deep-space missions and it has the ability to operate at -10 dB Es/No
(Energy per symbol divided by Noise spectral density), DVB-S2 operates down to -2.5 dB Es/No of course the
bitrate drops significantly to get the improved performance.

There are also some other changes to do with the types of baseband signalling that make it more flexible.

Currently the only easy to obtain DVB-S2X receivers support only a small subset of the new features.

Cubesats will probably operate at very low symbol rates ~ 10kbit/s (I think) which means receivers will probably
have to be Software Defined.

- Charles