Hi Jeroen,
Good work on generating a code for your application! In theory the LLRs
can have any value but since it's a log ratio, they are normally small.
You have probably seen David's use of the standard (BPSK) formula for
getting LLRs from SDs which scales in proportion to the SNR. Zero
indicates the bit is equally likely to be 1 or 0 and larger values
indicated increasing certainly. LLRs are often clipped to some
magnitude (eg not allowed to be > 10 for "1" or <-10 for "0") as large
values almost always cause numerical problems.
Cheers, Bill
On 16/6/20 8:48 am, Jeroen Vreeken wrote:
Thanks Bill, I'll definitly dive into that.
After figuring out what the current octave code is and isn't doing I
generated a 2/3 rate code (696 code bits, 434 data, 232 parity) and
got the mode 6000 unittests working with it.
I actually used a LDPC code generator I wrote a few years back when I
was looking into them the first time. (But never got around to
actually finishing it). It probably is not the best code, but at least
I can start testing on air again and if needed replace the code later.
As for making the modem softdecission what exactly is the range
run_ldpc_decoder() expects.
I see that the 'fixed' bits use -10.0. Is the value of the bits
important or is it completly relative?
Regards,
Jeroen
On 06/13/2020 07:31 AM, Bill Cowley wrote:
Hi Jeroen,
Further to David's reply, you might like to look at the AFF3CT
sitehttps://aff3ct.github.io/ which provides comprehensive info on
channel coding. David's codes have been Repeat Accumulate (RA) LDPC
codes - we used them initially as encoding is very easy (eg well
suited to a microController). Of course you probably don't need
this - so you might get slightly better performance with other
options. I haven't played with the AFF3CT software much, but it
does cover RA.
Cheers, Bill
Cc:
Bcc:
Date: Thu, 11 Jun 2020 23:30:58 +0200
Subject: [Freetel-codec2] Generating LDPC code for 6000 mode
Hi,
The experiments with mode 6000 are going pretty well.
Another local HAM (PE1ITR) even managed to work our repeater with
just a
HackRF.
Compared to analog FM it seems to perform pretty well.
But I would like to take it a bit further and try to introduce
some FEC.
Since the currently used codec2 mode only needs 3200bps there is
still
plenty of room.
I was thinking about a rate 2/3 code (687 code bits for 458 data
bits),
but I am a bit puzzled with the ldpc octave code.
How exactly do I generate the needed matrix (and ofcourse the C and
header files)?
Regards,
Jeroen
---------- Forwarded message ----------
From: David Rowe <[email protected] <mailto:[email protected]>>
To: [email protected]
<mailto:[email protected]>
Cc:
Bcc:
Date: Fri, 12 Jun 2020 07:29:34 +0930
Subject: Re: [Freetel-codec2] Generating LDPC code for 6000 mode
Hi Jeroen,
Hey that's great work with working the new mode on a HackRF, is that
about 50mW max?
Yes, FEC is a fine idea.
A friend has been kind enough to generate the codes for me,
however it's
not a process that is automated or straight forward. Couple of
options:
1/ Use an existing code.
2/ Use a code with more data bits than you require, and fill them
with
known bits at the encoder and decoder that you don't transmit. This
effectively lowers the code rate a little. I do that for FreeDV
2020.
3/ Peruse the CML library codes and see if they have one that
fits your
use case, e.g. ldpcut.m
There is an Octave utility to generate .[ch] files and run the
codes in C.
Cheers,
David
On 12/6/20 7:00 am, Jeroen Vreeken wrote:
> Hi,
>
> The experiments with mode 6000 are going pretty well.
> Another local HAM (PE1ITR) even managed to work our repeater
with just a
> HackRF.
> Compared to analog FM it seems to perform pretty well.
>
> But I would like to take it a bit further and try to introduce
some FEC.
> Since the currently used codec2 mode only needs 3200bps there
is still
> plenty of room.
> I was thinking about a rate 2/3 code (687 code bits for 458
data bits),
> but I am a bit puzzled with the ldpc octave code.
> How exactly do I generate the needed matrix (and ofcourse the C and
> header files)?
>
> Regards,
> Jeroen
>
>
>
> _______________________________________________
> Freetel-codec2 mailing list
> [email protected]
<mailto:[email protected]>
> https://lists.sourceforge.net/lists/listinfo/freetel-codec2
>
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