(cc-ed to list)
Sure, SM1000 purchase details here:
http://www.rowetel.com/?page_id=3902
The LDPC codes have much better performance (coding gain) than Hamming
codes. I've found Codec 2 (especially the lower rates like 700) fall
over at a few % BER.
Cheers,
David
On 08/10/17 23:58, Adrian Musceac wrote:
Hi David,
I just tried 700B today on VHF (2 km path) and I have to say I'm very
impressed. I got a completely readable and clear signal at only 3 dB
SNR, that's in the context where SSB required 2 dB SNR for a very
noisy copy. This without any channel coding. Do you have any idea if I
can still buy the SM1000 and from where, as I'd like to test my SSB
transceiver with the 700C mode against the full SDR mode I use
currently.
Regarding channel coding, I was wondering if you considered that LDPC
and similar codes have very abrupt slopes at high BER numbers,
compared to for example Hamming. I was actually able to understand
voice at 25% BER, but at these numbers there's not much channel coding
can do.
I'm very interested in your next steps :)
Thanks,
Adrian
On 10/6/17, David Rowe <da...@rowetel.com> wrote:
Hi Adrian,
There is Octave and C code for the mFSK modem we have developed in
codec2-dev, that we have tested against the ideal FSK performance
curves. Not sure I'd call it matched filter, but take a look at the code.
It's important to check your modem implementation against theory at a
couple of points on the BER versus Eb/No curves. Very easy for bugs to
creep in.
I'm currently working on improved quality Codec 2 at 700 bit/s, but it's
a slow process with no release date in mind.
- David
On 06/10/17 10:32, Adrian Musceac wrote:
Hi David,
Thanks very much for the tips! Would you suggest that the matched
filter approach is better for 4FSK as well? I am using it for 2FSK and
it works well, it's just that being lazy I wanted to avoid too much
complexity in the code for the 4FSK variant.
Regarding PSK: I have several things to try and real world tests will
show which one is more practical. Right now for PSK I am using Codec2
at 1400 bits as what I find a good compromise between quality and
bitrate. This gives me just enough space for synchronization bits and
other protocol data (which may span on multiple frames). I am
considering moving down to 700 bits per second and I wanted to ask you
if you think you will be making major changes to it's quality in the
near future. This would give me 3 additional dB to play with, but at
this point I don't think we can afford to have more than 1% errors per
frame, as each bit carries a lot of information.
I tried rate 1/2 convolutional encoding with real world tests and it
seems to give an additional 2 dB of space. The advantage is that frame
sizes are short, so we don't have large gaps when errors occur. On top
of that, Viterbi soft symbol decoding and trellis to 8PSK add to the
computational cost, which I have a low budget for.
Best regards,
Adrian
On 10/6/17, David Rowe <da...@rowetel.com> wrote:
Hi Adrian,
It's very important to avoid using an analog FM demodulator with FSK -
it's the reason C4FM/DMR are such a poor performers:
http://www.rowetel.com/?p=3799
http://www.rowetel.com/?p=4279
At 1% BER, Eb/Nos reqd are roughly:
2FSK 9dB
4FSK 6dB
PSK 4dB
The PSK results are for coherent demodulation, which is hard to do
without overhead (e.g. pilot symbols or a unique word). I suspect
non-coherent PSK is worse than FSK, so not worth doing unless you are
really concerned about bandwidth.
The FSK results are for non-coherent demodulators which are really
simple to implement and get real-world results right on ideal.
Convolutional codes are a bit old hat - we're getting gd results on HF
with short-ish LDPC codes.
But best to sort out your uncoded demodulator performance first.
Cheers,
David
On 05/10/17 20:05, Adrian Musceac wrote:
Hi David,
Thanks for the answer! I have just simulated a 2FSK modem on AWGN
channel, but this time without using FM demodulation. It performs just
like you said, ~2 dB worse than QPSK (at 5% frames dropped). This
means that the FM demodulator I used for 2400A must be introducing
some symbol errors.
What I can't figure out is the 10 dB difference to analog FM. My
experimental results (test in urban environment, with distances
between 500 meters and 1 km between sender and receiver) show ~6 dB
between QPSK and analog FM (with 2.5 kHz deviation) and no more than 4
dB between 4FSK and FM. Could the non-coherent demodulation explain
this?
I know I can obtain up to 6 dB SNR improvement by going to Codec2 700
bits/sec and using Viterbi soft symbol decoding, but I'd like to get
the optimal results before. Would convolutional encoding in 2400A be
worth considering?
Thanks,
Adrian
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