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
------------------------------------------------------------------------------
Check out the vibrant tech community on one of the world's most
engaging tech sites, Slashdot.org! http://sdm.link/slashdot
_______________________________________________
Freetel-codec2 mailing list
Freetel-codec2@lists.sourceforge.net
https://lists.sourceforge.net/lists/listinfo/freetel-codec2
------------------------------------------------------------------------------
Check out the vibrant tech community on one of the world's most
engaging tech sites, Slashdot.org! http://sdm.link/slashdot
_______________________________________________
Freetel-codec2 mailing list
Freetel-codec2@lists.sourceforge.net
https://lists.sourceforge.net/lists/listinfo/freetel-codec2
