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 >> >> ------------------------------------------------------------------------------ >> 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 > ------------------------------------------------------------------------------ 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
