Considering we are not bandwidth constrained, why is everyone so enamoured with 4 level waveforms like 4FSK , When two level/ two state waveforms like BPSK and 2FSK are far more robust in a mobile environment ????
Differential BPSK is an easy demod , 2FSK is also an easy demod. BPSK having the slight edge due to being a antipodal waveform compared to being an orthogonal waveform. (though the full 3dB gain (BER = 1e-04) of an antipodal waveform only occurs with coherent demod) . (gain at very low SNR closer to 2dB) complexity of a DBPSK incoherent demod and incoherent 2FSK demod are similar DBPSK leaves open the option of a highly productive coherent demod, and 2FSK leaves open the option of highly simplified demodulators (slicing FM demod). -glen Vk1xx On 6/10/2017 11:02 AM, 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 >>> >>> ------------------------------------------------------------------------------ >>> 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 > ------------------------------------------------------------------------------ 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
