If implemented correctly, 4FSK gets you 3dB over 2FSK on an AWGN
channel. The FSKs can use class C PAs.
For DRM/C4FSK they _are_ bandwidth constrained due to the high bit rate
of AMBE/standards (which is black box with a bit rate so they have no
choice over). I presume this is the reason for poorly spaced 4FSK tones
and a performance hit of 6-ish dB. Or maybe low SNR performance doesn't
matter in the use-cases for those standards.
Why is 2FSK more robust than 4FSK (both non-coherent demods) on a
UHF/VHF mobile channel?
- David
On 06/10/17 10:57, glen english wrote:
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
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