On 27/04/2019 18:06, Игорь Ч via wsjt-devel wrote:
There are also implications for ramp up and
down of the transmission boundaries since the GFSK filtering will
naturally do this without extra implementation complications.
Hi Igor,
I was wrong in respect of the above. Steve, K9AN, pointed out that we
actually do waveform envelope shaping, to limit the bandwidth of the
transmission start and end, separately after converting to a GFSK
signal. This is as a consequence of the way that we convolve the
Gaussian function to smooth the frequency transitions which is all done
in the frequency domain with padding symbols (later discarded in the FT8
case) added to the begin and end. The smoothing is done by convolving
the instantaneous frequencies with a Gaussian across a three symbol
window. The resulting frequency time series is converted to a phase
angle time series, which in turn has the ramp up and down applied. The
resulting signal is later used to generate the transmitted PCM audio stream.
The envelope shaping is done by applying a raised cosine ramp up/down
amplitude function to the phase angle time series signal at the
beginning/end respectively. The FT4 waveform explicitly includes one
symbol extensions at the beginning and end of the waveform with values
equal to the first and last symbols respectively (the padding mentioned
above). These two extra symbols have the ramp up and down applied across
their full duration. For FT8 we need to maintain exact compatibility
with the existing FSK waveform, at least in length and timing, so a
different envelope shaping scheme is applied where the first 1/8 of the
first symbol and the last 1/8 of the last symbol have a raised cosine
ramp up/down function applied in the time domain.
Lastly, going back to your initial question. I was probably a bit over
zealous in saying there are only compatibility issues and no sensitivity
issues. For a single signal communication in the presence of noise
alone, using GFSK will reduce sensitivity compared with FSK due to
information being spread between symbols (inter-symbol interference or
ISI for short). The aim is to use an amount of frequency change
smoothing so that, on a busy channel, the overall interference between
signals is reduced, by limiting their bandwidths, such that net
sensitivity is increased. Choosing the exact amount of smoothing to
apply is a complex problem since it depends on the number of signals,
the distribution of signal strengths, the frequency distribution of
signals, as well as the distribution of time-synchronization accuracies.
All of these characteristics vary continuously during real world FT8/4
communication on the Amateur bands, so all we can really do is carry out
empirical tests and try to pick an optimum filter bandwidth which gives
best results for the whole community.
73
Bill
G4WJS.
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