On 10/01/2019 14:58, Frederic Beaulieu wrote:
Hi,
I am new to amateur radio but I have a good general understanding of
digital communication. I am enjoying FT8 at the moment and I would
like to understand it better.
I wonder if someone can explain me (or point me to related
documentation) the reasons for using 4-FSK instead of any other
modulations.
Thanks in advance to share your knowledge with me.
73,
Fred, VE2WFB
Hi Fred,
firstly a correction, FT8 uses 8-FSK modulation.
Have you read the references on the WSJT-X project web site:
http://physics.princeton.edu/pulsar/k1jt/refs.html .
Numbers 29 and 30 have some information on the design constraints that
contributed to the choices made with FT8. Note that FT8 targeted
multi-hop sporadic E propagation on 6m and possibly higher VHF bands.
The key constraints that were considered were a shorter T/R period than
JT65 or JT9 due to the sometimes very short openings. The sensitivity
was not needed to be as good as JT65 or JT9 since signals are usually
relatively strong on these paths so sacrificing sensitivity for speed
was one design trade-off.
FSK in general is chosen because it allows the use of constant amplitude
and near continuous phase signals, as such linear RF stages are not
necessary this making virtually any transmitter suitable. FSK signals
are inherently narrow without having to apply complicated pre-modulation
filtering. They are also relatively easy to analyse in the frequency
domain within a decoder routine. Basic characteristics that have to be
designed are bandwidth (increases with greater N in N-FSK) vs. higher
symbol rates (increases with greater N in N-FSK). Given the typical
frequency stability and propagation stability on the bands targeted,
8-FSK turns out to be a good choice for a modulation bandwidth around 50
Hz for 15s T/R periods and the alphabet, and symbol rate required for
WSJT-X style QSO messages plus parity bits and checksum bits.
The popularity of FT8 on HF is largely a happy accident in that it
fulfilled a latent frustration of many potential users of WSJT-X with
the slow rate of QSOs using 1 minute T/R periods. Clearly that
frustrated group of users did not require the greater sensitivity of
JT65 or JT9 and once the level activity grew greatly the opportunities
for many QSOs, even with limited equipment, caused a near exponential
growth since launch.
73
Bill
G4WJS.
_______________________________________________
wsjt-devel mailing list
wsjt-devel@lists.sourceforge.net
https://lists.sourceforge.net/lists/listinfo/wsjt-devel
