[Mich VHF UHF Society] FT8, FT4? Try FST4

Sun Sep 27 19:01:51 CDT 2020

Interesting in what is coming down the pike....

New mode FST4 – WSJT-X

Steve Franke, K9AN; Bill Somerville, G4WJS; and Joe Taylor, K1JT

Version 2.3 of WSJT-X introduces a new family of digital modes we 
designate FST41

These modes have a wide range of capabilities and potential 
applications. Their fundamental sensitivities are better
than most older modes in WSJT-X and are close to the theoretical limits 
for the same information
throughput rates. Particular target applications include

▪ WSPR-like, quasi-beacon transmissions on LF and MF bands
▪ QSOs with very weak signals on the LF and MF bands
▪ EME on VHF and UHF bands with T/R sequences both shorter and longer 
than 60 s

… but other uses will surely be found, as well. We expect that FST4 will 
serve some of its intended
purposes better than older modes in WSJT-X, and without the strict time- 
and frequency-locking
requirements of modes such as EbNaut.

FST4 uses 4-GFSK modulation and offers T/R sequence lengths of 15, 30, 
60, 120, 300, 900, and 1800
s. Submodes A, B, C, and D provide different tone spacings, to allow for 
use with a variety of propagation types and Doppler spreads. Forward 
error correction uses a low density parity check (LDPC) code with 240 
information and parity bits.

Transmissions consist of a sequence of 120 information-carrying symbols 
(2 bits each) interspersed with 40 synchronizing symbols. The underlying 
message payloads contain either 77 bits (the standard payload for FT4, 
FT8, and MSK144) or 50 bits (the standard for quasi-beacon, WSPR-style 
messages). With WSPR-like messages the mode
designation becomes FST4W and the A submode is always used. In all 
cases, message formats
displayed to the user are the same as in the other 50-bit and 77-bit 
modes in WSJT-X.

Basic parameters of the A submodes are given in the following table. The 
measured sensitivities are based on simulations on the additive white 
Gaussian noise (AWGN) channel, and depend on oscillator
drifts and path Doppler spreads being smaller than the tone spacing over 
the full transmission length.

Do not confuse FST4 with FT4 — a digital mode with 7.5 s T/R sequences 
designed for HF and VHF contesting.

Submodes are given full names like FST4-60A, where 60 indicates 
60-second sequences and A impliesminimum tone spacing. Submodes B, C, 
and D have larger tone spacings and occupied bandwidths by factors of 2, 
4, and 8, respectively.

Sensitivities of the wider submodes are degraded by amounts that depend 
on Doppler spread and other details of the propagation path. As with 
other recently developed modes in WSJT-X, a feature called a priori (AP) 
decoding improves sensitivity by several additional dB as information 
accumulates during a standard minimal QSO.

Suggested test applications for some of the FST4 and FST4W submodes 
include the following:

FST4W-120A, -300A, -900A, -1800A WSPR-like transmissions on the 630 and 
2200 m bands
FST4-120A, -300A, … QSOs on the 630 and 2200 m bands
FST4-15A Ionospheric scatter (mid-day is best) on 50 MHz
FST4-15A, -30B, -30C, -30D Fast (contest?) EME QSOs on 144 – 1296 MHz ?
FST4-120B, -120C, -300C, -300D EME on 50, 144, 432 MHz with weakest 
possible signals

Many other combinations are possible, and users will surely think of 
some we have not considered. As
just one example: VK7MO and VK7ZBX have had good success with 
non-line-of-sight optical
scattering communication over obstructed paths as long as 43 km, using 
LED arrays and FST4W.

Operators familiar with WSJT-X will find using the FST4 modes 
straightforward, so we touch here only
on a few differences. The decoder searches for decodable signals only 
within the selected F Tol rangeeither side of Rx Freq.

We suggest operating with File → Settings → General → Single decode
checked. (Note, however, that if more than one signal is present 
multiple decodes can occur within the F Tol range.) Other on-screen 
controls including Auto Seq behave as expected in other WSJT-X modes.

For EME, be sure to check the options Enable VHF and submode features 
and Decode after EME delay. Doppler tracking for the EME path is 
accomplished in the same way as with other WSJT-X modes.

To see the presently suggested default FST4 and FST4W frequencies in the 
drop-down band-change
control you may need to do a one-time reset. Go to File → Settings → 
Frequencies, then right-click on
the Working Frequencies table and select Reset.

FST4W behaves in most ways like the familiar WSPR mode. Sequence lengths 
of 120, 300, 900, and
1800 s are available. Rx Freq is always set to 1500 Hz and F Tol to 100, 
so the active FST4W range is
1400 to 1600 Hz. A new drop-down control just under the one for T/R 
sequence lengths provides a
round robin mode for scheduling transmissions (see Figure 1). If three 
operators agree in advance to
select the options 1/3, 2/3, and 3/3, their FST4W transmissions will 
occur in fixed sequence with no two
stations transmitting at once. Sequence 1 is the first sequence after 
00:00:00 UTC. For WSPR-like
scheduling behavior, select Random.

Optional noise blanking is available and appears to be effective in 
handling typical atmospheric noise on
LF and MF bands. A spinner control labeled NB nn % is located on the 
FST4 and FST4W main
window, just below the band-change control. Set this control to a 
suggested percentage of data samples
to be blanked. We have found that levels in the 10 – 15% range work well 
in summer conditions on LF/
MF bands, but you will probably want to experiment. Data displayed on 
the waterfall and saved to .wav
files do not have blanking applied.

Figure 1: Controls for “Round Robin” scheduling and other features for 
FST4W mode.
As an example of capabilities of the new modes, Figure 2 shows decodes 
of FST4W-300A signals received on the 2200 m band at NO3M on July 23, 
2020. The weakest of these signals are close to the decoding threshold 
for this submode, SNR = -35 dB. Numbers at the end of each decoded line 
are measurements of path Doppler spread in Hz. To activate this feature, 
create a file named plotspec in the current working directory and start 
WSJT-X from the command line there. As a general rule, decoding requires 
Doppler spreads less than the submode tone spacing. Sensitivity is best 
when Doppler pread is no more than 1/8 of the tone spacing.

As with other modes in WSJT-X, our development of FST4 started by coding 
a simulator to generate Signals in any of the submodes with specified 
SNR, Doppler spread, and multi-path delays. The simulator is called 
fst4sim, and is part of the test distribution package. Feel free to use 
it for your own tests.In early testing we will appreciate your detailed 
feedback on both successes and failures. The FST4 decoder makes internal 
adaptations that depend on the degree of coherence found in received 
signals. Final optimizations will depend on having representative 
recordings of signals in propagation conditions
of particular interest. Please use Save All or Save Decoded and preserve 
the recorded .wav files for laterstudy.

Members of the initial test group fall in two broad and overlapping 
categories according to current
interests known to us, as follows:

LF and MF: 2E0ILY KA1R K3MF K3RWR N1BUG N6LF NO3M VK4YB W1VD W7IUV
EME: G3WDG HB9Q K3MF K5DOG KA1GT PA2V VK4EME VK7MO W2HRO

Many useful tests will need pre-arrangement by two or more group 
members. Among our highest
priority needs are saved .wav files from two-way QSOs over significant 
distances on the 2200 and 630 m
bands, and decodable EME signals on all bands — but especially 50, 144, 
432, and 1296 MHz, where
FST4 is likely to be most useful SOURCE:LINK

https://jl1jvt.cocolog-nifty.com/.../fst4_quick_start.pdf 
<https://l.facebook.com/l.php?u=https%3A%2F%2Fjl1jvt.cocolog-nifty.com%2Fblog%2Ffiles%2Ffst4_quick_start.pdf%3Ffbclid%3DIwAR15N7jUhBcZLe4uEvrx2G9e1Xbp3poBqbGAs6S1cX2oGoDkzJk_ZZ2Dq9Y&h=AT0Mlk5nXMCFTfNzEwRRmAQdhYLbod468Hv7K7TRKwNFOyMccEpJi09bFimdP78CxzlCVvEpQml_fJ7aaEU05qXXp5ULloz-uM4niVwNOxNo0hIziLxr2uej-b-6S37Z&__tn__=-UK-y0.g-R&c[0]=AT14X9sZry6Eci45cj4YTvOiuaXEPJI-tb4y4Xyq8QgYg-K76ZSaqLaN1Pw9sZbwhzZwAjWp9ERz9Ewep4KSkhGHf6QY6-xY4ci1L2-d4wEErtOCTYWR5tGYLQal0H7cr6FEoto_sdQwv41vvMo09u51uolYpADlkSuRu0u3D0_8URL2bfN7R5BZczOemj2yFfPM1daZKg>
#hamradio 
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#Digital_modes 
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#FT8 
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#FT4 
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#FST4 
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