WSJT-X/doc/user_guide/en/new_features.adoc

[[NEW_FEATURES]]
=== New in Version {VERSION}

_WSJT-X 2.3.0_ introduces *FST4* and *FST4W*, new digital protocols
designed particularly for the LF and MF bands.  Decoders for these
modes can take advantage of the very small Doppler spreads present at
these frequencies, even over intercontinental distances.  As a
consequence, fundamental sensitivities of FST4 and FST4W are better
than other _WSJT-X_ modes with the same sequence lengths, approaching
the theoretical limits for their rates of information throughput.  The
FST4 protocol is optimized for two-way QSOs, while FST4W is for
quasi-beacon transmissions of WSPR-style messages.  FST4 and FST4W do
not require the strict, independent phase locking and time
synchronization of modes like EbNaut.

The new modes use 4-GFSK modulation and share common software for
encoding and decoding messages.  FST4 offers T/R sequence lengths of
15, 30, 60, 120, 300, 900, and 1800 seconds, while FST4W omits the
lengths shorter than 120 s.  Submodes are given names like FST4-60,
FST4W-300, etc., the appended numbers indicating sequence length in
seconds.  Message payloads contain either 77 bits, as in FT4, FT8, and
MSK144, or 50 bits for the WSPR-like messages of FST4W.  Message
formats displayed to the user are like those in the other 77-bit and
50-bit modes in _WSJT-X_.  Forward error correction uses a low density
parity check (LDPC) code with 240 information and parity bits.
Transmissions consist of 160 symbols: 120 information-carrying symbols
of two bits each, interspersed with five groups of eight predefined
synchronization symbols.

*We recommend that on the 2200 and 630 m bands FST4 should replace JT9
for making 2-way QSOs, and FST4W should replace WSPR for propagation
tests*.  Operating conventions on these LF and MF bands will
eventually determine the most useful T/R sequence lengths for each
type of operation. We also expect that the 60 second variant of FST4
(FST4-60) will outperform JT9 for DX QSOs on HF bands due, in part,
to the FST4 decoder's ability to use AP decoding for messages received
from a QSO partner. In addition, FST4 provides the added benefits 
associated with 77-bit messages and auto-sequencing.