[[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.  On these bands their
fundamental sensitivities are better than other _WSJT-X_ modes with the
same sequence lengths, approaching the theoretical limits for their
rates of information throughput.  FST4 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 time
synchronization and phase locking 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.
