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

[[PROTOCOL_OVERVIEW]]
=== Overview

All QSO modes except ISCAT benefit from the use of structured
messages.  Each such message consists of two 28-bit fields for
callsigns and a 15-bit field for a grid locator, report,
acknowledgment, or a "`73`" sign-off indicator.  Alternatively, a
72^nd^ bit flags a message containing arbitrary alphanumeric text, up
to 13 characters.  Special formats allow other information such as
add-on callsign prefixes (e.g., ZA/K1ABC) or suffixes (e.g., K1ABC/4)
to be encoded. The basic aim is to compress the most common messages
used for minimally valid QSOs into a fixed 72-bit length.  To be
useful, this kind of lossless message compression requires use of a
strong forward error correcting (FEC) code.  Different FEC codes are
used for each mode.  These modes require good synchronization of time
and frequency between transmitting and receiving stations.  As an aid
to the decoders, each protocol includes a "`synch vector`" of known
symbols along with the information-carrying symbols.  Generated
waveforms for all of the _WSJT-X_ modes have continuous phase and
a constant envelope.

[[JT4PRO]]
=== JT4

FEC in JT4 uses a strong convolutional code with constraint length
K=32, rate r=1/2, and a zero tail. This choice leads to an encoded
message length of (72+31) x 2 = 206 information-carrying bits.
Modulation is 4-tone frequency-shift keying (4-FSK) at 11025 / 2520 =
4.375 baud.  Each symbol carries one information bit (the most
significant bit) and one synchronizing bit.  The pseudo-random sync
vector is the following sequence:

 000011000110110010100000001100000000000010110110101111101000
 100100111110001010001111011001000110101010101111101010110101
 011100101101111000011011000111011101110010001101100100011111
 10011000011000101101111010

The two 32-bit polynomials used for convolutional encoding have
hexadecimal values f2d05351 and e4613c47.

[[JT9PRO]]
=== JT9

FEC in JT9 uses the same strong convolutional code aa JT4: constraint
length K=32, rate r=1/2, and a zero tail, leading to an encoded
message length of (72+31) × 2 = 206 information-carrying
bits. Modulation is nine-tone frequency-shift keying, 9-FSK at
12000.0/6912 = 1.736 baud.  Eight tones are used for data, one for
synchronization. Eight data tones means that three data bits are
conveyed by each transmitted information symbol. Sixteen symbol
intervals are devoted to synchronization, so a transmission requires a
total of 206 / 3 + 16 = 85 (rounded up) channel symbols. The sync
symbols are those numbered 1, 2, 5, 10, 16, 23, 33, 35, 51, 52, 55,
60, 66, 73, 83, and 85 in the transmitted sequence.  Tone spacing of
the 9-FSK modulation for JT9A is equal to the keying rate, 1.736 Hz.
The total occupied bandwidth is 9 × 1.736 = 15.6 Hz.

[[JT65PRO]]
=== JT65

A detailed description of the JT65 protocol was published in
{jt65protocol} for September-October, 2005. A Reed Solomon (63,12)
error-control code converts 72-bit user messages into sequences of 63
six-bit information-carrying symbols.  These are interleaved with
another 63 symbols of synchronizing information according to the
following pseudo-random sequence:

 100110001111110101000101100100011100111101101111000110101011001
 101010100100000011000000011010010110101010011001001000011111111


The synchronizing tone is normally sent in each interval having a
"`1`" in the sequence. Modulation is 65-FSK at 11025/4096 = 2.692
baud.  Frequency spacing between tones is equal to the keying rate for
JT65A, and 2 and 4 times larger for JT65B and JT65C.  For EME QSOs the
signal report OOO is sometimes used instead of numerical signal
reports. It is conveyed by reversing sync and data positions in the
transmitted sequence.  Shorthand messages for RO, RRR, and 73 dispense
with the sync vector entirely and use time intervals of 16384/11025 =
1.486 s for pairs of alternating tones. The lower frequency is the
same as that of the sync tone used in long messages, and the frequency
separation is 110250/4096 = 26.92 Hz multiplied by n for JT65A, with n
= 2, 3, 4 used to convey the messages RO, RRR, and 73.

[[QRA64_PROTOCOL]]
=== QRA64

Still to come ...

[[SLOW_SUMMARY]]
=== Slow Mode Summary

[[SLOW_TAB]]
.Parameters of Slow Modes
[width="90%",cols="3h,^3,^2,^1,^2,^2,^2,^2,^2,^2",frame=topbot,options="header"]
|===============================================================================
|Mode  |FEC Type   |(k,n)   | Q|  Mod | Baud |BW (Hz)|fSync|TxT (s)|S/N (dB)
|JT4A  |K=32, r=1/2|(206,72)| 2| 4-FSK| 4.375|  17.5 | 0.50| 47.1   | -23
|JT9A  |K=13, r=1/2|(206,72)| 8| 9-FSK| 1.736|  15.6 | 0.19| 49.0   | -27
|JT65A |RS         |(63,12) |64|65-FSK| 2.692| 177.6 | 0.50| 46.8   | -25
|QRA64A|QRA        |(63,12) |64|64-FSK| 1.736| 111.1 | 0.25| 48.4   | -28
| WSPR |K=32, r=1/2|(162,50)| 2| 4-FSK| 1.465|   5.9 | 0.50|110.6   | -29
|===============================================================================

Frequency spacing between tones, total occupied bandwidth, and
approximate threshold signal-to-noise ratios are given for the various
submodes of JT4, JT9, JT65, and QRA64 in the following table:

 Submode Spacing   BW    S/N
           (Hz)   (Hz)    dB
 ----------------------------
 JT4A     4.375   17.5   -23
 JT4B     8.75    35.0   -22
 JT4C    17.5     70.0   -21
 JT4D    39.375  157.5   -20
 JT4E    78.75   315.0   -19
 JT4F    157.5   630.0   -18
 JT4G    315.0  1260.0   -17

 JT9    1.7361  15.625   -27

 JT65A  2.6917   177.6   -25
 JT65B  5.3833   355.3   -24
 JT65C  10.767   710.6   -23

 QRA64A  1.736   111.1   -28?
 QRA64B  3.472   222.2   
 QRA64C  6.944   444.4
 QRA64D 13.889   888.9
 QRA64E 27.228  1777.8

JT4 and JT65 signal reports are constrained to the range –1 to –30
dB. This range is more than adequate for EME purposes, but not enough
for optimum use at HF. S/N values displayed by the JT4 and JT65
decoders are clamped at an upper limit –1 dB, and the S/N scale
becomes significantly nonlinear above –10 dB.  JT9 allows signal
reports in the range –50 to +49 dB. It manages this by taking over a
small portion of "`message space`" that would otherwise be used for
grid locators within 1 degree of the south pole. The S/N scale of the
present JT9 decoder is reasonably linear (although it's not intended
to be a precision measurement tool).

=== ISCAT

ISCAT messages are free-form, up to 28 characters in length.
Modulation is 42-tone frequency-shift keying at 11025 / 512 = 21.533
baud (ISCAT-A), or 11025 / 256 = 43.066 baud (ISCAT-B).  Tone
frequencies are spaced by an amount in Hz equal to the baud rate.  The
available character set is

----
 0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ /.?@-
----

Transmissions consist of sequences of 24 symbols: a synchronizing
pattern of four symbols at tone numbers 0, 1, 3, and 2, followed by
two symbols with tone number corresponding to (message length) and
(message length + 5), and finally 18 symbols conveying the user's
message, sent repeatedly character by character.  The message always
starts with +@+, the beginning-of-message symbol, which is not
displayed to the user.  The sync pattern and message-length indicator
have a fixed repetition period, recurring every 24 symbols.  Message
information occurs periodically within the 18 symbol positions set
aside for its use, repeating at its own natural length.

For example, consider the user message +CQ WA9XYZ+.  Including the
beginning-of-message symbol +@+, the message is 10 characters long.
Using the character sequence displayed above to indicate tone numbers,
the transmitted message will therefore start out as shown in the first
line below:

----
 0132AF@CQ WA9XYZ@CQ WA9X0132AFYZ@CQ WA9XYZ@CQ W0132AFA9X ...
 sync##                  sync##                 sync##
----

Note that the first six symbols (four for sync, two for message
length) repeat every 24 symbols.  Within the 18 information-carrying
symbols in each 24, the user message +@CQ WA9XYZ+ repeats at its own
natural length, 10 characters.  The resulting sequence is extended as
many times as will fit into a Tx sequence.

=== MSK144

Standard MSK144 messages are structured in the same way as those in
the slow modes, with a 72 bits of user information.  Forward error
correction is implemented by first augmenting the 72 message bits with
an 8-bit CRC calculated from the message bits. The CRC is used to
detect and eliminate most false decodes at the receiver. The resulting
80-bit augmented message is mapped to a 128-bit codeword using a
(128,80) binary low-density-parity-check (LDPC) code designed by K9AN
specifically for this purpose.  Two 8-bit synchronizing sequences are
added to make a message frame 144 bits long.  Modulation is Offset
Quadrature Phase-Shift Keying (OQPSK) at 2000 baud. Even-numbered bits
are conveyed over the in-phase channel, odd-numbered bits on the
quadrature channel.  Individual symbols are shaped with half-sine
profiles, thereby ensuring a generated waveform with constant
envelope, equivalent to a Minimum Shift Keying (MSK) waveform.  Frame
duration is 72 ms, so the effective character transmission rate for
standard messages is up to 250 cps.

MSK144 also supports short-form messages that can be used after QSO
partners have exchanged both callsigns.  Short messages consist of 4
bits encoding a signal report, R+report, RRR, or 73, together with a
12-bit hash code based on the ordered pair of "`to`" and "`from`"
callsigns.  Another specially designed LDPC (32,16) code provides
error correction, and an 8-bit synchronizing vector is appended to
make up a 40-bit frame.  Short-message duration is thus 20 ms, and
short messages can be decoded from very short meteor pings.

The 72 ms or 20 ms frames of MSK144 messages are repeated without gaps
for the full duration of a transmission cycle. For most purposes, a
cycle duration of 15 s is suitable and recommended for MSK144.

The modulated MSK144 signal occupies the full bandwidth of a SSB
transmitter, so transmissions are always centered at audio frequency
1500 Hz. For best results, transmitter and receiver filters should be
adjusted to provide the flattest possible response over the range
300Hz to 2700Hz. The maximum permissible frequency offset between you
and your QSO partner ± 200 Hz.

=== Fast Mode Summary

.Parameters of Fast Modes
[width="90%",cols="3h,^3,^2,^1,^2,^2,^2,^2,^2,^2",frame="topbot",options="header"]
|=============================================================================
|Mode     |FEC Type   |(k,n)   | Q|  Mod | Baud |BW (Hz)|fSync|TxT (s)|S/N (dB)
|ISCAT-A  |   -       |  -     |42|42-FSK| 21.5 |  905  | 0.17| 1.176  | 
|ISCAT-B  |   -       |  -     |42|42-FSK| 43.1 | 1809  | 0.17| 0.588  | 
|JT9E     |K=32, r=1/2|(206,72)| 8| 9-FSK| 25.0 |  225  | 0.19| 3.400  |  
|JT9F     |K=32, r=1/2|(206,72)| 8| 9-FSK| 50.0 |  450  | 0.19| 1.700  |  
|JT9G     |K=32, r=1/2|(206,72)| 8| 9-FSK|100.0 |  900  | 0.19| 0.850  |  
|JT9H     |K=32, r=1/2|(206,72)| 8| 9-FSK|200.0 | 1800  | 0.19| 0.425  |  
|MSK144   |LDPC       |(128,72)| 2| OQPSK| 2000 | 2000  | 0.11| 0.072  | -5
|MSK144 Sh|LDPC       |(32,16) | 2| OQPSK| 2000 | 2000  | 0.20| 0.020  | -5
|=============================================================================