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Starting to fill in the "Protocol Specifications" section of User Guide.

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git-svn-id: svn+ssh://svn.code.sf.net/p/wsjt/wsjt/branches/wsjtx@6657 ab8295b8-cf94-4d9e-aec4-7959e3be5d79
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Joe Taylor 2016-04-28 18:59:34 +00:00
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doc/user_guide/en/protocols.adoc
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@ -90,7 +90,7 @@ Hz, the inverse of the symbol duration. The total occupied bandwidth
is 9 × 1.736 = 15.6 Hz.
[[PROTOCOL_SUMMARY]]
=== Summary
=== Comparison of Slow Modes
Frequency spacing between tones, total occupied bandwidth, and
approximate decoding thresholds are given for the various submodes of
@ -135,3 +135,59 @@ JT9 is an order of magnitude better than JT65 in spectral
efficiency. On a busy HF band, the conventional 2-kHz-wide JT65
sub-band is often filled with overlapping signals. Ten times as many
JT9 signals can fit into the same frequency range, without collisions.
=== 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 the message length, 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:
----
0132AA@CQ WA9XYZ@CQ WA9X0132AAYZ@CQ WA9XYZ@CQ W0132AAA9X ...
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.
=== JTMSK
The letters MS are often used to abbreviate meteor scatter; the three
letters MSK mean "Minimum Shift Keying", the modulation scheme used in
JTMSK. This mode uses the same standard message structure as the slow
modes JT4, JT9, and JT65. User information is "`source encoded`" to
72 bits; a 15-bit cyclic redundancy check (CRC) is appended, and a
convolutional code with constraint length K=13 and rate r=1/2 is then
applied. This procedure makes for a total of (72+15+12)*2 = 198
information bits for the encoded message. Three copies of the
"`Barker-11`" code and three even-parity bits are added for
synchronization, making a total of 198+33+3 = 234 channel symbols.
Modulation is carried out using a constant-envelope, continuous-phase
minimum-shift keying (MSK) waveform, with tone frequencies of 1000
and 2000 Hz.