A few more updates tothe User Guide, making it "minimally acceptable" for the GA release of v2.0.

This commit is contained in:
Joe Taylor 2018-12-04 15:05:47 -05:00
parent e9aebea074
commit 6517adbfbc
2 changed files with 38 additions and 42 deletions

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@ -8,9 +8,9 @@ false decodes.
For example: when you decide to answer a CQ, you already know your own
callsign and that of your potential QSO partner. The software
therefore "`knows`" what to expect for 57 of the 72 message bits (28
bits for each of two callsigns, 1 bit for message type) in the next
received message. The decoder's task can thus be reduced to
therefore "`knows`" what might be expected for at least 57 message
bits (28 for each of two callsigns, 1 or more for message type) in the
next received message. The decoder's task can thus be reduced to
determining the remaining 15 bits of the message and ensuring that the
resulting solution is reliable.

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@ -2,18 +2,20 @@
=== Overview
All QSO modes except ISCAT use structured messages that compress
user-readable information into fixed-length packets of 72 bits. Each
message consists of two 28-bit fields normally used for callsigns and
a 15-bit field for a grid locator, report, acknowledgment, or 73. An
additional bit flags a message containing arbitrary alphanumeric text,
up to 13 characters. Special cases allow other information such as
add-on callsign prefixes (e.g., ZA/K1ABC) or suffixes (e.g., K1ABC/P)
to be encoded. The basic aim is to compress the most common messages
used for minimally valid QSOs into a fixed 72-bit length. The
information payload in FT8 includes 3 additional bits (75 bits total).
One of the added bits is used to flag special messages used by the
DXpedition station in FT8 DXpedition Mode. Uses for the remaining two
bits are yet to be defined.
user-readable information into fixed-length packets. JT4, JT9, JT65,
and QRA64 use 72-bit payloads. Standard messages consist of two
28-bit fields normally used for callsigns and a 15-bit field for a
grid locator, report, acknowledgment, or 73. An additional bit flags
a message containing arbitrary free text, up to 13 characters.
Special cases allow other information such as add-on callsign prefixes
(e.g., ZA/K1ABC) or suffixes (e.g., K1ABC/P) to be encoded. The basic
aim is to compress the most common messages used for minimally valid
QSOs into a fixed 72-bit length.
The information payload for FT8 and MSK144 contains 77 bits. The 5
additional bits are used to flag special message types used for FT8
DXpedition Mode, contesting, nonstandard callsigns, and a few other
special types.
A standard amateur callsign consists of a one- or two-character
prefix, at least one of which must be a letter, followed by a digit
@ -42,22 +44,16 @@ additional information is sent in place of the grid locator or by
encoding additional information into some of the 6 million available
slots mentioned above.
As a convenience for sending directed CQ messages, the compression
algorithm supports messages starting with `CQ AA` through `CQ ZZ`.
These message fragments are encoded internally as if they were the
callsigns `E9AA` through `E9ZZ`. Upon reception they are converted
back to the form `CQ AA` through `CQ ZZ`, for display to the user.
As a convenience for sending directed CQ messages, the 72-bit
compression algorithm supports messages starting with `CQ AA` through
`CQ ZZ`. These message fragments are encoded internally as if they
were the callsigns `E9AA` through `E9ZZ`. Upon reception they are
converted back to the form `CQ AA` through `CQ ZZ`, for display to the
user.
The FT8 and MSK144 modes support a special feature allowing convenient
transmission and acknowledgment of four-character grid locators, the
required exchanges in most North American VHF contests. With this
Contest Mode enabled, _WSJT-X_ supports messages of the form `W9XYZ
K1ABC R FN42` by converting the grid locator to that of its
diametrically opposite point on Earth. The receiving program
recognizes a locator implying a distance greater than 10,000 km, does
the reverse transformation, and inserts the implied "`R`". Obviously,
this mode should not be used on the HF bands or under other
circumstances where world-wide propagation is possible.
The new FT8 and MSK144 protocols use a different lossless compression
algorithm with features to generate and recognize the special messages
used for contesting and the like. (More to come, here ...)
To be useful on channels with low signal-to-noise ratio, this kind of
lossless message compression requires use of a strong forward error
@ -75,9 +71,9 @@ _WSJT-X_ modes have continuous phase and constant envelope.
==== FT8
Forward error correction (FEC) in FT8 uses a low-density parity check
(LDPC) code with 75 information bits, a 12-bit cyclic redundancy check
(CRC), and 87 parity bits making a 174-bit codeword. It is thus
called an LDPC (174,87) code. Synchronization uses 7×7 Costas arrays
(LDPC) code with 77 information bits, a 14-bit cyclic redundancy check
(CRC), and 83 parity bits making a 174-bit codeword. It is thus
called an LDPC (174,91) code. Synchronization uses 7×7 Costas arrays
at the beginning, middle, and end of each transmission. Modulation is
8-tone frequency-shift keying (8-FSK) at 12000/1920 = 6.25 baud. Each
transmitted symbol carries three bits, so the total number of channel
@ -231,7 +227,7 @@ which the probability of decoding is 50% or higher.
|===============================================================================
|Mode |FEC Type |(n,k) | Q|Modulation type|Keying rate (Baud)|Bandwidth (Hz)
|Sync Energy|Tx Duration (s)|S/N Threshold (dB)
|FT8 |LDPC, r=1/2|(174,87)| 8| 8-FSK| 6.25 | 50.0 | 0.27| 12.6 | -21
|FT8 |LDPC, r=1/2|(174,91)| 8| 8-FSK| 6.25 | 50.0 | 0.27| 12.6 | -21
|JT4A |K=32, r=1/2|(206,72)| 2| 4-FSK| 4.375| 17.5 | 0.50| 47.1 | -23
|JT9A |K=32, r=1/2|(206,72)| 8| 9-FSK| 1.736| 15.6 | 0.19| 49.0 | -27
|JT65A |Reed Solomon|(63,12) |64|65-FSK| 2.692| 177.6 | 0.50| 46.8 | -25
@ -329,13 +325,13 @@ For details see Table 4, below.
==== MSK144
Standard MSK144 messages are structured in the same way as those in
the slow modes, with 72 bits of user information. Forward error
correction is implemented by first augmenting the 72 message bits with
an 8-bit cyclic redundancy check (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
Standard MSK144 messages are structured in the same way as in FT8,
with 77 bits of user information. Forward error correction is
implemented by first augmenting the 77 message bits with a 13-bit
cyclic redundancy check (CRC) calculated from the message bits. The
CRC is used to detect and eliminate most false decodes at the
receiver. The resulting 90-bit augmented message is mapped to a
128-bit codeword using a (128,90) 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
@ -379,6 +375,6 @@ and your QSO partner ± 200 Hz.
|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 |(128,80)| 2| OQPSK| 2000 | 2400 | 0.11| 0.072
|MSK144 |LDPC |(128,90)| 2| OQPSK| 2000 | 2400 | 0.11| 0.072
|MSK144 Sh|LDPC |(32,16) | 2| OQPSK| 2000 | 2400 | 0.20| 0.020
|=====================================================================