337 lines
16 KiB
Plaintext
337 lines
16 KiB
Plaintext
[[PROTOCOL_OVERVIEW]]
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=== Overview
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All QSO modes except ISCAT use structured messages that compress
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user-readable information into fixed-length packets of 72 bits. Each
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message consists of two 28-bit fields normally used for callsigns and
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a 15-bit field for a grid locator, report, acknowledgment, or 73. An
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additional bit flags a message containing arbitrary alphanumeric text,
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up to 13 characters. Special cases allow other information such as
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add-on callsign prefixes (e.g., ZA/K1ABC) or suffixes (e.g., K1ABC/P)
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to be encoded. The basic aim is to compress the most common messages
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used for minimally valid QSOs into a fixed 72-bit length. Information
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payloads in FT8 include 3 additional bits (75 bits total), with
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definitions yet to be defined.
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A standard amateur callsign consists of a one- or two-character
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prefix, at least one of which must be a letter, followed by a digit
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and a suffix of one to three letters. Within these rules, the number
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of possible callsigns is equal to 37×36×10×27×27×27, or somewhat over
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262 million. (The numbers 27 and 37 arise because in the first and
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last three positions a character may be absent, or a letter, or
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perhaps a digit.) Since 2^28^ is more than 268 million, 28 bits are
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enough to encode any standard callsign uniquely. Similarly, the number
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of 4-digit Maidenhead grid locators on earth is 180×180 = 32,400,
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which is less than 2^15^ = 32,768; so a grid locator requires 15 bits.
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Some 6 million of the possible 28-bit values are not needed for
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callsigns. A few of these slots have been assigned to special message
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components such as `CQ`, `DE`, and `QRZ`. `CQ` may be followed by three
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digits to indicate a desired callback frequency. (If K1ABC transmits
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on a standard calling frequency, say 50.280, and sends `CQ 290 K1ABC
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FN42`, it means that s/he will listen on 50.290 and respond there to
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any replies.) A numerical signal report of the form `–nn` or
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`R–nn` can be sent in place of a grid locator. (As originally
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defined, numerical signal reports `nn` were required to fall between -01
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and -30 dB. Recent program versions accommodate reports between
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-50 and +49 dB.) A country prefix or portable suffix may be
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attached to one of the callsigns. When this feature is used the
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additional information is sent in place of the grid locator or by
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encoding additional information into some of the 6 million available
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slots mentioned above.
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Finally, the message compression algorithm supports messages starting
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with `CQ AA` through `CQ ZZ`. Such messages are encoded by
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sending `E9AA` through `E9ZZ` in place of the first callsign of a
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standard message. Upon reception these calls are converted back to
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the form `CQ AA` through `CQ ZZ`.
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To be useful on channels with low signal-to-noise ratio, this kind of
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lossless message compression requires use of a strong forward error
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correcting (FEC) code. Different codes are used for each mode.
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Accurate synchronization of time and frequency is required between
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transmitting and receiving stations. As an aid to the decoders, each
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protocol includes a "`sync vector`" of known symbols interspersed with
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the information-carrying symbols. Generated waveforms for all of the
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_WSJT-X_ modes have continuous phase and constant envelope.
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[[SLOW_MODES]]
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=== Slow Modes
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[[FT8PRO]]
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==== FT8
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Forward error correction (FEC) in FT8 uses a low-density
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parity check (LDPC) code with 75 information bits, a 12-bit cyclic
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redundancy check (CRC), and 174 channel symbols. It is thus called an
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LDPC (174,87) code. Synchronization uses 7×7 Costas arrays at the
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beginning, middle, and end of each transmission. Modulation is 8-tone
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frequency-shift keying (8-FSK) at 12000/1920 = 6.25 baud. Each
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transmitted symbol carries three bits, so the total number of channel
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symbols is 174/3 + 21 = 79. The total occupied bandwidth is
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8 × 6.25 = 50 Hz.
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[[JT4PRO]]
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==== JT4
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FEC in JT4 uses a strong convolutional code with constraint length
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K=32, rate r=1/2, and a zero tail. This choice leads to an encoded
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message length of (72+31) x 2 = 206 information-carrying bits.
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Modulation is 4-tone frequency-shift keying (4-FSK) at 11025 / 2520 =
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4.375 baud. Each symbol carries one information bit (the most
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significant bit) and one synchronizing bit. The two 32-bit
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polynomials used for convolutional encoding have hexadecimal values
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0xf2d05351 and 0xe4613c47, and the ordering of encoded bits is
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scrambled by an interleaver. The pseudo-random sync vector is the
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following sequence (60 bits per line):
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000011000110110010100000001100000000000010110110101111101000
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100100111110001010001111011001000110101010101111101010110101
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011100101101111000011011000111011101110010001101100100011111
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10011000011000101101111010
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[[JT9PRO]]
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==== JT9
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FEC in JT9 uses the same strong convolutional code as JT4: constraint
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length K=32, rate r=1/2, and a zero tail, leading to an encoded
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message length of (72+31) × 2 = 206 information-carrying
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bits. Modulation is nine-tone frequency-shift keying, 9-FSK at
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12000.0/6912 = 1.736 baud. Eight tones are used for data, one for
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synchronization. Eight data tones means that three data bits are
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conveyed by each transmitted information symbol. Sixteen symbol
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intervals are devoted to synchronization, so a transmission requires a
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total of 206 / 3 + 16 = 85 (rounded up) channel symbols. The sync
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symbols are those numbered 1, 2, 5, 10, 16, 23, 33, 35, 51, 52, 55,
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60, 66, 73, 83, and 85 in the transmitted sequence. Tone spacing of
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the 9-FSK modulation for JT9A is equal to the keying rate, 1.736 Hz.
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The total occupied bandwidth is 9 × 1.736 = 15.6 Hz.
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[[JT65PRO]]
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==== JT65
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A detailed description of the JT65 protocol was published in
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{jt65protocol} for September-October, 2005. A Reed Solomon (63,12)
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error-control code converts 72-bit user messages into sequences of 63
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six-bit information-carrying symbols. These are interleaved with
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another 63 symbols of synchronizing information according to the
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following pseudo-random sequence:
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100110001111110101000101100100011100111101101111000110101011001
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101010100100000011000000011010010110101010011001001000011111111
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The synchronizing tone is normally sent in each interval having a
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"`1`" in the sequence. Modulation is 65-FSK at 11025/4096 = 2.692
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baud. Frequency spacing between tones is equal to the keying rate for
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JT65A, and 2 and 4 times larger for JT65B and JT65C. For EME QSOs the
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signal report OOO is sometimes used instead of numerical signal
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reports. It is conveyed by reversing sync and data positions in the
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transmitted sequence. Shorthand messages for RO, RRR, and 73 dispense
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with the sync vector entirely and use time intervals of 16384/11025 =
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1.486 s for pairs of alternating tones. The lower frequency is the
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same as that of the sync tone used in long messages, and the frequency
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separation is 110250/4096 = 26.92 Hz multiplied by n for JT65A, with n
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= 2, 3, 4 used to convey the messages RO, RRR, and 73.
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[[QRA64_PROTOCOL]]
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==== QRA64
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QRA64 is an experimental mode intended for EME and other extreme
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weak-signal applications. Its internal code was designed by IV3NWV.
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The protocol uses a (63,12) **Q**-ary **R**epeat **A**ccumulate code
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that is inherently better than the Reed Solomon (63,12) code used in
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JT65, yielding a 1.3 dB advantage. A new synchronizing scheme is based
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on three 7 x 7 Costas arrays. This change yields another 1.9 dB
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advantage.
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In most respects the current implementation of QRA64 is operationally
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similar to JT65. QRA64 does not use two-tone shorthand messages, and
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it makes no use of a callsign database. Rather, additional
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sensitivity is gained by making use of already known information as a
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QSO progresses -- for example, when reports are being exchanged and
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you have already decoded both callsigns in a previous transmission.
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QRA64 presently offers no message averaging capability, though that
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feature may be added. In early tests, many EME QSOs were made using
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submodes QRA64A-E on bands from 144 MHz to 24 GHz.
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[[SLOW_SUMMARY]]
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==== Summary
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Table 2 provides a brief summary parameters for the slow modes in
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_WSJT-X_. Parameters K and r specify the constraint length and rate
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of the convolutional codes; n and k specify the sizes of the
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(equivalent) block codes; Q is the alphabet size for the
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information-carrying channel symbols; Sync Energy is the fraction of
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transmitted energy devoted to synchronizing symbols; and S/N Threshold
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is the signal-to-noise ratio (in a 2500 Hz reference bandwidth) above
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which the probability of decoding is 50% or higher.
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[[SLOW_TAB]]
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.Parameters of Slow Modes
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[width="90%",cols="3h,^3,^2,^1,^2,^2,^2,^2,^2,^2",frame=topbot,options="header"]
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|===============================================================================
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|Mode |FEC Type |(n,k) | Q|Modulation type|Keying rate (Baud)|Bandwidth (Hz)
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|Sync Energy|Tx Duration (s)|S/N Threshold (dB)
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|FT8 |LDPC, r=1/2|(174,87)| 8| 8-FSK| 6.25 | 50.0 | 0.27| 12.6 | -21
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|JT4A |K=32, r=1/2|(206,72)| 2| 4-FSK| 4.375| 17.5 | 0.50| 47.1 | -23
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|JT9A |K=32, r=1/2|(206,72)| 8| 9-FSK| 1.736| 15.6 | 0.19| 49.0 | -27
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|JT65A |Reed Solomon|(63,12) |64|65-FSK| 2.692| 177.6 | 0.50| 46.8 | -25
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|QRA64A|Q-ary Repeat Accumulate|(63,12) |64|64-FSK|1.736|111.1|0.25|48.4| -26
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| WSPR |K=32, r=1/2|(162,50)| 2| 4-FSK| 1.465| 5.9 | 0.50|110.6 | -28
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|===============================================================================
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Submodes of JT4, JT9, JT65, and QRA64 offer wider tone spacings for
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circumstances that may require them, such significant Doppler spread.
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Table 3 summarizes the tone spacings, bandwidths, and approximate
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threshold sensitivities of the various submodes when spreading is
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comparable to tone spacing.
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[[SLOW_SUBMODES]]
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.Parameters of Slow Submodes
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[width="50%",cols="h,3*^",frame=topbot,options="header"]
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|=====================================
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|Mode |Tone Spacing |BW (Hz)|S/N (dB)
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|FT8 |6.25 | 50.0 |-21
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|JT4A |4.375| 17.5 |-23
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|JT4B |8.75 | 30.6 |-22
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|JT4C |17.5 | 56.9 |-21
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|JT4D |39.375| 122.5 |-20
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|JT4E |78.75| 240.6 |-19
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|JT4F |157.5| 476.9 |-18
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|JT4G |315.0| 949.4 |-17
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|JT9A |1.736| 15.6 |-27
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|JT9B |3.472| 29.5 |-26
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|JT9C |6.944| 57.3 |-25
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|JT9D |13.889| 112.8 |-24
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|JT9E |27.778| 224.0 |-23
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|JT9F |55.556| 446.2 |-22
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|JT9G |111.111|890.6 |-21
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|JT9H |222.222|1779.5|-20
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|JT65A |2.692| 177.6 |-25
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|JT65B |5.383| 352.6 |-25
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|JT65C |10.767| 702.5 |-25
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|QRA64A|1.736| 111.1 |-26
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|QRA64B|3.472| 220.5 |-25
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|QRA64C|6.944| 439.2 |-24
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|QRA64D|13.889| 876.7 |-23
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|QRA64E|27.778|1751.7 |-22
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|=====================================
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[[FAST_MODES]]
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=== Fast Modes
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==== ISCAT
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ISCAT messages are free-form, up to 28 characters in length.
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Modulation is 42-tone frequency-shift keying at 11025 / 512 = 21.533
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baud (ISCAT-A), or 11025 / 256 = 43.066 baud (ISCAT-B). Tone
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frequencies are spaced by an amount in Hz equal to the baud rate. The
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available character set is:
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----
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0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ /.?@-
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----
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Transmissions consist of sequences of 24 symbols: a synchronizing
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pattern of four symbols at tone numbers 0, 1, 3, and 2, followed by
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two symbols with tone number corresponding to (message length) and
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(message length + 5), and finally 18 symbols conveying the user's
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message, sent repeatedly character by character. The message always
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starts with `@`, the beginning-of-message symbol, which is not
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displayed to the user. The sync pattern and message-length indicator
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have a fixed repetition period, recurring every 24 symbols. Message
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information occurs periodically within the 18 symbol positions set
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aside for its use, repeating at its own natural length.
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For example, consider the user message `CQ WA9XYZ`. Including the
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beginning-of-message symbol `@`, the message is 10 characters long.
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Using the character sequence displayed above to indicate tone numbers,
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the transmitted message will therefore start out as shown in the first
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line below:
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----
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0132AF@CQ WA9XYZ@CQ WA9X0132AFYZ@CQ WA9XYZ@CQ W0132AFA9X ...
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sync## sync## sync##
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----
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Note that the first six symbols (four for sync, two for message
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length) repeat every 24 symbols. Within the 18 information-carrying
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symbols in each 24, the user message `@CQ WA9XYZ` repeats at its own
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natural length, 10 characters. The resulting sequence is extended as
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many times as will fit into a Tx sequence.
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==== JT9
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The JT9 slow modes all use keying rate 12000/6912 = 1.736 baud. By contrast, with
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the *Fast* setting submodes JT9E-H adjust the keying rate to match the
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increased tone spacings. Message durations are therefore much
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shorter, and they are sent repeatedly throughout each Tx sequence.
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For details see Table 4, below.
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==== MSK144
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Standard MSK144 messages are structured in the same way as those in
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the slow modes, with 72 bits of user information. Forward error
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correction is implemented by first augmenting the 72 message bits with
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an 8-bit cyclic redundancy check (CRC) calculated from the message
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bits. The CRC is used to detect and eliminate most false decodes at
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the receiver. The resulting 80-bit augmented message is mapped to a
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128-bit codeword using a (128,80) binary low-density-parity-check
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(LDPC) code designed by K9AN specifically for this purpose. Two 8-bit
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synchronizing sequences are added to make a message frame 144 bits
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long. Modulation is Offset Quadrature Phase-Shift Keying (OQPSK) at
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2000 baud. Even-numbered bits are conveyed over the in-phase channel,
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odd-numbered bits on the quadrature channel. Individual symbols are
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shaped with half-sine profiles, thereby ensuring a generated waveform
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with constant envelope, equivalent to a Minimum Shift Keying (MSK)
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waveform. Frame duration is 72 ms, so the effective character
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transmission rate for standard messages is up to 250 cps.
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Contest Mode in MSK144 conveys an additional acknowledgment bit (the
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"`R`" in a message of the form `W9XYZ K1ABC R FN42`) by using the fact
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that meteor scatter and other propagation modes usable with MSK144 are
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generally effective only out to distances of order 2500 km. To convey
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the message fragment `R FN42`, WSJT-X encodes the locator as that of
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its antipodes. The receiving program recognizes a locator with
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distance greater than 10,000 km, does the reverse transformation, and
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inserts the implied "`R`".
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MSK144 also supports short-form messages that can be used after QSO
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partners have exchanged both callsigns. Short messages consist of 4
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bits encoding R+report, RRR, or 73, together with a 12-bit hash code
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based on the ordered pair of "`to`" and "`from`" callsigns. Another
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specially designed LDPC (32,16) code provides error correction, and an
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8-bit synchronizing vector is appended to make up a 40-bit frame.
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Short-message duration is thus 20 ms, and short messages can be
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decoded from very short meteor pings.
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The 72 ms or 20 ms frames of MSK144 messages are repeated without gaps
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for the full duration of a transmission cycle. For most purposes, a
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cycle duration of 15 s is suitable and recommended for MSK144.
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The modulated MSK144 signal occupies the full bandwidth of a SSB
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transmitter, so transmissions are always centered at audio frequency
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1500 Hz. For best results, transmitter and receiver filters should be
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adjusted to provide the flattest possible response over the range
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300Hz to 2700Hz. The maximum permissible frequency offset between you
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and your QSO partner ± 200 Hz.
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==== Summary
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.Parameters of Fast Modes
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[width="90%",cols="3h,^3,^2,^1,^2,^2,^2,^2,^2",frame="topbot",options="header"]
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|=====================================================================
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|Mode |FEC Type |(n,k) | Q|Modulation Type|Keying rate (Baud)
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|Bandwidth (Hz)|Sync Energy|Tx Duration (s)
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|ISCAT-A | - | - |42|42-FSK| 21.5 | 905 | 0.17| 1.176
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|ISCAT-B | - | - |42|42-FSK| 43.1 | 1809 | 0.17| 0.588
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|JT9E |K=32, r=1/2|(206,72)| 8| 9-FSK| 25.0 | 225 | 0.19| 3.400
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|JT9F |K=32, r=1/2|(206,72)| 8| 9-FSK| 50.0 | 450 | 0.19| 1.700
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|JT9G |K=32, r=1/2|(206,72)| 8| 9-FSK|100.0 | 900 | 0.19| 0.850
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|JT9H |K=32, r=1/2|(206,72)| 8| 9-FSK|200.0 | 1800 | 0.19| 0.425
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|MSK144 |LDPC |(128,80)| 2| OQPSK| 2000 | 2400 | 0.11| 0.072
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|MSK144 Sh|LDPC |(32,16) | 2| OQPSK| 2000 | 2400 | 0.20| 0.020
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|=====================================================================
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