mirror of https://github.com/saitohirga/WSJT-X.git
404 lines
20 KiB
Plaintext
404 lines
20 KiB
Plaintext
//status: edited
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[[PROTOCOL_OVERVIEW]]
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=== Overview
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All QSO modes use structured messages that compress
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user-readable information into fixed-length packets. JT4, JT9, and JT65
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use 72-bit payloads. Standard messages consist of two
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28-bit fields normally used for callsigns and a 15-bit field for a
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grid locator, report, acknowledgment, or 73. An additional bit flags
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a message containing arbitrary free text, up to 13 characters.
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Special cases allow other information such as add-on callsign prefixes
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(e.g., ZA/K1ABC) or suffixes (e.g., K1ABC/P) to be encoded. The basic
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aim is to compress the most common messages used for minimally valid
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QSOs into a fixed 72-bit length.
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Information payloads for FST4, FT4, FT8, Q65, and MSK144 contain 77 bits.
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The 5 additional bits are used to flag special message types used for
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nonstandard callsigns, contest exchanges, FT8 DXpedition Mode, and a
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few other possibilities. Full details have been published in QEX, see
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{ft4_ft8_protocols}.
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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 are 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 such as 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 2.3 and later 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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As a convenience for sending directed CQ messages, the 72-bit
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compression algorithm supports messages starting with `CQ AA` through
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`CQ ZZ`. These message fragments are encoded internally as if they
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were the callsigns `E9AA` through `E9ZZ`. Upon reception they are
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converted back to the form `CQ AA` through `CQ ZZ`, for display to the
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user.
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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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[[FST4PRO]]
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==== FST4
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FST4 offers T/R sequence lengths of 15, 30, 60, 120, 300, 900, and
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1800 seconds. Submodes are given names like FST4-60, FST4-120, etc.,
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the appended numbers indicating sequence length in seconds. A 24-bit
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cyclic redundancy check (CRC) is appended to the 77-bit message
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payload to create a 101-bit message-plus-CRC word. Forward error
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correction is accomplished using a (240,101) LDPC code. Transmissions
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consist of 160 symbols: 120 information-carrying symbols of two bits
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each, interspersed with five groups of eight predefined
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synchronization symbols. Modulation uses 4-tone frequency-shift
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keying (4-GFSK) with Gaussian smoothing of frequency transitions.
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[[FT4PRO]]
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==== FT4
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Forward error correction (FEC) in FT4 uses a low-density parity check
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(LDPC) code with 77 information bits, a 14-bit cyclic redundancy check
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(CRC), and 83 parity bits making a 174-bit codeword. It is thus
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called an LDPC (174,91) code. Synchronization uses four 4×4 Costas
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arrays, and ramp-up and ramp-down symbols are inserted at the start
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and end of each transmission. Modulation is 4-tone frequency-shift
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keying (4-GFSK) with Gaussian smoothing of frequency transitions. The
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keying rate is 12000/576 = 20.8333 baud. Each transmitted symbol
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conveys two bits, so the total number of channel symbols is 174/2 + 16
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+ 2 = 105. The total bandwidth is 4 × 20.8333 = 83.3 Hz.
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[[FT8PRO]]
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==== FT8
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FT8 uses the same LDPC (174,91) code as FT4. Modulation is 8-tone
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frequency-shift keying (8-GFSK) at 12000/1920 = 6.25 baud.
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Synchronization uses 7×7 Costas arrays at the beginning, middle, and
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end of each transmission. Transmitted symbols carry three bits, so
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the total number of channel symbols is 174/3 + 21 = 79. The total
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occupied bandwidth is 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, respectively.
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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, respectively.
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[[Q65_PROTOCOL]]
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==== Q65
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Q65 is intended for scatter, EME, and other extreme weak-signal
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applications. Forward error correction (FEC) uses a specially
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designed (65,15) block code with six-bit symbols. Two symbols are
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“punctured” from the code and not transmitted, thereby yielding an
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effective (63,13) code with a payload of k = 13 information symbols
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conveyed by n = 63 channel symbols. The punctured symbols consist of
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a 12-bit CRC computed from the 13 information symbols. The CRC is
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used to reduce the false-decode rate to a very low value. A 22-symbol
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pseudorandom sequence spread throughout a transmission is sent as
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“tone 0” and used for synchronization. The total number of channel
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symbols in a Q65 transmission is thus 63 + 22 = 85. Q65 offers T/R
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sequence lengths of 15, 30, 60, 120, and 300 s, and submodes A - E
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have tone spacings 1, 2, 4, 8, and 16 times the symbol rate. Submode
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designations include a number for sequence length and a letter for
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tone spacing, as in Q65-15A, Q65-120C, etc. Occupied bandwidths are
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65 times the tone spacing, ranging from 19 Hz (Q65-300A) to a maximum
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of 1733 Hz (Q65-15C, Q65-30D, and Q65-60E).
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[[WSPR_PROTOCOL]]
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==== WSPR
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WSPR is designed for probing potential radio propagation paths using
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low power beacon-like transmissions. WSPR signals convey a callsign,
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Maidenhead grid locator, and power level using a compressed data
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format with strong forward error correction and narrow-band 4-FSK
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modulation. The protocol is effective at signal-to-noise ratios as low
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as –31 dB in a 2500 Hz bandwidth.
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WSPR messages can have one of three possible formats illustrated by
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the following examples:
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- Type 1: K1ABC FN42 37
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- Type 2: PJ4/K1ABC 37
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- Type 3: <PJ4/K1ABC> FK52UD 37
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Type 1 messages contain a standard callsign, a 4-character Maidenhead
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grid locator, and power level in dBm. Type 2 messages omit the grid
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locator but include a compound callsign, while type 3 messages replace
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the callsign with a 15-bit hash code and include a 6-character locator
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as well as the power level. Lossless compression techniques squeeze
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all three message types into exactly 50 bits of user
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information. Standard callsigns require 28 bits and 4-character grid
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locators 15 bits. In Type 1 messages, the remaining 7 bits convey the
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power level. In message types 2 and 3 these 7 bits convey power level
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along with an extension or re-definition of fields normally used for
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callsign and locator. Together, these compression techniques amount to
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“source encoding” the user message into the smallest possible number
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of bits.
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WSPR uses a convolutional code with constraint length K=32 and rate
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r=1/2. Convolution extends the 50 user bits into a total of (50 + K –
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1) × 2 = 162 one-bit symbols. Interleaving is applied to scramble the
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order of these symbols, thereby minimizing the effect of short bursts
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of errors in reception that might be caused by fading or interference.
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The data symbols are combined with an equal number of synchronizing
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symbols, a pseudo-random pattern of 0’s and 1’s. The 2-bit
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combination for each symbol is the quantity that determines which of
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four possible tones to transmit in any particular symbol
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interval. Data information is taken as the most significant bit, sync
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information the least significant. Thus, on a 0 – 3 scale, the tone
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for a given symbol is twice the value (0 or 1) of the data bit, plus
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the sync bit.
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[[FST4WPRO]]
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==== FST4W
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FST4W offers T/R sequence lengths of 120, 300, 900, and 1800 seconds.
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Submodes are given names like FST4W-120, FST4W-300, etc., the appended
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numbers indicating sequence length in seconds. Message payloads
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contain 50 bits, and a 24-bit cyclic redundancy check (CRC) appended
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to create a 74-bit message-plus-CRC word. Forward error correction
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is accomplished using a (240,74) LDPC code. Transmissions consist of
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160 symbols: 120 information-carrying symbols of two bits each,
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interspersed with five groups of eight predefined synchronization
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symbols. Modulation uses 4-tone frequency-shift keying (4-GFSK) with
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Gaussian smoothing of frequency transitions.
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[[SLOW_SUMMARY]]
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==== Summary
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Table 7 provides a brief summary of 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="100%",cols="3h,^3,^2,^1,6*^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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|FST4-15 |LDPC | (240,101)| 4| 4-GFSK| 16.67 | 66.7 | 0.25| 9.6 | -20.7
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|FST4-30 |LDPC | (240,101)| 4| 4-GFSK| 7.14 | 28.6 | 0.25| 22.4 | -24.2
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|FST4-60 |LDPC | (240,101)| 4| 4-GFSK| 3.09 | 12.4 | 0.25| 51.8 | -28.1
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|FST4-120 |LDPC | (240,101)| 4| 4-GFSK| 1.46 | 5.9 | 0.25| 109.3 | -31.3
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|FST4-300 |LDPC | (240,101)| 4| 4-GFSK| 0.558 | 2.2 | 0.25| 286.7 | -35.3
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|FST4-900 |LDPC | (240,101)| 4| 4-GFSK| 0.180 | 0.72 | 0.25| 887.5 | -40.2
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|FST4-1800 |LDPC | (240,101)| 4| 4-GFSK| 0.089 | 0.36 | 0.25| 1792.0| -43.2
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|FT4 |LDPC |(174,91)| 4| 4-GFSK| 20.83 | 83.3 | 0.15| 5.04 | -17.5
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|FT8 |LDPC |(174,91)| 8| 8-GFSK| 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 | -26
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|JT65A |RS|(63,12) |64|65-FSK| 2.692| 177.6 | 0.50| 46.8 | -25
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|Q65-15A |QRA|(63,13) |64|65-FSK|6.667|433|0.26| 12.8| -22.2
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|Q65-30A |QRA|(63,13) |64|65-FSK|3.333|217|0.26| 25.5| -24.8
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|Q65-60A |QRA|(63,13) |64|65-FSK|1.667|108|0.26| 51.0| -27.6
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|Q65-120A|QRA|(63,13) |64|65-FSK|0.750| 49|0.26|113.3| -30.8
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|Q65-300A|QRA|(63,13) |64|65-FSK|0.289| 19|0.26|293.8| -33.8
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| WSPR |K=32, r=1/2|(162,50)| 2| 4-FSK| 1.465| 5.9 | 0.50|110.6 | -31
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|FST4W-120 |LDPC | (240,74)| 4| 4-GFSK| 1.46 | 5.9 | 0.25| 109.3 | -32.8
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|FST4W-300 |LDPC | (240,74)| 4| 4-GFSK| 0.558 | 2.2 | 0.25| 286.7 | -36.8
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|FST4W-900 |LDPC | (240,74)| 4| 4-GFSK| 0.180 | 0.72 | 0.25| 887.5 | -41.7
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|FST4W-1800 |LDPC | (240,74)| 4| 4-GFSK| 0.089 | 0.36 | 0.25| 1792.0| -44.8
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|===
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LDPC = Low Density Parity Check
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RS = Reed Solomon
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QRA = Q-ary Repeat Accumulate
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Submodes of JT4, JT9, and JT65 offer wider tone spacings for
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circumstances that may require them, such as significant Doppler spread.
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Table 8 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 JT4, JT9, and JT65 with Selectable Tone Spacings
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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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|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 |-26
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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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|===
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.Parameters of Q65 Submodes
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[width="100%",cols="h,5*^",frame=topbot,options="header"]
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|===
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|T/R Period (s) |A Spacing Width (Hz)|B Spacing Width (Hz)|C Spacing Width (Hz)|D Spacing Width (Hz)|E Spacing Width (Hz)
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|15|6.67     4.33|13.33     867|26.67     1733|N/A|N/A
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|30|3.33     217|6.67     433|13.33     867| 26.67     1733| N/A
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|60|1.67     108|3.33     217|6.67     433|13.33     867|26.67     1733
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|120|0.75     49|1.50     98|3.00     195|6.00     390| 12.00     780
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|300|0.29     19|0.58     38|1.16     75|2.31     150|4.63     301
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|===
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[[FAST_MODES]]
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=== Fast Modes
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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 9, below.
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==== MSK144
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Standard MSK144 messages are structured in the same way as in FT8,
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with 77 bits of user information. Forward error correction is
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implemented by first augmenting the 77 message bits with a 13-bit
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cyclic redundancy check (CRC) calculated from the message bits. The
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CRC is used to detect and eliminate most false decodes at the
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receiver. The resulting 90-bit augmented message is mapped to a
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128-bit codeword using a (128,90) 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
|
||
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 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.
|
||
|
||
==== Summary
|
||
|
||
.Parameters of Fast Modes
|
||
[width="90%",cols="3h,^3,^2,^1,5*^2",frame="topbot",options="header"]
|
||
|===
|
||
|Mode |FEC Type |(n,k) | Q|Modulation Type|Keying rate (Baud)
|
||
|Bandwidth (Hz)|Sync Energy|Tx Duration (s)
|
||
|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 |(128,90)| 2| OQPSK| 2000 | 2400 | 0.11| 0.072
|
||
|MSK144 Sh|LDPC |(32,16) | 2| OQPSK| 2000 | 2400 | 0.20| 0.020
|
||
|===
|