WSJT-X/jt8.txt

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JT8 is a mode designed for amateur QSOs and beacon-like transmissions
at MF and LF. The mode uses the same 72-bit user messages as JT65,
augmented by a 12-bit cyclic redundancy check (CRC). Error-control
coding uses a convolutional code with constraint length K=16, rate
r=1/2, and a zero tail, leading to an encoded message length of
(72+12+15)*2 = 198 bits. Modulation is 8-FSK, so a transmission
requires 198/3 = 66 information-carrying channel symbols. Two 8x8
Costas arrays are added to each transmission for purposes of time and
frequency synchronization. A full transmission thus contains 66 + 16
= 82 symbols. Tone spacing df of the 8-FSK modulation is equal to the
keying rate; symbol duration tsym = 1/df, and the total occupied
bandwidth is 8*df. The actual transmission length TxT is slightly
less than the T/R sequence time, to allow for possible message
decoding before the next transmission starts.
Parameters of the five JT8 sub-modes are summarized in the following
table, along with S/N thresholds measured by simulation on an AWGN
(additive white Gaussian noise) channel. Parameter nsps is the number
of samples per symbol at sample rate 1500 Hz.
-----------------------------------------------------
Mode T/R TxT nsps df tsym BW S/N*
(m) (s) (Hz) (s) (Hz) (dB)
-----------------------------------------------------
JT8-1 1 52.5 960 1.562 0.64 12.6 -26.9
JT8-2 2 112.0 2048 0.732 1.37 5.9 -30.2
JT8-5 5 293.9 5376 0.279 3.58 2.2 -34.4
JT8-10 10 587.8 10752 0.140 7.17 1.1 -37.5
JT8-30 30 1791.3 32768 0.046 21.85 0.4 -42.3
-----------------------------------------------------
* Noise power measured in a 2500 Hz bandwidth.