WSJT-X/jt9.txt

JT9 is a mode designed for amateur QSOs at MF and LF.  The mode uses
the same 72-bit structured messages as JT65.  Error control coding
(ECC) uses a convolutional code with constraint length K=32, rate
r=1/2, and a zero tail, leading to an encoded message length of
(72+31)*2 = 206 information-carrying bits.  Modulation is 9-FSK: 8
tones for data, one for synchronization.  Sixteen symbol intervals are
used for synchronization, so a transmission requires a total of 207/3
+ 16 = 85 channel symbols.  Symbol durations tsym are approximately
(TRperiod-8)/85, where TRperiod is the T/R sequence length in seconds.
Exact symbol lengths are chosen so that nsps, the number of samples
per symbol (at 12000 samples per second) is a number with no prime
factor greater than 7.  This choice makes for efficient FFTs.  Tone
spacing of the 9-FSK modulation is df=1/tsym=12000/nsps, the same as
the keying rate.  The total occupied bandwidth is 9*df.

Parameters of five JT9 sub-modes are summarized in the following
table, along with S/N thresholds measured by simulation on an AWGN
channel.

-------------------------------------------------------------------------
Mode     nsps nsps2   df    tsym  BW    S/N*   Tdec  Tfree   Factors 
        12000  1500  (Hz)   (s)  (Hz)   (dB)   (s)    (s)    of nsps
-------------------------------------------------------------------------
JT9-1    6912   864  1.736  0.58 15.6  -26.9   52.5   7.5   2^8 3^3
JT9-2   15360  1920  0.781  1.28  7.0  -30.2  112.3   7.7   2^10 3 5
JT9-5   40960  5120  0.293  3.41  2.6  -34.4  293.6   6.4   2^13 5
JT9-10  82944 10368  0.145  6.91  1.3  -37.5  591.0   9.0   2^10 3^4
JT9-30 252000 31500  0.048 21.00  0.4  -42.3 1788.5  11.5   2^5 3^2 5^3 7
-------------------------------------------------------------------------
* Noise power measured in a 2500 Hz bandwidth.


Transmitting
------------
1. Source encode the structured message to 72 bits
2. Apply convolutional ECC (K=32, r=1/2) to yield (72+31)*2 = 206 bits
3. Interleave to scramble the bit order
4. Assemble 3-bit groups to make (206+1)/3 = 69 symbols
5. Gray-code the symbol values
6. Insert 16 sync symbols ==> 69+15=81 channel symbols, values 0-8


Receiving
---------
1.  Apply noise blanking with the timf2 method
2.  Filter to 1000 Hz bandwidth and downsample (1/8) to 1500 Hz, saving
    complex data to array c0(1350000).
3.  Compute symbol-length spectra at half-symbol steps.  Use for
    waterfall display s(15750) and save in ss(184,15750) and 
    savg(15750) for detecting sync vectors.
4.  At time Tdec, find sync vectors in ss(); get estimates of DF, DT
5.  Do full-length FFT, NFFT1=96*nsps2, zero-pad as required.
6.  For each candidate signal, do inverse FFT of length 1536.  This 
    yields 16 complex samples per symbol, and sync tone should be 
    close to zero frequency.
7.  Use afc65b method to get improved values of DF, DT.
8.  Tweak freq and time offset to 0.
9.  Compute 8-bin spectra of 69 data symbols: s2(8,69).  Re-order bins
    by removing Gray code.
10. Compute soft symbols for 206 bits.
11. Remove interleaving
12. Pack bits into bytes, send to Fano decoder
13. If Fano succeeds, remove source encoding and display user message.