2012-09-28 14:07:07 -04:00
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JT9 is a mode designed for amateur QSOs and beacon-like transmissions
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at MF and LF. The mode uses the same 72-bit user messages as JT65.
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Convolutional error-control coding (ECC) uses constraint length K=32,
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rate r=1/2, and a zero tail, which leads to an encoded message length
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of (72+31)*2 = 206 bits. Modulation is 9-FSK: 8 tones for data, one
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for synchronization. Sixteen symbol intervals are used for
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synchronization, so a transmission requires a total of 207/3 + 16 = 85
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channel symbols. Symbol durations tsym are set to approximately
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(TRperiod-10)/85, where TRperiod is the T/R sequence length in
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seconds. The exact symbol lengths are chosen so as to make nsps, the
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number of samples at 12000 samples per second, a number rich in
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factors less than 7. This choice makes for efficient FFTs. Tone
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spacing of the 9-FSK modulation is df=1/tsym, the same as the keying
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rate. The total occupied bandwidth is 9*df.
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Parameters of the five JT9 sub-modes are summarized in the following
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table, along with S/N thresholds measured by simulation on an AWGN
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(additive white Gaussian noise) channel.
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-------------------------------------------------------------------------
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Mode nsps nsps2 df tsym BW S/N* Tdec Tfree Factors
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12000 750 (Hz) (s) (Hz) (dB) (s) (s) of nsps
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-------------------------------------------------------------------------
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JT9-1 6912 432 1.736 0.58 15.6 -26.9 52.5 7.5 2^8 3^3
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JT9-2 15360 960 0.781 1.28 7.0 -30.2 112.3 7.7 2^10 3 5
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JT9-5 40960 2560 0.293 3.41 2.6 -34.4 293.6 6.4 2^13 5
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JT9-10 82944 5184 0.145 6.91 1.3 -37.5 591.0 9.0 2^10 3^4
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JT9-30 250880 15750 0.048 20.91 0.4 -42.3 1788.5 11.5 2^5 3^2 5^3 7
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-------------------------------------------------------------------------
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* Noise power measured in a 2500 Hz bandwidth.
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Transmitting
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------------
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1. Source encode the structured message to 72 bits
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2. Convolutionally encode (K=32, r=1/2) to (72+31)*2 = 206 bits
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3. Interleave to scramble the bit order
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4. Assemble 3-bit groups to make (206+1)/3 = 69 symbols
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5. Gray-code the symbol values
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6. Insert 16 sync symbols ==> 69+15=81 channel symbols, values 0-8
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Receiving
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---------
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1. Apply noise blanking with the timf2 method
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2. Filter to 500 Hz bandwidth, downsample (1/16) to 750 Hz complex
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(FIR filter with 61 taps). Complex data to array c0 (max 1.35M)
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3. Compute symbol-length spectra at half-symbol steps. Use for
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waterfalls s(15750) and detecting sync vectors; save in ss(184,15750)
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and savg(15750)
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4. At time Tdec, look for sync vectors in ss(); get estimates of DF, DT
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5. Do full-length FFTs, NFFT1=96*nsps2
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6. For each candidate signal, do inverse FFT of length 1536. This
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will provide 16 complex samples per symbol, and sync tone should be
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close to f=0.
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7. Use afc65b method to get improved values of DF, DT.
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8. Tweak freq and time offset to 0.
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9. Compute 8-bin spectra of 69 data symbols: s2(8,69). Re-order the
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bins by removing Gray code.
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10. Compute soft symbols for 206 bits
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11. Re-order the bits by removing interleaving.
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12. Pack bits into bytes, send to Fano decoder
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13. If Fano succeeds, remove source encoding and display user message.
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