mirror of https://github.com/saitohirga/WSJT-X.git
51 lines
2.1 KiB
Plaintext
51 lines
2.1 KiB
Plaintext
JTMS v3.0: Possible New Mode for Meteor Scatter
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1. Transmitting
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Messages are sent character-by character, 6 bits plus even parity.
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Message length can be one of {5 7 9 11 13 17 19 23 29}; messages are
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padded with blanks up to the next available length.
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Modulation is BPSK at 2000 baud. The baseband waveform is built by
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inserting a tapered sinc function for each bit, then multiplying by a
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sine wave at frequency f0 = 10000.0/7 = 1428.57 Hz. At fsample=48000
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Hz there are 24 samples per PSK symbol and 7*24=168 samples per
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character. Carrier phase increment over one character is
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f0*(168/48000) = 5 cycles, exactly.
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2. Receiving
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a. Pings are detected (or mouse-picked data is selected) as in
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WSJT9. Pings can also be detected from the squared analytic
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signal. Experience may tell which is better.
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b. Compute real-to-complex FFT. Zap birdies, remove frequency
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components outside the range 300 - 2700 Hz, and convert to analytic
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time-domain signal. (analytic)
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c. Square the complex signal, cx2=cx*cx, and compute FFT. Look for
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carrier at frequency 3000 + 2*DF +/- 2*Tol. (msdf)
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d. If carrier is found, measure frequency f and phase phi. Multiply
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cx by exp(-twopi*i*f*t - phi) to recover the real baseband signal
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x() to within a sign ambiguity. (tweak1)
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e. Apply matched filter for the Tx pulse shape to x(). This is
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essentially a rectangular BPF, -1000 to +1000 Hz ? (Or convolve
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with the generated PSK pulse shape, the tapered sinc() function.)
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f. Establish symbol and character sync by cross-correlating with
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conjg(cwb), where cwb is the baseband PSK waveform for the
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<space> character.
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g. Find message length by computing ACF (of what? cdat? soft
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symbol values?)
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h. Decode the message by cross-correlating character-length segments
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of cdat against complex waveforms for each possible character.
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i. If msglen is established and long enough, try folding the data and
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determining best-fit characters as above.
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