WSJT-X/jtms3.txt

	JTMS v3.0: Possible New Mode for Meteor Scatter
	-----------------------------------------------

1. Transmitting

Messages are sent character-by character, 6 bits plus even parity.
Message length can be one of {5 7 9 11 13 17 19 23 29}; messages are
padded with blanks up to the next available length.  

Modulation is BPSK at 2000 baud.  The baseband waveform is built by
inserting a tapered sinc function for each bit, then multiplying by a
sine wave at frequency f0 = 10000.0/7 = 1428.57 Hz.  At fsample=48000
Hz there are 24 samples per PSK symbol and 7*24=168 samples per
character.  Carrier phase increment over one character is
f0*(168/48000) = 5 cycles, exactly.

2. Receiving

  a. Pings are detected (or mouse-picked data is selected) as in
     WSJT9.  Pings can also be detected from the squared analytic
     signal.  Experience may tell which is better.

  b. Compute real-to-complex FFT.  Zap birdies, remove frequency 
     components outside the range 300 - 2700 Hz, and convert to analytic
     time-domain signal.  (analytic)

  c. Square the complex signal, cx2=cx*cx, and compute FFT.  Look for
     carrier at frequency 3000 + 2*DF +/- 2*Tol.  (msdf)

  d. If carrier is found, measure frequency f and phase phi.  Multiply
     cx by exp(-twopi*i*f*t - phi) to recover the real baseband signal
     x() to within a sign ambiguity.  (tweak1)

  e. Apply matched filter for the Tx pulse shape to x().  This is
     essentially a rectangular BPF, -1000 to +1000 Hz ?  (Or convolve
     with the generated PSK pulse shape, the tapered sinc() function.)

  f. Establish symbol and character sync by cross-correlating with
     conjg(cwb), where cwb is the baseband PSK waveform for the
     <space> character.

  g. Find message length by computing ACF (of what?  cdat?  soft
     symbol values?)

  h. Decode the message by cross-correlating character-length segments
     of cdat against complex waveforms for each possible character.

  i. If msglen is established and long enough, try folding the data and
     determining best-fit characters as above.