mirror of
https://github.com/saitohirga/WSJT-X.git
synced 2024-11-18 01:52:05 -05:00
0a3c96fab0
git-svn-id: svn+ssh://svn.code.sf.net/p/wsjt/wsjt/branches/jtms3@2482 ab8295b8-cf94-4d9e-aec4-7959e3be5d79
76 lines
3.4 KiB
Plaintext
76 lines
3.4 KiB
Plaintext
JTMS v3.0: Possible New Mode for Meteor Scatter
|
|
-----------------------------------------------
|
|
|
|
1. Transmitting
|
|
|
|
Type 1 messages are 72 user-information bits, source encoded as in
|
|
JT65. Convolutional FEC (K=32, r=1/2) increases the number of bits to
|
|
(72+31)*2 = 206. Nine bits are sent twice, extending the array to 215
|
|
bits. These are interleaved by bit-reversal of index values. Then 43
|
|
sync bits are inserted, spread evenly so as to fall at positions 1, 7,
|
|
13, ... 253. Frame size is 258 bits: 215 information-carrying bits
|
|
and 43 sync bits. Frame duration is 129 ms.
|
|
|
|
[Optional: Type 2 messages convey 4 user information bits (report,
|
|
R+report, RRR, 73) encoded with a (15,4,8) block code, plus a 12-bit
|
|
CRC for each callsign encoded with the Golay (23,12) code. This
|
|
makes for 15 + 2*23 = 61 information-carrying bits. One dummy bit is
|
|
added, and the 62 bits are interspersed with 31 sync bits, making a
|
|
frams of 93 bits and frame time 46.5 ms.]
|
|
|
|
2. Modulation is BPSK at 2000 baud. For sample rate 48000 Hz, this
|
|
means nsps = 48000/2000 = 24 samples per symbol. The baseband
|
|
waveform is built as follows:
|
|
|
|
a. Replicate each bit nsps times into array y(npts=30*48000),
|
|
substituting -1 for 0. Repeat the whole message enough times
|
|
to fill npts, then pad with zeros to length 2*npts.
|
|
|
|
b. Compute real-to-complex FFT of y(2*nsym*nsps). Roll off the complex
|
|
spectrum at f=1000 Hz. Translate the half-band 0-1000 upward to
|
|
1500-2500 Hz, and insert conjugate values at 1500 down to 500 Hz.
|
|
|
|
c. The inverse (complex-to-real) FFT yields the Tx audio waveform.
|
|
|
|
3. Receiving
|
|
|
|
a. Compute real-to-complex windowed FFTs, N=12288 (t=256 ms),
|
|
stepped by 128 ms (say). Zap birdies, remove frequency
|
|
components outside the range 300 - 2700 Hz, and convert to an
|
|
analytic time-domain signal.
|
|
|
|
b. Square the complex signal, cx2=cx*cx, and compute N=12288 FFT of
|
|
cx2 (resolution = 3.9 Hz). Look for carrier at 3000 + 2*DF Hz
|
|
+/- 2*Tol.
|
|
|
|
c. 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.
|
|
|
|
d. Apply matched filter for the Tx pulse shape to x(). (This is just
|
|
a rectangular BPF, 500 - 25-- Hz ?)
|
|
|
|
e. Establish PSK symbol sync (offset i0, 0 to nsps-1 samples) by finding
|
|
maximum of Sum(sum*sum) over groups of nsps consecutive samples.
|
|
|
|
f. Read off the soft symbols, sym(1:512), and compute CCF with 3
|
|
versions of the 43-bit sync vector (rotated by 0, 14, 29 out of
|
|
its 43 positions) and three of the 31-bit sync vector (rotated by
|
|
0, 10, 20 of 31).
|
|
|
|
g. If the best CCF abs(peak) exceeds a specified threshold, the sign
|
|
of peak resolves the sign ambiguity.
|
|
|
|
h. For Type 1 messages: Gather the proper set of 215
|
|
information-carrying soft symbols. Form averages using the 9
|
|
extra symbols, reducing the number to 206, and remove
|
|
interleaving to re-order the symbols. Then run the fano232
|
|
decoder. If decoding fails, add soft symbols into an
|
|
accumulation array and (if nsum is 2 or more) try decoding the
|
|
average.
|
|
|
|
i. For Type 2 messages: Gather the proper set of 62 soft symbols.
|
|
Decode Nrpt using an exhaustive search over all possibilities.
|
|
For the CRCs, also do exhaustive searches -- and make sure that
|
|
the expected values are best (or fall in the top few, anyway).
|