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66 lines
3.0 KiB
Plaintext
66 lines
3.0 KiB
Plaintext
JTMS v3.0: Possible New Mode for Meteor Scatter
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1. Transmitting
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Type 1 messages are 72 user-information bits, source encoded as in
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JT65. Convolutional FEC (K=32, r=1/2) increases the number of bits to
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(72+31)*2 = 206; nine bits are sent twice, extending the array to 215
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bits. These are interleaved by bit-reversal of index values. Then 43
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sync bits are inserted, spread evenly so as to fall at positions 1, 7,
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13, ... 253. Frame size is 258 bits: 215 information-carrying bits
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and 43 sync bits. Frame duration is 129 ms.
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Type 2 messages convey 4 user information bits (report, R+report, RRR,
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73) encoded with a (15,4,8) block code, plus a 12-bit CRC for each
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callsign, encoded with the Golay (23,12) code. This makes for 15 +
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2*23 = 61 information-carrying bits. One dummy bit is added, and the
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62 bits are interspersed with 31 sync bits, making a frame of 93 bits
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and frame time 46.5 ms.
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2. Modulation is BPSK at 2000 baud, 24 samples per symbol at 48000 Hz
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asmple rate. The baseband waveform is built by inserting a tapered
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sinc function for each bit, then multiplying by a 1500 Hz sine wave.
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3. Receiving
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a. Compute real-to-complex windowed FFTs, N=12288 (t=256 ms),
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stepped by 128 ms (say). Zap birdies, remove frequency
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components outside the range 300 - 2700 Hz, and convert to an
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analytic time-domain signal.
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b. Square the complex signal, cx2=cx*cx, and compute N=12288 FFT of
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cx2 (resolution = 3.9 Hz). Look for carrier at 3000 + 2*DF Hz
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+/- 2*Tol.
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c. 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.
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d. Apply matched filter for the Tx pulse shape to x(). (This is just
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a rectangular BPF, 500 - 25-- Hz ?)
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e. Establish PSK symbol sync (offset i0, 0 to nsps-1 samples) by finding
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maximum of Sum(sum*sum) over groups of nsps consecutive samples.
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f. Read off the soft symbols, sym(1:512), and compute CCF with 3
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versions of the 43-bit sync vector (rotated by 0, 14, 29 out of
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its 43 positions) and three of the 31-bit sync vector (rotated by
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0, 10, 20 of 31).
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g. If the best CCF abs(peak) exceeds a specified threshold, the sign
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of peak resolves the sign ambiguity.
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h. For Type 1 messages: Gather the proper set of 215
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information-carrying soft symbols. Form averages using the 9
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extra symbols, reducing the number to 206, and remove
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interleaving to re-order the symbols. Then run the fano232
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decoder. If decoding fails, add soft symbols into an
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accumulation array and (if nsum is 2 or more) try decoding the
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average.
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i. For Type 2 messages: Gather the proper set of 62 soft symbols.
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Decode Nrpt using an exhaustive search over all possibilities.
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For the CRCs, also do exhaustive searches -- and make sure that
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the expected values are best (or fall in the top few, anyway).
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