priyom.org

Protocol

P07 is one waveform supported by a proprietary mixed-mode, multicarrier FSK/PSK digital system. Transmissions of this digital system feature different variable elements organized around a common structure. P07 transmissions proceed as follows:

  1. A short, 4.1-second callup sequence of 1800/1200 Hz alternating FSK tones:
    • 1800 Hz tone for 500 ms, 1200 Hz tone for 1500 ms, 1800 Hz tone for 500 ms, 1200 Hz tone for 1500 ms, and 1000 Hz tone for 100 ms
  2. An FSK Barker sequence(15.625 Bd, 875/2375 Hz) possibly for synchronization purposes:
    • 875 Hz tone for 320 ms, 2375 Hz tone for 128 ms, 875 Hz tone for 128 ms, 2375 Hz tone for 64 ms, 875 Hz tone for 64 ms, 2375 Hz tone for 64 ms, 875 Hz tone for 64 ms
  3. A preamble consisting of three successive PSK sections, using each a different carrier frequency:
    • 500 Bd BPSK for 512 ms (256 bits)
    • 40 Bd BPSK for 1200 ms (48 bits)
    • 40 Bd BPSK for 2400 ms (96 bits)
  4. A main section composed of any number of consecutive 12.8-second blocks, each made of:
    • 250 Bd BPSK for 512 ms (128 bits), using one of five different subcarriers, chosen randomly
    • 8-tone OFDM, 62.5 Bd QPSK for 12288 ms (12288 bits), 125 Hz subcarrier spacing (from 1125 to 2000 Hz)
  5. An outro FSK Barker sequence (15.625 Bd, 1500/1750 Hz):
    • 1500 Hz tone for 320 ms, 1750 Hz tone for 128 ms, 1500 Hz tone for 128 ms, 1750 Hz tone for 64 ms, 1500 Hz tone for 64 ms, 1750 Hz tone for 64 ms, 1500 Hz tone for 64 ms

Within each block, the OFDM section is suspected of making use of data interleaving. The last block of the transmission, presumably padded with 0 bits after the end of the message, often lets appear 8 (as many as channels) temporal sections of unmodulated subcarriers, hinting at this interleaving. The exact data structure, or encoding within those blocks, is unknown.

Known variations

Apart from its F07 counterpart which uses a different block structure based on 5 x 16FSK, a number of experimental variations on this OFDM-based waveform have been heard through the years, and still are. For example:

  • Instead of the short callup, a more classic 2-minute, two-tone callup (1600/2100 Hz); or even no callup at all, the transmission then starting with the Barker sequence
  • 2 QPSK OFDM channels instead of 8 (1500 and 1625 Hz subcarriers)
  • Lack of 250 Bd BPSK bursts at the beginning of each data block
  • Different Barker sequence tone frequencies
  • Different preamble section mode arrangements

Known waveforms of this digital system can be found here.