priyom.org

Protocol

F07 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. F07 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 10.112-second blocks, each made of:
    • 250 Bd BPSK for 512 ms (128 bits), using one of five different subcarriers, chosen randomly
    • 5 adjacent, parallel channels of 10 Bd 16-tone MFSK for 9600 ms (1920 bits), 20 Hz tone spacing (from 800 to 2380 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 MFSK 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 5 (as many as channels) temporal sections of continuous lowest-tone subcarrier, hinting at this interleaving. The exact data structure, or encoding within those blocks, is unknown.

Known variations

P07 is a counterpart waveform of this digital system, which uses a different block structure based on 8-tone OFDM. There also exists another, experimental MFSK variant, using a single 16-tone MFSK channel instead of 5, with 40 Hz tone spacing and supporting rates of either 10, 20 or 40 Bd (10 Bd being the most common). As further variations, such a single-channel MFSK mode has also been heard using alternative frequency ranges for the MFSK tones, or featuring FSK bursts instead of PSK ones at the beginning of each data block.

Known waveforms of this digital system can be found here.