updated
Unid transmissions heard on 8120, 8130, 8140, and 8150 KHz (the second moved to 8160 Khz) thanks to the KiwiSDR located at N4BUT Orlando, FL [1].
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| Fig. 1 |
At first sight the signal seemed a MPSK modulation, but working the signal along with my friend cryptomaster any another interesting features came out. Each transmission consists of 4 100 Hz separated channels (a,b,c,d) each occupying a band of about 900 Hz, for a full bandwidth business of about 3900 Hz (Figure 2).
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| Fig. 2 |
In turn, each of the 4 channels consists of 4 sub-channels with a modulation rate of 49.6 Baud, the utilized modulation seems to be ASK2/OOK [2].
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| Fig. 3 |
Each of the 4 sub-channels shows strong ACF peaks of 1290 ms corresponding to a 64-bit dimension frame: the "aggregate" frame so has a dimension equal to precisely 1 Kb, i.e. 1024 bits or 128 bytes (64x4x4).
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| Fig. 4 |
Below in Figure 5 is simply a comparison of the 4 channels obtained from the analysis of my friend cryptomaster.
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| Fig. 5 |
Although I have kept an eye on that first condition of the 8 MHz band (fixed/mobile band, shared with marine for simplex purposes), those transmissions have not appeared again (at least until today): hard to specify their intent and user(s).
Below the interesting comment sent me by my friend Nicola, who I thanks for the collaboration:
"The interesting signal discussed in the blogpost “Unid ASK2/OOK
transmissions” dated 1 February is most likely a Frequency-Time Matrix
(FTM) system, where a data symbol (bit string) is represented by
FT-matrices, i.e. combinations of frequency and time domain
positions. The 'secret' of this robust mode is that no frequency is
repeated within each matrix. This characteristic is utilized to enhance
the resilience against frequency domain broadband sound (or fading)
affecting a broad scope of frequencies and time domain narrowband
sound (or fading)."
https://disk.yandex.com/d/7u74pzY-bPntXg
[1] http://sdr.n4but.com:8173/?f=8120.00iqz10&pbw=10000
[2] https://en.wikipedia.org/wiki/Amplitude-shift_keying
