13 December 2016

unid FSK 300Bd/500, 255/360 bit period

The signal was spotted on two frequencies 06775.0 and 12572.0 on different days and different formats as continue stream or tone-separated bursts. The FSK-2 modulation exhibits a constant 300 symbols/sec speed and shift of 500 Hz. 
The most interesting feature is the switch of the period lenght most likely in corrispondence of switching from idle/sync and traffic mode. In the fist case the ACF has the value of 850.55 msec that corresponds to a 255-bit period length, in the second case, traffic mode, the ACF exhibits the value of 200.13 msec that corresponds to a 360-bit period length or, better, to a 60-bit frame which is repeated six times.

The two 252 and 255 -bit patterns can be descrambled using the polynomial x^6+x^5+1.
The T=360 bit data block can be descrambled using the polynomial x^18+x^15+x^3+1: after removing the "0" columns a 6-bit structure is obtained where the 6th bit is the parity bit:

Same results for the 300Bd/500 waveform and 60-bit period:


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