6 October 2015

CIS-128, OFDM 128-tone 21Bd

CIS-128 is an OFDM 128-tone +  1 'off' tone in the middle of the grid (then 64+64 carriers), it spreads a 3000Hz bandwidth with a step between carriers of 23.5 Hz, manipulation speed of ~ 21 Baud and modulation QAM-16 (Pic. 1). The signal was heard at 0948z on 13399.0 KHz on USB. This waveform  is discussed in radioscanner.ru here: http://signals.radioscanner.ru/base/signal250/
Analyzing the signal, is clearly visible that each 5 symbols a special symbol is sent (Pic. 2) and then the expected ACF value should be 5 symbols; but there is a discrepancy because ACF is effectively doubled, i.e. 10 symbols for a ~477 ms length (Pic. 2).

Pic 1
Pic. 2 - a "special symbol" is sent every 5 symbols
Pic. 3 - 10 Symbols ACF
It seems that during the special symbol, i.e. every 5 clocks, AM modulation is lost since the "shape" of these symbols are pretty the same.
In sync-mode are sent only the special symbols and each 5 symbols all the tones do not change their phase (Pic. 4). The ACF is always 10 symbols lenght but in this case the symbols markers are more evident (Pic. 5). 
Pic. 4
Pic. 5 - ACF of sync mode
Dduring a same transmission is possible to see both the data and sync mode, alternatively.

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