2 May 2016

(CIS) OFDM 64-tone QAM-16, 40Bd


this is an interesting burst waveform composed of a two-parts preamble phase and a data phase; bursts have a duration of 2800ms and are 980ms spaced.

Preamble phase (pic. 1)
Part one
has a duration of 8 symbol element periods (~200ms) and consist of LFM pulse modulated data. Part two has a duration of 24 symbol element periods (~600ms) and consist of four unmodulated data tones with frequencies of 350, 1350, 2350 and 3350 Hz. During this part, the transmitted level of the 1350 Hz tone is 7 dB higher than the level of the 350 Hz and 2350 Hz tones and the transmitted level of the 3350 Hz tone is 7 dB lower than the level of the 350 Hz and 2350 Hz tones (pic. 2). Since the lack of the typical 3350 Hz tone, it's possible that the  1350 Hz tone could be used as Doppler correction.

pic. 1
pic. 2
Data phase (pic. 3)
The data phase has a duration of 2000 ms and consists of 64 QAM-16 data tones with constant modulation rate of 40 Baud. The 64 tones are 46.8 Hz shifted and spread about 2955 Hz bandwidth.

pic. 3
Detailed analysis in the OFDM module indicates that one special/service symbol is sent each five symbols (pic 4) and, as logically expected, it is confirmed by the 125ms value of the ACF (pic. 5).

pic. 4
pic. 5
Most likely this is a Russian signal: my friend KarapuZ sent me a recording with the evidence of a serdolik MFSK-32 ALE just before the OFDM 64-tone (pic. 6). That same ALE, characterized by the LFM period, was meet here (waveform s3):
http://i56578-swl.blogspot.it/2015/10/prob-new-serdolik-mfsk-burst-waveforms.html

pic. 6

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