11 December 2021

CIS 2400Bd serial bursts

 Just to point out some differences bewteen these two types of CIS bursts (figure 1) :

* the ones sent in group or in front of the OFDM-121
* the ones sent individually or before Serdolik MFSK/OFDM 

Fig. 1
Both the bursts are 2400 Baud keyed and have the same 20ms ACF value as well as - likely - the used modulation, although their constellations and state transitions are not much help in identifying the type of modulation (figure 2): most likely PSK2 in preambles and reinserted probes. The number of the transmitted frames is different, 33 Vs 22, but since they have the same 48-symbol length framing (same ACF) their duration differs (about 760 and 476 ms).

Fig. 2

For what concerns the structure of the frames, the classical method can be misleading, showing two different structures consisting of 33 unknown symbols + 15 known symbols for a type of burst and 32 unknown symbols + 16 known symbols for the other (figure 3a); the plain raster method anyway shows the same 32 + 16 framing for both the burst (figure 3b).
Fig. 3a

Fig. 3b

Given the different systems in which these bursts are used, and also the modalities of their transmission (isolated, in group or in front of OFDM), I think they also have different purposes and functionalities.


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