Wavecom W74PC V8.7.0 - Manual (Page 109)

WAVECOM Decoder W74PC, W-PCI/e, W-CODE, W-CLOUD Manual V8.7.0 Analysis Tools

99
If two or more different systems are identified, or if more than five proven transmission errors occur, an
automatic switch to a mode will not take place.
The software then proceeds with code and system analysis. The incoming bit stream is tested and com-
pared against known MFSK modes.
The name of each identified system is displayed in the output window. An identified system may be imme-
diately started by double-clicking the mode name in the output window – for instance double-clicking
COQUELET-13 in the figure above will interrupt MFSK code checking and immediately start COQUELET-13.
Hits-Checks shows how many times a mode was successfully identified during the code check. If multiple
modes are listed, the mode with most hits should be tried first.
Some MFSK modes are very difficult to distinguish, especially when the system is in idle mode.
Autocorrelation
Autocorrelation is used for determining the periodicity of bit patterns. Periodicity implies a constant repeti-
tion of a specific bit pattern. If a station transmits the idle pattern 0010011011 0010011011..., the perio-
dicity is said to be 10 bits. HNG-FEC and RUM-FEC have a periodicity of 15 and 16 bits respectively. The
periodicity could also be 11250 bits, i.e., after 11250 bits the same constantly repeated pattern occurs
again. Periodicity becomes very important in the classification of unknown transmissions and the analysis
of unknown modes and systems.
First of all, the Auto option from the Demodulator menu field or the Auto button should be used to de-
termine the exact baud rate and frequency shift. If the exact baud rate is unknown, the IAS measurement
function can be used for this purpose with an accuracy of 0.001 Bd. This is done by activating the IAS set-
ting in the Demodulator menu field. Autocorrelation is then initiated by selecting and programming the
baud rate menu field. If the baud rate deviates by more than 0.5 Baud, a bit slip may occur and therefore
the autocorrelation must be restarted with the exact baud rate.