Sync Found Index From MT Demodulate PSK VI Does not Stay Consistent

Updated May 3, 2018

Reported In


  • Modulation Toolkit
  • LabVIEW
  • LabVIEW Communications System Design Suite

Issue Details

When I run my USRP application, I see that the "sync found index" from my Demodulation VI changes every time I run my application. How should I use the "sync found index" to locate my message bits?


  • The sync found index returns the symbol index within the input complex waveform where the peak correlation to the sync sequence was found. 
  • The sync found index will change between runs because the sync bits will not be at the same place every time. When you are in an environment with low noise, you may be able to find the sync sequence very quickly, but if there's a lot of noise in the environment, it may take longer to find the sync sequence and therefore, the sync found index will vary between runs.
  • The output bit stream from the MT PSK Demodulate VI is a bit stream that includes just the sync bits and the message bits. The VI will trim the guard bits and any other signal that was picked up before the guard bits.
  • To locate your message bits, take your sync bits and compare it to the output bit stream from the MT PSK Demodulate VI. The start of the output bit stream should match your sync bits and the bits after your sync bits is your message.


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