Symbol Sync Block Still in Sync but Symbols are Incorrect

[Ceren Karaköse]

Hi all,

First off, thank you for all the amazing work you've put into this community.
 
I'm working with 1.024M baud QPSK signal with sample rate of 4.096M. This symbol sync block works perfectly 99% of time. I can successfully demodulate input symbols, perform convolutional decoding and find sync word using Sync and Create PDU block. However, when I close and re-open the flow while the source is still transmitting, sometimes it stops finding sync word despite the constellation being correct. In other words, constellation sink shows perfect 4 dots on the screen. I tried delaying constellation decoder output for 1 symbol time to cover for bit shifts. It did not work. I want to highlight that this only happens when I close and re-open the flow, but it never stops finding sync word while running. Also, it never starts finding the sync word once it fails to find one. What might the problem be? GNU radio version is 3.10.7.0 (Python 3.10.12). Flow is attached.

Thanks in advance!

Ceren.

symbol_sync.png
Description: symbol_sync.png