[Discuss-gnuradio] QPSK Synchro

[samuel verdon]

Hello,

 

I am blocked with a QPSK problem that I would like to share, and maybe you will find my mistake.

 

I have build a QPSK transciever based the gnuradio tutorial for a project. The Tx part is not done by myself modulation is already made, I just have to emit the signal. Here is the spec :

Bitrate : 2Mbps

Sym rate : 1 Mbps

Symbol Oversampling ratio: 9

Hardware: USRP b200mini

The data send are two symbol, exactly 0x22 or 00 10 00 10 repeatly

 

The reception use a LimeSDR. This part receive de signal but I can clearly see the the lock on is not working. I post some image of my flograph:

 

Flowgraph: https://ibb.co/fJQxSo

 

Data sent: https://ibb.co/mLJvDT

 

Data received: https://ibb.co/dLNcSo

 

My questions are:

• How can I correctly lock the rotation? Playing with the “loop bandwidth does not really affect it”.
• I feel like my sample rate are wrong, when I see the “received data”, it is like the point moving constantly on the circle of the phase and then shift the signal slowly and at the end I lost the good lock of my constellation. Is it possible?

 

Thanks in advance and it needs more explaination, let me know I will post it.

 

Cheers

 

Sam