# Require help with synchronisation (using USRPs)

**URL:** <https://www.ruby-forum.com/t/require-help-with-synchronisation-using-usrps/241890>\
**Category:** GNU Radio\
**Created:** [May 8, 2015, 11:06pm UTC](https://www.ruby-forum.com/t/require-help-with-synchronisation-using-usrps/241890 "2015-05-08T23:06:53Z")\
**Posts on this page:** 1\
**Page:** 1

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**Author:** ![musicdenotation](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/ea666f/32.png) [@musicdenotation](https://www.ruby-forum.com/u/musicdenotation)\
**Post date:** [May 8, 2015, 11:06pm UTC](https://www.ruby-forum.com/t/require-help-with-synchronisation-using-usrps/241890/1 "2015-05-08T23:06:53Z")

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Hello

I am implementing time synchronisation in OFDM tx-rx. In my matlab  
simulations I manually chose a start point in the receive signal which  
was not the first sample. And then I assumed that I don’t know what that  
start point was and let my algorithm find it.

In GNURadio, I know that the scheduler takes care of the buffering of  
samples. So in my code, I can use the forecast function to decide how  
many minimum samples I need before I start looking for the synch  
sequence.

I was just curious, in the practical case (with USRPs), is it the  
hardware that decides the start point? Once I have my buffer full of  
samples, I can code it to start from the first sample so I’m unable to  
visualize where the timing offset comes from in the real scenario.

Does the USRP source (at the receiver) add some sort of tag to the  
sample which it deems to be the first sample?

Thanks  
Anil