# Transmit time synchronization

**URL:** <https://www.ruby-forum.com/t/transmit-time-synchronization/232504>\
**Category:** GNU Radio\
**Created:** [October 13, 2014, 2:55pm UTC](https://www.ruby-forum.com/t/transmit-time-synchronization/232504 "2014-10-13T14:55:14Z")\
**Posts on this page:** 4\
**Page:** 1

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**Author:** ![addis\_a](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/a/c4cdca/32.png) [@addis\_a](https://www.ruby-forum.com/u/addis_a)\
**Post date:** [October 13, 2014, 2:55pm UTC](https://www.ruby-forum.com/t/transmit-time-synchronization/232504/1 "2014-10-13T14:55:14Z")

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Hello all,

I am trying to implement a multi-relay cooperative network using  
gnuradio  
and USRP B200. I cannot figure out how to synchronize the relay  
transmissions using GNU-Radio (without editing FPGA code or using  
external  
clock source).

I created an out of tree module derived from the tags\_demo example of  
uhd  
but i encounter a problem with the packet encoder.

my flow graph is something like

USRP source —\> tagging\_block —\> DBPSK demod --\>packet decoder --\>  
encoder --\> mod --\> USRP sink.

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**Author:** ![Muhammad\_Shahmeer\_O](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/76d3ee/32.png) [@Muhammad\_Shahmeer\_O](https://www.ruby-forum.com/u/Muhammad_Shahmeer_O)\
**Post date:** [October 13, 2014, 2:58pm UTC](https://www.ruby-forum.com/t/transmit-time-synchronization/232504/2 "2014-10-13T14:58:13Z")

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sorry disregard previous mail

I am trying to implement a multi-relay cooperative network using  
gnuradio  
and USRP B200. I cannot figure out how to synchronize the relay  
transmissions using GNU-Radio (without editing FPGA code or using  
external  
clock source).

I created an out of tree module derived from the tags\_demo example of  
uhd  
but i encounter a problem with the packet encoder.

my flow graph is something like

USRP source —\> tagging\_block —\> DBPSK demod --\>packet decoder --\>  
encoder --\> mod --\> USRP sink.

when i attach tag debug block, i see that the packet decoder is not  
propogating my tags. Are their any substitutes to the encoder or is  
there  
any problem with my code which is attached. Any help would be very  
welcome.  
Thanks!

On Mon, Oct 13, 2014 at 5:53 AM, Muhammad Shahmeer Omar \<

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**Author:** ![Muhammad\_Shahmeer\_O](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/76d3ee/32.png) [@Muhammad\_Shahmeer\_O](https://www.ruby-forum.com/u/Muhammad_Shahmeer_O)\
**Post date:** [October 13, 2014, 4:06pm UTC](https://www.ruby-forum.com/t/transmit-time-synchronization/232504/3 "2014-10-13T16:06:49Z")

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On 10/13/2014 08:57 AM, Muhammad Shahmeer Omar wrote:

> my flow graph is something like
> 
> USRP source —\> tagging\_block —\> DBPSK demod --\>packet decoder →   
> encoder → mod → USRP sink.
> 
> when i attach tag debug block, i see that the packet decoder is not  
> propogating my tags. Are their any substitutes to the encoder or is

The packet\_decoder block has a message queue in the middle of its  
implementation, so tags are not propagated. Not sure how to get around  
this.

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<div class="post-metadata">

**Author:** ![Muhammad\_Shahmeer\_O](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/76d3ee/32.png) [@Muhammad\_Shahmeer\_O](https://www.ruby-forum.com/u/Muhammad_Shahmeer_O)\
**Post date:** [October 13, 2014, 6:08pm UTC](https://www.ruby-forum.com/t/transmit-time-synchronization/232504/4 "2014-10-13T18:08:52Z")

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thanks for the input Jeff. I am fairly new to GNURadio so any  
suggestions  
on how i can create the said network. My main goal is to implement the  
multiple relay network and demonstrate its advantages. As i understand  
it,  
a differential modulation scheme is my only option if i want to achieve  
diversity gain on rx without having to estimate the channel. Any  
suggestions on how i go about doing this?