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[Discuss-gnuradio] Syncronising transmitting and receiving streams to an

From: Will Thompson
Subject: [Discuss-gnuradio] Syncronising transmitting and receiving streams to and from a USRP
Date: Tue, 19 May 2015 15:42:10 +0000

Hi All
I'm using N210 USRP with XCVR2450 (Rev2) hardware.

I'm writing an out of tree module for GRC in C++ for a consensus synchronisation system.

The block takes in a certain number of samples from the USRP, then estimated when it needs to transmit its signal.
I set the transmit time for the outputted data using the tx_time tag.
I then need to start recapturing the data after the transmission has ended.

I was wondering what is the best way to do this?

My current plan is to use nitems_read to count how many samples have been in-putted into the block.
So I would have the fist set of data (call this BlockA) that I process to estimate the required Tx time, I would then have a certain period of guard samples (lets call this BlockB) that would occupy the time between the end of BlockA and the start of the transmission. (I have tried to illustrate the frame structure below for two repetitions, with sample number increasing along the page).

|   BlockA(1)   |   BlockB(1)  |  Transmit(1)  |    BlockA(2)   |    BlockB(2)    |   Transmit(2)   | .......

I need to know when to expect the first sample of the next BlockA.
If the USRP sent a stream tag when it switched from a transmitter to a receiver this would be easy, is this possible?? I thought the USRP are meant to produce a tag on each stream disruption, but I have not been able to see any!?

If not, I need to use nitems_read. I can estimate the length of BlockB by seeing how many samples would exist between the end of BlockA and the requested transmit time.
But what would happen if I set the transmit time to not be an integer multiple of the sample period (1/rx_rate) from the start (I use the initial tag to set a time zero). i.e. Say the samples are arriving at 1us, 2us, 3us... and I set a transmit time of 8.5us.
Will the length of BlockB be rounded down to the number of complete samples between the end of BlockA and the start of transmission?

I'm worried that if I get this wrong I will start to mix up samples from BlockA and BlockB when the block is run for a long time.

Any help with this would be much appreciated.
(This question was originally posted in address@hidden but I was advised to post it here as well)

Kind regards

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