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Re: [Discuss-gnuradio] Time synchronization between two USRPs without ex

From: Harry Zhang
Subject: Re: [Discuss-gnuradio] Time synchronization between two USRPs without external signal
Date: Sat, 12 Oct 2013 16:10:19 +0800
User-agent: Mozilla/5.0 (Windows NT 6.1; rv:17.0) Gecko/20130801 Thunderbird/17.0.8

Dear Miklos,
    I'm glad to hear from you.
    The idea of this experiment is quite similar to the core of your honored paper "The flooding time synchronization protocol". It's a transmitter-receiver sync method using precious tx/rx timestamp to synchronize transmitter's and receiver's local timer.
    On the transmitter side, sync message is transmitted every 1 sec. Using rx tags, it's easy to get the average receive interval is 1.0003sec and the jitter is around 320us. Considering the interval jitter is 2*(rx jitter+rx jitter), the sync accuracy is 160us.
    I wanna break into USRP FPGA to achieve 1us or less accuracy. And I don't understand your "continuously transmission". Could give me some details.

2013/10/12 9:03, Miklos Maroti wrote:
Hi Harrz,

What do you mean by 160us precision? How did you measure it or compute
it exactly? I do not understand your goal, but it is quite simple to
synchronize two usrps with continuous transmission to within one
sample and if you continuously receive the transmitted signal on the
transmitter side, then you can avoid all time stamping problems and
effectively synchronize the tx and rx chains of a single usrp.


On Thu, Oct 10, 2013 at 3:51 PM, Harry Zhang <address@hidden> wrote:
I have implemented time synchronization between two USRPs without GPSDO,
MIMO cable or referring to computer's time.It's a sender-receiver method
based on message exchange. It will be included in my next paper soon.
I use the tx_time and tx_sob tag to transmit the message at the planned
time. When this message researches the receiver, I can get the receive
time via rx_time tags. The transmit and receive time of tx tags and rx
tags are recorded in USRP motherboard without the jitter of Ethernet
cable or operating system. So I think it could achieve the best accuracy
without modifying FPGA.
The experiment shows the accuracy is around 160us. I think it's mostly
caused by the jitter of the tx tag's time. I wanna achieve better
accuracy than this and the best way is adding a hardware timestamp
module in FPGA. Is this way feasible?
As for now, I wanna get a depth understanding of the implementing of tx
tag,so I will know the accuracy limit of this method. But I'm not
familiar with the FPGA, so could anyone describe how tx_time tag
implemented or give me some documents about this?
Thanks in advance.

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