[Discuss-gnuradio] Time synchronization between two USRPs without external signal

[Harry Zhang]

Hi,
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.