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

[Miklos Maroti]

Hi Harry,

First, you should always transmit from node A, but when you want to be
silent, then transmit something very close to zero complex numbers.
This will ensure, that you have a nice continuous stream of data going
out, and you can plan to do anything you want with sampling rate
precision (better than 1us). Once you can do this, then transmit some
pseudo random sequence from node A, e.g. BPSK with 2 samples per bit,
and it is possible to synchronize to that with sampling rate precision
again. Now comes the trick: node A not only transmits continuously,
but it also receives continuously just like node B (with an antenna or
just overhearing in the board). Both A and B synchronizes to the
signal transmitted by A. In case of node A you do not have to worry of
slightly different clocks, so once you are synchronized you will never
get out of sync if you count the number of samples. In the case of
node B it is harder, since node A might run a little faster or slower,
so you will get out of sync, so you have to maintain synchronization.
At this point, you have achieved synchronization of the two USRP
nodes: you can stop sending periodically (continue spending close to
zero samples) and then you can sample some data from node C, doing
beam forming (depends on modulation), or whatever. You can correlate
the received samples at node B with the received samples at node A
with close to one sample precision (better than 1us).

If you do not want to transmit all the time, then you can use TX tags,
but it gets a little trickier, and I think there is some bug in the
FPGA hardware to cause very rarely one sample shift between the TX and
RX chain. I am not absolutely sure about this, but I could not explain
something in any other way.

Best,
Miklos

On Sat, Oct 12, 2013 at 10:10 AM, Harry Zhang <address@hidden> wrote:
> 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.
>
> Miklos
>
> On Thu, Oct 10, 2013 at 3:51 PM, Harry Zhang <address@hidden> wrote:
>
> 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.
>
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