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

[Harry Zhang]

Dear Miklos,
    Thank you for your inspiring reply.
    1.I do think this method sounds like a receiver-receiver sync while 
sync message's transmitter A also doing beacon node C's function ( 
(1)sending sync message and (2)recording receive time which would be 
sended to B for sync).Is it correct?
    2.For 1us accuracy, does it mean the sample rate must be more than 1e6?
    3.Does "close to zero samples" means the sample_rate*sample_offset 
produces larger error when I use it for getting sync message's receive time?
    4.The 1us is the jitter caused by sample duration.What about the 
jitter produced by tx/rx tags? I do think 160us is mainly caused by the 
difference between actual time when message leaving/arriving antenna and 
tx/rx tags's time.What's your opinion?

Best,
Harry

2013/10/12 19:17, Miklos Maroti wrote:
> 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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