Hi Stephan
Thanks for your reply.
It’s good to hear you’re having similar issues with tags.
Actually in part of our larger system we do actually synchronise transmissions 
as well, but at that point ours system is transmitting OFDM so is less 
sensitive to small delays.

I was just wondering, as you are using the MIMO cable, do you request the 
current time from both usrps? Or just the one?
As we initially had an OOT block that polled both USRPs and found that the 
start up time was a fraction out, but as they both share a clock and time 
source (via-MIMO cable), we used the time from just one of the nodes and it 
seemed to work. In our case I think there was just a bit of delay in reporting 
the time from each USRP (I think it was done sequentially in C++). – Our system 
then just tagged bursts with a tx_time tag, we haven’t used messages for 
synchronising transmission bursts.

If you get a response from Ettus please put it up on the forum, or if possible 
forward it to me, as I would be really interested in finding out where these 
delays are coming from.

All the best
Will


--
William Thompson Ph.D.
Research Engineer
Toshiba Research Europe Limited
32 Queen Square, Bristol, BS1 4ND, UK
Tel: +44 (0) 117 906 0734

From: Ludwig Stephan (CR/AEH4) [mailto:stephan.ludw...@de.bosch.com]
Sent: 17 November 2015 08:38
To: Will Thompson <will.thomp...@toshiba-trel.com>; discuss-gnuradio@gnu.org
Subject: AW: USRP Latency.

Hi Will,

we have a similar problem with our set-up, and hence I can somehow confirm your 
findings, although we do not have a solution/explanation (yet), either.

We have a 2x2 MIMO N210 system (with MIMO cable and w/o GPSDO) in burst-mode 
(PDU messages from message strobe are issued every 500ms, converted into tagged 
stream and transmitted identically on both Tx antennae) and experience an 
offset of some 12µs between signals of each antenna. Internally, we take the 
current time from the USRPs directly before we transmit, then add some buffer 
time and use this as a tx_time tag’s value.

Since we do not rely on the original rx_time tag, we (and maybe you as well) 
can - at least - rule out this as a source.

In our case the offset changes between the antennae, i.e. one time ant1 is 12µs 
ahead, the other time ant2 is 12µs ahead. So it seems to be not constant :-( 
But the 12µs are pretty constant and independent from the chosen sampling 
frequency.

For the file: Up to now, we have not been able to synchronize samples for MIMO 
burst transmission with message strobes, while it synchronizes out of the box 
for continuous streams.

Unfortunately, we haven’t got an explanation, either. But I am about to contact 
Ettus support for that.

Mit freundlichen Grüßen / Best regards

Stephan Ludwig

Robert Bosch GmbH
Communication Technology (CR/AEH4)
Renningen
70465 Stuttgart
GERMANY
www.bosch.com<http://www.bosch.com>

Tel. +49(711)811-8809
Fax +49(711)811-5187845
Mobile +49(172)5630639
stephan.ludw...@de.bosch.com<mailto:stephan.ludw...@de.bosch.com>

Registered Office: Stuttgart, Registration Court: Amtsgericht Stuttgart, HRB 
14000;
Chairman of the Supervisory Board: Franz Fehrenbach; Managing Directors: Dr. 
Volkmar Denner,
Dr. Stefan Asenkerschbaumer, Dr. Rolf Bulander, Dr. Stefan Hartung, Dr. Markus 
Heyn, Dr. Dirk Hoheisel,
Christoph Kübel, Uwe Raschke, Dr. Werner Struth, Peter Tyroller
Von: 
discuss-gnuradio-bounces+stephan.ludwig2=de.bosch....@gnu.org<mailto:discuss-gnuradio-bounces+stephan.ludwig2=de.bosch....@gnu.org>
 [mailto:discuss-gnuradio-bounces+stephan.ludwig2=de.bosch....@gnu.org] Im 
Auftrag von Will Thompson
Gesendet: Freitag, 13. November 2015 17:18
An: discuss-gnuradio@gnu.org<mailto:discuss-gnuradio@gnu.org>
Betreff: [Discuss-gnuradio] USRP Latency.

Hi
I have question about the latency of transmissions in the N210 USPR.

A bit of back ground in my system:
I have three nodes.
One (Node1) that periodically transmits a signal.
The one of the other nodes (Node2) tries to synchronise its transmission with 
that of Node 1.
Node3 is simply used to record the transmissions and check that they align.

When Node2 starts up, it saves the rx_time of the initial stream item Tag from 
the USRP source.
It then listens for the periodic transmission from Node1. Node2 then estimates 
the timing offset of Node1’s transmission and starts to transmits its signal 
that should be synchronised with Node1’s.
It adds a tx_time tag to its burst which is based on the rx_time tag of its 
first arriving sample and a multiple of the sample time and then number of 
sample bins needed to align with Node1’s transmissions.
(the system is working with a sample rate of 1Ms, centre frequency of 2.48GHz, 
and using N210 with XCVR2450)

The issue I’m having is that the actual transmission of Node2 is about 26us 
behind that of Node1, and I’m wondering why this might be… is this a 
predictable delay, or might it change with other systems?

Possible issues might be that it is the delay between the Tx burst being 
released from the buffers and actually being transmitted, but I feel that 26us 
is quite a long time really.
Or it might be from the original rx_time tag somehow.

Any advice or insight into this would be greatly appreciated.

Thanks.

Will

--
William Thompson Ph.D.
Research Engineer
Toshiba Research Europe Limited
32 Queen Square, Bristol, BS1 4ND, UK
Tel: +44 (0) 117 906 0734


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