[Discuss-gnuradio] N210 MIMO packet TX time alignment when using message strobe

[Ludwig Stephan (CR/AEH4)]

Hello list,

I have a really tricky question that we have been working on during the last 
few weeks. I am asking for any advice how to handle this issue:

In short: Samples input into the USRP sink do not appear at both hardware 
outputs at the same time, but are set off by several sampling periods. 
Meanwhile, we got the system synchronized when using tagged streams, but if we 
use a message strobe as a periodic trigger, sync is lost again.

The long story:
We are trying to synchronize samples (not carrier phase) in time at a 
transmitter, which consists of 2 N210 (SBX daughter board, w/o GPSDO) connected 
by a MIMO cable. We managed to synchronize the samples for the transmission of 
a "packet_len"-tagged stream, which does not work out-of-the-box, by adapting 
the source code of 
https://github.com/gnuradio/gnuradio/blob/master/gr-uhd/lib/usrp_sink_impl.cc#L476
We changed the USRP_sink_impl.cpp file in such a way that is reads the current 
URSP time right before transferring the data to UHD/USRP and attach this time 
plus a buffer time as a tx_time tag.

We inserted the following lines after line 476
_metadata.time_spec = get_time_now() + ::uhd::time_spec_t(0.15);
_metadata.has_time_spec = true;
, where 0.15 [ms] is the time buffer which shall care for the transmission 
latency, in order to have the hardware clock have a time smaller the tag's 
value.

Question #1: Do you see any negative effects by this change?

This works for a simple transmission with tagged streams.

But: As soon as we switch to a transmission, which is triggered by a periodic 
message strobe, the time alignment is lost. The setup is:
message strobe (with PDU content) -> PDU to tagged stream (packet_len) -> 
unpack K Bits (convert 1 Byte to 8 Bits) -> tagged stream multiply length tag 
(by 8) -> UChart to Float -> Float to Complex (as real part) -> both data 
inputs of USRP sink

By measuring both antenna RF output signals (modulated at 500 MHz carrier 
frequency) with an oscilloscope, we determine a timing offset of some 12µs for 
long (<500) packet lengthes, which is pretty much constant; but which signal is 
ahead of the other seems to be random: one time antenna 1 is 12µs ahead of 
antenna 2, the other time it is vice versa.

The timing offset depends on the the packet_len,if it is chosen small (1..500); 
the 12µs seem to be an asymptote.
Even more strange is that this delay is constant during a run of the model, but 
changes (randomly?) from execution of the model to execution.

The timing offset is independent from the chosen sampling frequency (at least 
for large packets).
Since we intend to use 10 MHz of sampling frequency, 12µs offset results in a 
shift of several samples

Today, there has been raised a similar but different issue by Will. Basically, 
I am asking the same questions but from a different background. Since I am not 
sure whether our topics have the same cause, I ask in a separate thread.

We have an open ticket with NI/Ettus support, but they asked me to also post 
this issue here, since there exists some overlap.

Question #2: Do you have any hint to help us solve this issue?

I noticed similar questions, which did not help us so far:
http://lists.ettus.com/pipermail/usrp-users_lists.ettus.com/2014-June/009842.html
http://lists.ettus.com/pipermail/usrp-users_lists.ettus.com/2014-April/009274.html,
 which might be a hint to a work-around.

Any help is appreciated

Best regards,

 Stephan Ludwig

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

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