Hi List, thank you all for your comments and suggestions. I found the solution. Viewed from now, it was pretty simple: If one changes the code as stated in the original post, the time tags will only be inserted sometimes, i.e. only if noutput_items is not a multiple of the packet_len value, because of the if clause in l. 475. My_tag_count basically is the global time index of the packet_len tag and samp0count is the index of the first input sample. This code makes the output packets be aligned to packet_len's value.
Now, if the packets are already aligned (and this was the case when using the message strobe), the if clause is never entered, and the time tag is never added. Solution: Add the two code lines below l. 485 such that the tags are always added. The break in line 477 ensures that the tag is only inserted, if the packet_len tag is at the first sample of the input buffer. Maybe this might be a thing to change in mainstream code, because at least I expected packet transmission to be synchronized in the MIMO case (everything else is useless). While it is a necessity for MIMO, there might be a performance drawback in a single transmitter scenario. Thank you all again for all your help. Mit freundlichen Grüßen / Best regards Stephan Ludwig Communication Technology (CR/AEH4) Robert Bosch GmbH | Renningen | 70465 Stuttgart | GERMANY | www.bosch.com Tel. +49(711)811-8809 | Mobile +49(172)5630639 | Fax +49(711)811-5187845 | 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 -----Ursprüngliche Nachricht----- Von: discuss-gnuradio-bounces+stephan.ludwig2=de.bosch....@gnu.org [mailto:discuss-gnuradio-bounces+stephan.ludwig2=de.bosch....@gnu.org] Im Auftrag von Ludwig Stephan (CR/AEH4) Gesendet: Dienstag, 17. November 2015 16:56 An: discuss-gnuradio@gnu.org Betreff: [Discuss-gnuradio] N210 MIMO packet TX time alignment when using message strobe 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 Tel. +49(711)811-8809 Fax +49(711)811-5187845 Mobile +49(172)5630639 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 _______________________________________________ Discuss-gnuradio mailing list Discuss-gnuradio@gnu.org https://lists.gnu.org/mailman/listinfo/discuss-gnuradio _______________________________________________ Discuss-gnuradio mailing list Discuss-gnuradio@gnu.org https://lists.gnu.org/mailman/listinfo/discuss-gnuradio
