Hi guys, Maybe I could jump in and share some related results. My group has been developing a MIMO system with N310 units. We did a test sounding recently where we sent 4, length 4096, orthogonal multitone signals from the transmitters to the receivers and processed the data by finding the channel response between each transmitter and receiver pair (16 in total) and recording the magnitude and phase of the arrival spikes between each pair.
We took several seconds of data and processed it in length 4096 chunks (around 1500 chunks in total) and looked at the phase difference between transmitter pairs as time progressed. Since transmitters 1 and 2 are sharing an LO and our setup was not moved during the sounding we expected to see a constant phase difference between transmitters 1 and 2 and a single receiver (same with tx 3 and 4), but we saw some drift. Worse yet, not all LO sharing pairs drifted in the same way, some didn't drift much at all while some drifted in linear or non-linear patterns. If you're all fine with me breaking the rules I can attach some png images of what we recorded so you can see what it looks like. Later this week we'll repeat the experiment but leave the machines running longer to see if the drift diminishes as the machines run longer. Cheers, -Mark On Thu, Nov 8, 2018 at 9:03 AM Daniel Jepson via USRP-users < usrp-users@lists.ettus.com> wrote: > Hi Serge, > > Are you measuring the phase offset between the TX0 and TX2 signals in a > steady-state case, or the time difference in the start of those signals? > > In the former case, your results could be impacted by the lack of internal > LO sharing between daughterboards. I would fully expect an unknown phase > offset between channels 0 and 2 every time you reconfigure the device. In > the latter case, it sounds like a start trigger mismatch like Marcus > mentioned. > > Can you share more details as to how you're measuring the phase offset? > > Thanks, > -Daniel > > > > On Thu, Nov 8, 2018 at 5:30 AM Serge Malo via USRP-users < > usrp-users@lists.ettus.com> wrote: > >> Yes: we are using UHD 3.13.1.0 RC1, with the latest file system image >> >> I can try to use lower tx start times to see if the time offset changes >> with that. >> >> Thanks, >> Serge >> >> On Wed, 7 Nov 2018 at 21:44, Marcus D. Leech <patchvonbr...@gmail.com> >> wrote: >> >>> On 11/07/2018 09:31 PM, Serge Malo wrote: >>> >>> Yes: >>> We only use one streamer for all RF outputs, and send time_spec with >>> each call to the streamer's send method. >>> We reset the internal time with set_time_unkown_pps(0), and program the >>> first samples to be streamed at a time of 0.800s. >>> It is basically the same code we used on the X300/X310. >>> >>> Thanks, >>> Serge >>> >>> Well, that is quite strange--the magnitude of the time offsets is larger >>> than I would expect. >>> >>> Perhaps someone from the N310 team can comment? >>> >>> Serge, are you using the latest UHD and system image versions for the >>> N310? >>> >>> >>> >>> On Wed, 7 Nov 2018 at 21:03, Marcus D. Leech via USRP-users < >>> usrp-users@lists.ettus.com> wrote: >>> >>>> On 11/07/2018 08:53 PM, Serge Malo via USRP-users wrote: >>>> >>>> Hi all, >>>> >>>> We are trying to send 4 synchronous signals from the 4 Tx ports of the >>>> N310. >>>> We are using UHD 3.13.1.0 RC1 under Ubuntu. >>>> Central Freq = 1575.42 GHz and 1227.6 MHz >>>> Master Clock rate = 153.6 MHz >>>> >>>> We would expect to have less than 3ns offset between all TX ports of >>>> the N310, like we do with the X300/X310. However, we have measured 4700ns >>>> between TX RF ports 0 and port 2. >>>> We have tried the next things with no more success: >>>> - Sampling rates of 25.6MSps, 38.4Msps, 76.8Msps >>>> - Init with the device options "init_cals=ALL" and "force_reinit=1" >>>> - Use the internal GPSDO >>>> - Use clock_source=external and time_source=external (from an >>>> Octoclock). >>>> >>>> Can you tell us: >>>> -What time offset between TX RF ports we should expect to achieve? >>>> -Is there anything else we can try to reduce this offset to less than >>>> 3ns? >>>> >>>> Best regards, >>>> Serge >>>> >>>> >>>> How are you setting up your TX streamer? Is it time-tagged to start >>>> at a particular device time? >>>> >>>> >>>> >>>> _______________________________________________ >>>> USRP-users mailing list >>>> USRP-users@lists.ettus.com >>>> http://lists.ettus.com/mailman/listinfo/usrp-users_lists.ettus.com >>>> >>> >>> _______________________________________________ >> USRP-users mailing list >> USRP-users@lists.ettus.com >> http://lists.ettus.com/mailman/listinfo/usrp-users_lists.ettus.com >> > > > -- > > Daniel Jepson > > Digital Hardware Engineer > > National Instruments > > > > O: +1.512.683.6163 > > daniel.jep...@ni.com > _______________________________________________ > USRP-users mailing list > USRP-users@lists.ettus.com > http://lists.ettus.com/mailman/listinfo/usrp-users_lists.ettus.com > -- Mark Wagner University of California San Diego Electrical and Computer Engineering
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