Marcus - I am thinking about sending a training signal using one TX antenna and figure out the phases of other antennas by looking at the maximum correlation. Does this sound like a reasonable solution for a practical mimo system?
On Tue, Mar 3, 2015 at 11:44 AM, <mle...@ripnet.com> wrote: > Pengyu: > > I think people tend to do it differently, depending on the particular > application. In radio astronomy, they typically point the array at a > well-characterized "calibrator source" and make phase measurements, and > then tweak based on known baseline geometries. > > For commercial MIMO, using a distant transmitter of known properties and > arrival angles might be useful, or a local calibrator source. > > > > > > > On 2015-03-03 11:41, Pengyu Zhang wrote: > > Martin - I am thinking about doing beam forming around 900MHz. The number > of antennas that I want to use is around 8x8. I am thinking about using > USRP N210+SBX+OctoClock-G suggested by ettus. But not sure how much phase > mis-alignment will I observe. > > Marcus - Thanks for the tips. Maybe I should look at how other folks do > phase calibration on USRP. Any recommendation of sample projects? Ideally, > I hope that this process can be done automatically in software or in > hardware. > > Pengyu > > On Tue, Mar 3, 2015 at 11:32 AM, <mle...@ripnet.com> wrote: > >> Indeed, I should have emphasized that despite having features like >> "phase resynch", in any *practical* phased-array, you'll still need some >> mechanism for calibrating the phase centers of all your antenna. >> Differential environmental factors can, for example, affect the phasing of >> the antenna feed paths, even with your LOs all agreeing quite-well on phase. >> >> In Radio Astronomy, they do this all the time--there's always a >> phase-calibration process that is undertaken prior to observations, because >> despite careful layout of common LOs, carefully-measured feedlines from the >> same batch of cable, etc, differentials creep in. >> >> >> >> >> >> >> On 2015-03-03 11:26, Martin Braun wrote: >> >> Can you give us some details (frequency, number of antennas) that you want >> to have? >> >> A lot of people have done MIMO with USRPs, up to 100x100 antennas, so there >> is some precedent. For beam forming, you'll need very good phase alignment, >> depending on what you're planning to set up. This is always difficult, >> because all sorts of things affect phase. You should, in any case, have a >> way to test phase alignment. >> >> Cheers, >> Martin >> >> On 03.03.2015 08:18, mle...@ripnet.com wrote: >> >> If you use the phase-resynch feature of the SBX card, and you have common >> refclock and 1PPS across all your USRPs in the array, then you should be >> OK. In *general*, synthesized downconverters use fractional-N synthesizers >> where their starting phase after a re-tune is random with respect to any >> other synthesizer using the same reference clock. That's just the way >> fractional- N synthesis works. It happens that SBX uses an ADF4351 >> synthesizer, which has a (rare!) hardware feature to allow phase-resynch >> after tuning. In order to make this work, all SBXes in your "array" must be >> tuned at exactly the same time, so the timed_commands feature must be used >> to do the tuning. See: http://files.ettus.com/manual/page_sync.html On >> 2015-03-03 11:05, Pengyu Zhang wrote: >> >> Hi Guys, I am planning to build a mimo system which supports beam >> forming. Is USRP a good platform for my target? I do not want to use WARP >> because I need to operate at frequencies lower than 2.4GHz. A potential >> problem of USRP is phase alignment. As the following doc suggests, phase >> alignment can be done by MANUALLY tuning the phase. This process can be >> labor intensive when the number of antennas increases. I am also worried >> about the problem that the manual phase calibration might need to be done >> each time you use the mimo system. Is there any approach that can make this >> process much easier? >> http://www.ettus.com/content/files/kb/mimo_and_sync_with_usrp_updated.pdf >> Would be great if you can give me some suggestions about what devices >> should I use for building a mimo system for beam forming. Thanks. Pengyu >> _______________________________________________ Discuss-gnuradio mailing >> list Discuss-gnuradio@gnu.org <mailto: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 >> >> _______________________________________________ >> Discuss-gnuradio mailing >> listDiscuss-gnuradio@gnu.orghttps://lists.gnu.org/mailman/listinfo/discuss-gnuradio >> >>
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