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, [email protected] 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 [1] 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 [2] 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 [email protected] <mailto:[email protected]> https://lists.gnu.org/mailman/listinfo/discuss-gnuradio [3] _______________________________________________ Discuss-gnuradio mailing list [email protected] https://lists.gnu.org/mailman/listinfo/discuss-gnuradio [3] _______________________________________________ Discuss-gnuradio mailing list [email protected] https://lists.gnu.org/mailman/listinfo/discuss-gnuradio [3] Links: ------ [1] http://files.ettus.com/manual/page_sync.html [2] http://www.ettus.com/content/files/kb/mimo_and_sync_with_usrp_updated.pdf [3] https://lists.gnu.org/mailman/listinfo/discuss-gnuradio
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