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Re: [Discuss-gnuradio] mimo beam forming on USRP

From: mleech
Subject: Re: [Discuss-gnuradio] mimo beam forming on USRP
Date: Tue, 03 Mar 2015 11:44:28 -0500
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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.

On Tue, Mar 3, 2015 at 11:32 AM, <address@hidden> 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.


On 03.03.2015 08:18, address@hidden 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 address@hidden <mailto:address@hidden> https://lists.gnu.org/mailman/listinfo/discuss-gnuradio
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