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

From: Martin Braun
Subject: Re: [Discuss-gnuradio] mimo beam forming on USRP
Date: Tue, 03 Mar 2015 08:26:27 -0800
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.4.0

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.



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?
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.

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