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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 09:41:28 -0800
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On 03.03.2015 08:41, 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.

In that case, SBX will work fine. How much phase alignment you'll observe depends on your actual setup... there's a lot of factors (which is why, as Marcus mentioned, overall systems usually get calibrated before usage).

If you have specific questions or concerns, I suggest you contact Neel (CC'd).


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
<mailto: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  <mailto: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?
    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
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