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Re: [Discuss-gnuradio] transmitting with 4 antennas

From: Martin Dvh
Subject: Re: [Discuss-gnuradio] transmitting with 4 antennas
Date: Thu, 30 Aug 2007 01:04:27 +0200
User-agent: Icedove (X11/20070607)

Ketan Mandke wrote:
> I am part of the Hydra group at UT Austin
> (http://hydra.ece.utexas.edu). We have developed a MIMO prototype
> utilizing GNU Radio & the USRP board. The physical layer for our
> design is based on the draft standard for IEEE 802.11n. Our current
> system is designed to support 2 antennas (i.e. utilizing one USRP
> board per node), however, we would like to extend this to 4 antennas.
> We are currently following the directions on the 'USRPClockingNotes'
> page for synchronizing the local oscillators of two USRP boards. The
> only problem that remains is aligning the samples. I have been looking
> through the GNU Radio codebase for some insight on how to do this. I
> found 'usrp_multi.py' which describes how to do this for the receive
> chain. However, I also have to be able to align samples for the
> transmit chain.
> Does anyone have any experience with transmitting waveforms using
> multiple USRP boards? Specifically, do you have to go through any
> special process to ensure that transmitted samples are properly
> aligned?
Yes you do need special measures to make sure the samples are aligned.
I am the author of the usrp_multi code (and the modified usrp firmware)
I was working on the transmit site but with the current framework this got 
quite complicated.

The m-blocks framework which is currently in development will make it much 
easier to synchronize multiple receive or multiple transmit streams
on seperate USRPs.
So this work has been on hold waiting for the m-blocks become available and 
usuable in the main tree.

One other thing is the phase relationship between the different USRPs.
At the moment we use the upconverters (DUCs) of the ADC/DAC chips.
Unfortunately the starting phase of these can not be set.
Only after power-up these are in a known state.
After you have used the usrp these are in an unknown state.
This means that every time you start a gnuradio script the phase relationship 
between the multiple USRPs will have a fixed unknown phase difference.

Depending on your application this can be a problem.

The only solution is not to use the DUCs in the DAC but do the upconversion in 
the FPGA.
This can be developed but you would probably also have to disable some other 
functionality because the FPGA is already very full.
(disable the RX halfbandfilters, or reduce the number of RX channels)

I hope this clears it up for you.

Martin DvH

Software Defined Radio, GnuRadio, USRP development

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