Re: [Discuss-gnuradio] 4x4 MIMO with USRP

[Ketan Mandke]

Martin,

Thanks for your response. It is disappointing to hear that the MIMO
support for 4 antennas will not be available for some time.

Could you provide some more details into why this problem of aligning
the transmit samples will be easier in the new m-block framework? Will
these new packet oriented m-blocks utilize a different interface to
the USRP which will make this alignment-task easier?

On 8/29/07, Martin Dvh <address@hidden> wrote:
> Ketan Mandke wrote:
> > Martin,
> >
> > I've been searching through the mailing list for more information
> > about synchronizing two USRP boards to create the transmitter for a
> > 4x4 MIMO node. From what I've found, it seems that this is not
> > currently implemented (see links below for relevant threads). What is
> > the current status of this effort,
>
> 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.
> > and when (if at all) do you think
> > that this functionality might be implemented?
> Since the m-blocks are maturing now, in theory the work on this could be 
> started soon.
> The problem is that I am very busy doing contract work on another part of 
> GnuRadio.
> So at the moment I don't have time for this.
> After this project it also depends on the amount of work I have, how much 
> time will be left for implementing 4x4MIMO.
> So at the soonest in the next few months.
> At the latest, in a year or two.
>
> It also depends on how many others will be working on this code. Until now I 
> have been the only one implementing the low-level MIMO stuff.
>
> If you have a budget for it. Matt, Eric and I could probably team up and 
> implement it more quickly.
> If you don't have a budget you could try to help out by doing the dirty work 
> yourself.
> We can help you on your way in this.
>
> >
> > links:
> > http://www.nabble.com/Question-about-4-x-4-MIMO-transmitting-and-receiving-tf3605863.html#a10074272
> > http://www.nabble.com/Re%3A-USRP-Tx-Question-tf1452214.html#a3924517
> >
>
>
> 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.
>
> Greetings,
> Martin DvH
>
>
>
>
> Olifantasia
> Software Defined Radio, GnuRadio, USRP development
>


-- 
Ketan Mandke