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Re: Re: [Discuss-gnuradio] MIMO Question

From: Eric Blossom
Subject: Re: Re: [Discuss-gnuradio] MIMO Question
Date: Mon, 17 Aug 2009 12:31:32 -0700
User-agent: Mutt/1.5.18 (2008-05-17)

On Mon, Aug 17, 2009 at 11:23:42AM -0400, Bruce McGuffin wrote:
> Eric Blossom wrote:
>> On Fri, Aug 14, 2009 at 04:55:56PM -0400, Bruce McGuffin wrote:
>>> A while ago (Nov. 14, 2008) Matt talked about using a USRP2 with
>>> two antennas and two external radio receivers driving the basicRx.
>>> My question is, is it possible to do that if the external radio IF and
>>> bandwidth are too high to sample at 25 MHz without mixing the
>>> signal to a lower freqeuncy first? I'm concerned that the reference
>>> frequency into the mixer is probably complex, and the mixing process
>>> will wind up putting energy from both formerly real signals onto each
>>> of the mixer outputs. Is that true, and if so is there a way around it
>>> that is already implemented? What gnuRadio modules would I use?
>>> Thanks
>>> Bruce
>> Bruce,
>> All of this depends on the center of your IF and how wide it is.
>> If the numbers are right, bandpass sampling definitely works.
>> The USRP2 samples at 100 MS/s.
>> What's your IF freq?
>> How wide is it?
>> Eric
> Eric
> My IF center frequency is 21.4 MHz, and my bandwidth is about 20 MHz.
> The good news is I don't need the full IF bandwidth. I will probably
> stick an analog filter in front of the USRP2 to reduce the aliased power  
> of the image.
> But, for the sake of flexibility (I might want to use wider band signals  
> in the future)
> and to keep costs down, I don't want to use an analog filter that is too  
> narrow.
> The numbers can be made to work.

> I'm trying to avoid extensive (well, really, any if I can) FPGA
> programming, and wonder if versions of the USRP2 firmware exist that
> can either bandpass filter, then down-sample, or mix the signal to a
> lower IF with a sinusiod rather than an exponential, and low-pass
> filter. I prefer the direct down sampling approach.


The standard firmware and fpga image should work fine for the single
channel case.  

In the single channel case this will capture the entire IF and
translate it to baseband:

   u.set_decim(4)               # 25MS/s complex across ethernet (100MB/s)
   u.set_center_freq(21.4e6)    # tune to the center of the IF

There's currently no support in the standard code (host/firmware/fpga)
for multiple channels.  This will change when we switch to over to the
VRT interface.


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