Re: [Discuss-gnuradio] Beamforming with GNU Radio and USRPs

On 07/23/2014 08:11 AM, Jawad Seddar wrote:
> Hi everyone,
>
> I am currently trying to get 2 N210 to transmit coherently to a third
> N210.
>
> All devices have GPSDO kits and the "gps_locked" status is on
> "locked", there is no MIMO cable in the setup.
>
> I am currently doing some preliminary tests where I feed both sinks
> with the same signal i.e a complex sine wave using the signal source
> from GNU Radio and I observe the signal received by the third USRP. I
> made sure that both transmitted signals are received with the same
> amplitude when taken independently.
>
> My understanding is that if both transmitters are locked to the GPS
> there should be no frequency offset between them and there should
> only be a constant phase offset between them which would give me a
> somewhat constant amplitude signal at the receiver.
>
> On the receiver, I observe the signal with both a time sink and a
> frequency sink.
> My problem is that the received signal seems to be varying a lot in
> amplitude, it looks like a slow AM modulation which is something I
> would get if both transmitters weren't perfectly aligned in frequency.
>
> Changing the frequency of one of the transmitters makes the problem worse.
>
> Does anyone have prior experience with this kind of setup and
> experienced the same issues?
>
> Thanks in advance,
> Jawad
To clarify, your transmitters are both locked to *different* GPSDO
units? If so, they won't be at "exactly the same frequency".



--
Marcus Leech
Principal Investigator
Shirleys Bay Radio Astronomy Consortium
http://www.sbrac.org


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