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

[Marcus D. Leech]

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