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

From: Marcus D. Leech
Subject: Re: [Discuss-gnuradio] Beamforming with GNU Radio and USRPs
Date: Wed, 23 Jul 2014 08:48:15 -0400
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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,
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

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