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

[Marcus Müller]

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Hi Jawad,

I think you might want to post follow-ups to this discussion to
address@hidden, since so far this has not much to do with
GNU Radio.

On 23.07.2014 14:11, 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.
So you put your N210s in an anechoic chamber and calibrated the
system, and where able to exclude any changing fading effects? That
sounds like a lot of work, so kudos!

A few parameters would be nice to know, though: What's the tuning
frequency, and did you actually generate sines in software (so your
over-the-air signal would have $f_carrier = f_tune +  f_sine$)  or did
you just send a constant value at a specific frequency
($f_carrier=f_tune$)?
Range, antenna and amplification used would be nice to know, too, and
daughterboard model of course.
> 
> 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.
Clever measurement setup!
> 
> 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.
"Slow AM modulation" does sound *a lot* like fading. Also, your
receivers are not "perfectly" synced, they're synced within the
specification of your GPSDO, which is quite good, but still >2ppb (if
I remember correctly). Since you add up sines, you get a resulting
sine with a phase that is the result of adding up all the multipath
signals of your two transmitters. The slightest variation in
environment might shift a lot of not-direct signals, which then might
be cancelling or constructively overlay with other paths, thus
shifting the overall phase and amplitude quite a bit.

Also notice that heat will change the the characteristics of the
transceiver's analog chain, so over time the signal of both TX USRPs
might differ due to amplifiers etc reacting differently at different
temperature.

A few numbers would be nice here, too: What's the received amplitude
in the single-TX cases, what's the range for a two-RX case? How fast
is "slow AM modulation"? Do you tune to the same frequency or do you
avoid DC-component problems by intentionally receive at a slightly
offset 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?
Usually, the frequency stability of the USRPs with GPSDOs is quite
good, and I really can't tell if the slow amplitude variation you're
seeing (but not quantifying) isn't even covered by the spec of the GPSDO.

I think you should actually measure the phase and phase derivative of
your received signal.
> 
> Thanks in advance, Jawad
> 

Greetings,
Marcus
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