Re: [Discuss-gnuradio] signal cancellation example using GRC and USRP B210

[Inkyu Bang]

Dear Marcus

Thank you for the comments.

I am trying to make a beam which is canceled out (i.e., null) at a specific location but is not elsewhere. 

I am aware of beamforming and half-wavelength things

(By the way, a wavelength of 6GHz is 5cm)

So I put two Tx antennas and one Rx antenna in proper locations to satisfy the condition which you said (i.e., half wavelength distance difference).

I do not know much about phase offset and USRP hardware things.

Can I ask a few more questions?

(It might be silly questions.)

I used USRP B210 which has two RF chains and it seems to use the same clock for those two RF chains.

Here is a link to brief specification of B210:

https://www.ettus.com/content/files/b200-b210_spec_sheet.pdf

Q1) In this case, is there a chance of different phase offsets between two antennas from the same USRP?

Q2) Then, how can we calibrate those different phase offsets between two antennas in the same USRP?

(What happens in MIMO transmission in real SDR devices?)

Thank you.

Regards,

Inkyu

2018. 4. 27. 오후 10:13, Müller, Marcus (CEL) <address@hidden> 작성:

Dear Inkyu,

there's no guarantee that the phases of both TX are exactly identical.
In fact, you should expect an unknown offset.

Also, you'll notice that your two antennas are at two different
positions in the room. You're accidentally building a beamforming
system! So, these nulls will not be everywhere in the room, but only in
a specific direction in the far field of the TX antenna. And with a
carrier frequency of nearly 6 GHz, a wavelength in air is

c/f ≈ (3·10⁸ m/s) / (6·10⁹ 1/s) = 0.05 m = 50 cm

and half that distance from a (single) transmit antenna would be the
distance at which two received signals would be exactly of opposite
phase.

So, I'm not sure what you want to demonstrate, but unless it's
beamforming, you're probably not doing it right :) Which is no shame,
but it would be interesting to hear what it is that you want to
achieve.

Best regards,

Marcus

On Fri, 2018-04-27 at 20:44 +0800, Inkyu Bang wrote:

Hi, all !!

I am trying to make a simple example of signal cancellation using GRC.
However, I did not get any good results.

Here is my setting

Plan: sending two sine wave with different phases and receiving canceled signal (only noise)
Center frequency: 5.89GHz (I am using the antenna supporting this center frequency)
Sampling rate: 5MHz
Frequency of sine wave: 1kHz

USRP: B210
Tx GRC flow graph: signal source block + USRP sink (two channels)
Rx GRC flow graph: USRP sink + QT GUI time sink

When I send two sine signals, I used two RF chains in the same USRP.
I received those signals at the other USRP.

I expected to see canceled signals (i.e., only noise level power in time domain)
but still, I see sine wave signal.

I thought frequency offset in the receiving USRP is applied to both sine waves.

Do I need to consider frequency offset first?
Could anyone help me to solve this problem?

Thank you.
Regards,

Inkyu
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