USRP 2922 IQ data are interfering with other

[Lollipop_man]

 I am working with usrp 2922,and  i transmit the data, the IQ  data that I receive are  interfering with other.

And the I data when accepted into Q data.The Q data.when accepted into I data.

I  do'not know why and how  to correct the mistake.

Thanks.
 

[psisterhen]

Different oscillators are used for Rx and Tx, so while they are frequency-locked, they may be in different phases.  The result is that your received signal will be:

I_rcvr = I*cos(phi) + Q*sin(phi)

Q_rcvr = I*sin(phi) - Q*cos(phi)

If the phase difference (phi) is 0, then there is no Q in I_rcvr and no I in Q_rcvr.  Otherwise, there is a mix.

This can be corrected by applying a complex phase shift (multiply signal by e^(-j*phi)) to the data.

[Lollipop_man]

Thanks for your answer.

But I am confused about  applying a complex phase shift (multiply signal by e^(-j*phi)) to the data.How to do this? 

[psisterhen]

There are many methods to determine the phase shift at the receiver.  You'll have to research the best one for your application.  One method is including some known, fixed data as a training sequence at the beginning and using this to determine the phase shift through the channel.

Here is how to implement the complex multiple to perform a phase shift.

[Lollipop_man]

When I tansmit the I data (all 1)and Q data(all 0),but the IQ data I receiver is an irregular wave.

Here are the IQ data that I transmit and receive.  This let me at a loss.

Thank you so much and I am  waiting for your reply.

[Lollipop_man]

When I tansmit the I data (all 1)and Q data(all 0),but the IQ data I receiver is an irregular wave.

Here are the IQ data that I transmit and receive.  This let me at a loss.

Thank you so much and I am  waiting for your reply.

[psisterhen]

If all I=1 and all Q=0, your complex baseband signal is just a DC signal.  So it is just a DC "tone" sitting at 0 Hz, which is right on top of your 2GHz carrier frequency.  What you are seeing in those plots is probably just noise.  All of your IQ signal is just at 0 Hz.  Phase differences b/w the Tx and Rx oscillator will change the amplitude of this DC signal.  Generally speaking, you want your IQ signal to be away from your carrier.

There are a lot of online resources to build your understanding of IQ signals, modulation, and RF basics.  Here are some links you may find useful:

http://www.ni.com/rf-academy/

http://www.ni.com/webcast/128/en/

http://www.ni.com/white-paper/3992/en/

http://www.ni.com/tutorial/4805/en/