USRP can only use IQ modulation? Why if I only use I data without Q data, and there is a envelop in the received waveform?

[Jay_c]

Hello, everyone!

I am on a project that I need to transmit a sinewave with USRP. But I konw USRP can only use I/Q modulation, which means that there must be a Q part. If I only use the I data to transmit, then when I receive the waveform, I found that there is always a envelop? I attach my vi below, and I really hope someone could help me!

Thank you very much!

[Anthony_F]

Hello Jay_C

Take a look at the NI USRP Help document:

Start Menu -> All Programs -> National Instruments -> NI USRP -> Documentation -> NI-USRP Help

Check the topic:

Digital Communication System Fundamentals -> Modulation -> Modulation Schemes -> Quadrature Modulation -> Quadradture Modulated Waveforms

s(t) is the output of the USRP, and so if you wish to send a sinewave (vs a modulated sine wave), send a DC value for either I or Q (I recommend a complex double array of 0 + 0.7j)

[Jay_c]

Hi, Anthony_F

You have helped me a lot, thank you very much!

[lf_fr]

Hello,

I have the same problem with the envelop. But I just can't see why the received signal right from the niUSRP Fetch Rx Data function has the envelope. And the frequency of the envelope changes with times around some hundreds of Hz. You can see this envelope in the photo "To Forum.jpg" here attached.

This envelope is there even if I sent a sine wave or a BPSK signal. I test it with the niUSRP EX PSK Tx.vi example and it's the same situation.

Here attached I putted the Graph showing the envelop in the I/Q data, and the file used to receive (is the example file with a little modification to show the I/Q right out from the niUSRP Fetch Rx Data function).

I know that after demodulating this I/Q Data with a "MT Demodulate PSK VI" function the envelope disapears, but I need the raw data for my application. I don't know everything that the Demodulate VI does.

I need to know how to takeout this envelope and have the pure I/Q Data.

If someone can help me I'll be very grateful.

Cheers

[ErikL]

Frequency offset between the TX and RX.

[lf_fr]

Sorry, I'm using a ni 2920 usrp and LabView version 13.0.1f2

[lf_fr]

Then, if I sinchronize with a MIMO it should disapear ???

[ErikL]

if you tx and rx in the same radio, then its just phase offset (time delay).  The signal is the same, just rotated around the unit circle so the instead of exisiting only on the I axis, its inbetween the i and q axis which on the RX side shares energy between the two incoming streams.  This is perfectly normal and not a problem.

[lf_fr]

Yup, got it. Thank you very much, you helped me a lot.

I did sync with a MIMO cable between Tx and Rx USRPs and the envelope desapeared, the turn in the I/Q around the unit circle angle is not a problem, just the envelope.

Do you know some way to take this envelope out ? or algorith ?

Normaly my Tx and Rx USRP are too far away so I can't MIMO sync between them...

Thanks again

[litan]

Hello,

Actually I am working in modulation and demodulation,I am facing same problem that you also faced,but still I am not clarify my doubt so pls send me correct sine wave TX and RX.vi .And also tell me initial basic datas about to learn modulation demodulation programme designing using usrp.