Issues with QAM USRP2920

[goden]

Hello community, i've browsed through the forum for quite some time but was unable to figure out what i'm doing wrong.

I've written two VIs for the USRP2920, one sends a entered text via QAM and the other sould receive and display it.

Due to simplicity, i've decided to use a frame with a 32 byte pesudo random preamble follow by one byte describing the actual length of the text message and maximum 32 bytes of ascii (so 65 bytes is the maximum, if a text longer than than 32 character is entered, it will simply ignore all characters from 33rd on).

Frame coding and decoding works, i've tested it on a VI that will just generate a bitstream out of my frame including some random bytes before and after and then decodes the bitstram again. this works fine.

The idea is to have the sender send the frame continously while the receiver should sample about 3x the maximum frame length to ensure at least one complete frame is in the output bitstream.

Then it  just scans the bitstream for the preamble and starts decoding. But i cant get it working. I've tried playing with different I/Q rates, different filter coefficients, different frame sizes and so on.

When i watch the receivers fetched data, its all messed up and no sign of the preamble. From time to time it almost works, then most of the preamble received correctly. when i try again the output array is completely messed up. Maybe something with the synchronization is going wrong?

Im trying this on a single USRP with the antenna cable and the 30db attenuator. The device itself shouldn't have any problems, as i did some VIs using AM and FM, which work just fine.

I've attached my VIs, hopefully somebody might have a clue whats going wrong.

Regards

[JuliaO]

Hello goden,

have you also watched the sent data?
Are they the same as the recieved data?

Regards,

Julia

[goden]

Hi Julia,

thanks for the reply,

no they're not the same. I monitor the data array before its passed to the modulation VI, whats being demodulated after the transmission is completely messed up data.

It worked once...once out of approx 100 tries. Pulse shaping and matched filter is applied on both sides and the preamble is used for generating synchronization data.

[MahaM]

Hi goden,

Are you still having a problem with your program? Did you try to run some of the examples from our community page and see if you get the same behavior?

Here are some examples you could try:

https://decibel.ni.com/content/docs/DOC-25756

https://decibel.ni.com/content/docs/DOC-25709

Best Regards

[goden]

I've been through lots of example code, most of them seemed to work. But this was part of a project i had to complete at my university and i needed to prepare a VI on my own, which has failed to work. As i had to hand it over to my professor by now and i don't have access to the usrp anymore i think i don't neet anymore help 😉

Thanks for your replies

[Lary_andr]

Hello Goden,

I know that happend to you a long time ago, but I'm trying the same thing that you right now. So, I have some questions to you, but the principal is: Is possible transmit some message and receive in text, or just in table ask? Did you finish this project?