[Discuss-gnuradio] qam modulation blocks in GRC cannot work btw two computers?

[jiajue ou]

Hi all,

 

I’m playing with qam modulation and demodulation blocks using GRC.

To begin with, I built a flow graph in one computer as follows:

File source---->packet encoder---->qam mod----->qam demod----> packet decoder----->file sink

It worked well. The file sink can get what is in the file source.

 

Then, I separated the flow graph in two computers connected to a USRP N210 each to test real environment performance:

Computer1: File source---->packet encoder---->qam mod----->usrp sink

Computer2:usrp source----> qam demod----> packet decoder----->file sink

However, the file sink cannot get  anything.(both with/without differential encoding )

 

To find out why, I pinted out the transmitted symbols in qam mod block in gnuradio/gnuradio-core/src/gengen/chunks_to_symbol_bc.cc in a txt file, and received symbols in qam demod block in gnuradio/gr-digital/lib/digital_constellation_recever_cb.cc in another txt file. Next, I demodulate those symbols based on closet Euclidean distance criterion by MATLAB, however,bit error rate turned to be around 0.5 (some shift is made to find the minimum ber ) which implies that received symbols and transmitted symbols are independent. AM I RIGHT ABOUT WHERE TO PRINTING OUT THE SYMBOLS?

 

One thing to notice, when I changed qam blocks to OFDM blocks, the file sink can get what is transmitted properly even in the case of  two computers.

 

Therefore, I’m wondering if there is any problem with the qam blocks, like synchronization or channel equalization etc. that result in transmission failure. How can I solve it? MY PURPOSE IS TO GET correct symbols to demap to bits by myself.

 

Sorry for sending such a long mail. Thank you very much for your help.

 

Best,

Jia