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

[Ben Reynwar]

On Thu, Jun 14, 2012 at 10:58 PM, jiajue ou <address@hidden> wrote:
> 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
>
>
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>

The synchronization can be slow, so if you're sending for only a short
time, you might not ever be synchronizing.
How does it work if you put a channel block with a frequency shift in
the one computer flow graph?