# Qam modulation blocks in GRC cannot work btw two computers?

**URL:** <https://www.ruby-forum.com/t/qam-modulation-blocks-in-grc-cannot-work-btw-two-computers/220649>\
**Category:** GNU Radio\
**Created:** [June 15, 2012, 7:59am UTC](https://www.ruby-forum.com/t/qam-modulation-blocks-in-grc-cannot-work-btw-two-computers/220649 "2012-06-15T07:59:40Z")\
**Posts on this page:** 2\
**Page:** 1

<div class="post-metadata">

**Author:** ![aris](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/a/258eb7/32.png) [@aris](https://www.ruby-forum.com/u/aris)\
**Post date:** [June 15, 2012, 7:59am UTC](https://www.ruby-forum.com/t/qam-modulation-blocks-in-grc-cannot-work-btw-two-computers/220649/1 "2012-06-15T07:59:40Z")

</div>

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

---

<div class="post-metadata">

**Author:** ![jiajue\_ou](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/j/47e85d/32.png) [@jiajue\_ou](https://www.ruby-forum.com/u/jiajue_ou)\
**Post date:** [June 15, 2012, 6:36pm UTC](https://www.ruby-forum.com/t/qam-modulation-blocks-in-grc-cannot-work-btw-two-computers/220649/2 "2012-06-15T18:36:40Z")

</div>

On Thu, Jun 14, 2012 at 10:58 PM, jiajue ou [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid)  
wrote:

> criterion by MATLAB, however,bit error rate turned to be around 0.5 (some
> 
> Best,
> 
> Jia
> 
> * * *
> 
> Discuss-gnuradio mailing list  
> [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid)  
> [Discuss-gnuradio Info Page](https://lists.gnu.org/mailman/listinfo/discuss-gnuradio)

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?