Re: [Discuss-gnuradio] USRP: is performance degradation of PSK OK?

[Alick Zhao]

On Wed, 25 Apr 2012 12:01:33 +0800, Alick Zhao wrote:
> On Tue, 24 Apr 2012 07:52:09 -0700, Nick Foster wrote:
> 
>>
>> Alick,
>>
>> You will have to instrument your flowgraph in order to find out. Add file
>> sinks to different stages in the transmitter and receiver to verify that
>> the data looks as expected. You can use MATLAB or Octave to visualize the
>> data. There are scripts in gnuradio-core/src/utils which will aid you in
>> loading sample data into MATLAB. Verify that the frequency offset is within
>> bounds and being handled appropriately. Verify that clock recovery is
>> keeping a lock on the incoming data.
>>
>> --n

I also make some plots with the logged data. I can see that consecutive
1000 points of data after freq_recov or time_recov are distributed
around the unit circle. I think the freq/time recovery may not work
well. Given static indoor environment, the coherence time should be
long. I'd expect that the constellation points after timing recovery
should stay around two certain points for some thousands of samples. Is
it so?

If the problem is with the recovery loop, I wonder how I can adjust the
parameters of the loop. I have read Tom's post about loop gain
values[1]. It seems that damping factor (0.707) should not be changed,
and loop bw is somewhere around pi/100 to 2*pi/100. So I find little
change can be done.

By plotting data directly output by usrp source I find that the
constellation points are disperse too. But I think it is due to lack of
synchronization between tx/rx. Am I wrong about this?

[1] http://www.trondeau.com/blog/2011/8/13/control-loop-gain-values.html

-- 
alick
Fedora 16 (Verne) user
https://fedoraproject.org/wiki/User:Alick