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Re: [Discuss-gnuradio] Very low packet loss rate for the discontinuous B

From: Alex Zhang
Subject: Re: [Discuss-gnuradio] Very low packet loss rate for the discontinuous BPSK communications- the analysis and looking for solution
Date: Wed, 6 Mar 2013 12:15:09 -0600

Thanks you all.
I will try to adjust th loop_bw in the fll firstly to see what happened.

And I am not understanding the function advance_loop(error) well:

gri_control_loop::advance_loop(float error)
  d_freq = d_freq + d_beta * error;
  d_phase = d_phase + d_freq + d_alpha * error;

I am not clear why d_freq and d_phase are updated in this way. Is the d_freq the frequency offset estimate every step?

According to the http://www.trondeau.com/blog/2011/8/13/control-loop-gain-values.html , i have a rough picture on this, but still confused why d_freq is only dependent on d_beta.

The other question is: Is it possible to turn off the loop when there is no input signal (only noise) and turn on the loop when signal is detected?
If possible, how to update hte d_freq and d_phase?

On Wed, Mar 6, 2013 at 7:08 AM, Tom Rondeau <address@hidden> wrote:
On Wed, Mar 6, 2013 at 6:42 AM, Sreeraj Rajendran
<address@hidden> wrote:
>>>I don't believe this problem can be solved by adjusting the loop
>>> bandwidth.
> Acquisition time is roughly inversely proportional to loop_bw and error
> variance is proportional to loop_bw. So there is always a tradeoff between
> acquisition time and tracking performance. You should try adjusting loop_bw
> (http://www.trondeau.com/blog/2011/8/13/control-loop-gain-values.html).
>>>Applying faster tracking algorithm may solve this problem, but I think it
>>> takes long time to develop for a new guy.
> If you have a good understanding of the algorithm, its really easy to
> prototype the alogorithm in GRC or  by using gnuradio python-block coding
> support. "gr_modtool" is always there to make block coding easier.
> -Sreeraj

Also hand tune to make sure the frequencies of the transmitter and
receiver are as close as you can get them. The slowest loop in this
chain is the frequency lock loop, so overcome that by hand and see
what happens.


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