Re: [Discuss-gnuradio] how to use the gr_mpsk_receiver_cc

[intermilan]

hi Max:
  Thank you for your help. I will look at the op25 to see if it can help me.


inter.

> Date: Thu, 12 May 2011 13:51:14 -0700
> From: address@hidden
> To: address@hidden
> Subject: Re: [Discuss-gnuradio] how to use the gr_mpsk_receiver_cc
>
>
> > Did anyone use it to receive pi/4 DQPSK signal before?
>
> In the op25-dev project <http://sedition.org.au/op25/> we see a
> very common variant of PI/4 DQPSK, so-called CQPSK/LSM.
>
> Experimentation has shown that the gr_mpsk_receiver_cc() is not
> optimal for CQPSK. There seem to be multiple causes, e.g.,:
>
> 1) PI/4 (D)QPSK apparently looks enough like 8-PSK that the standard
> GR QPSK phase detector has difficulty achieving phase lock. There's
> a hack to rotate the second of every two symbols by 45 degress which
> seems to give a large improvement [see Fig 10. of my LSM page
> at http://www.lightlink.com/mhp/lsm/ ]
>
> 2) At least for the specific case of LSM/CQPSK, it was found that a
> Gardner symbol timing loop was superior to the M&M loop (and to the
> GR PFB clock sync block) as used in the GR receivers.
>
> The op25 site has example code for using both the gr_mpsk_receiver_cc
> as well as a complete working example using Gardner's method, including
> the PI/4 hack mentioned above.
>
> Finally, you should also take a look at the new "magic" PFB receiver
> (dqpsk2.py in GR) for your application...
>
> Max
>
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