Re: [Discuss-gnuradio] Using a MPSK Demodulator or 4QAM for oQPSK

[Tom Rondeau]

On Sat, May 26, 2012 at 12:08 PM, Marius <address@hidden> wrote:
> Hi!
>
> I found a comment in the source code of the MPSK block, that it could
> be used for oQPSK demodulation. I think this block is very interesting
> implementation, however I'm not sure which parameters I should use for
> oQPSK. In the gr-ieee802_15_4 implementation from Thomas Schmid he
> uses a Quadrature Demod with an MM Clock Recovery where the parameters
> are:
>
> omega = self.sps
> gain_mu=0.03
> mu=0.5
> omega_relative_limit=0.0002
> gain_omega = .25*gain_mu*gain_mu        # critically damped
>
> This works just fine - with a Quadrature Demodulator.
>
> Can somebody suggest parameters for the MPSK Block, or give me some
> hints how I can calculate/test them at least? The MPSK block uses a
> Costas Loop for Demodulation, so the approach is quite different here.
> Also I wonder whether it's possible to use a 4QAM Demod, because
> practically a 4QAM mod produces the same wave-form like a QPSK.
> Practically at least ,)
>
> Best,
> Marius

Marius,

Take a look at the new implementations of the digital demodulators now
in gr-digital/python/generic_mod_demod.py. We've split out the
different functions from the mpsk receiver. You can adjust them a bit
more easily now, and this might help you. Specifically, the recovery
loops are now a bit more robust and take in a single loop bandwidth
factor instead of trying to set alpha and beta. The result is more
stable than before, in my experience.

Tom