Using a MPSK Demodulator or 4QAM for oQPSK

GNU Radio
1

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 S. 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 = .25gain_mugain_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

2

On Sat, May 26, 2012 at 12:08 PM, Marius [email protected]
wrote:

gain_mu=0.03
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