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[Discuss-gnuradio] MSK de-/modulation

From: Thomas Schmid
Subject: [Discuss-gnuradio] MSK de-/modulation
Date: Mon, 8 May 2006 15:56:54 -0700


I figured out by now that the problem is not on how I decode MSK
within GNURadio, but that the signal I receive from the USRP is very
strange. To verify what my test code does, I loaded the received
message in octave and plotted the I and Q phases. A sample plot is
added to this email. I sampled the signal at a decimation rate of 8
which should give me 8 samples/symbol for the I and Q channel.

As one can see from the plot, this is not the case and there are a lot
of anomalies:

- First two green peaks are exactly 6 samples from each other, same
for the two red peaks just after that or the first red transition from
low to high.
- At around 30, the red signal has a bump in it. It could be decoded
as two 0 or only one.
- Just before 40, the green signal has a very wide swing, again
questionable if it is two or one 0
- Before 60, red swings towards low, though stops and goes back up.
Again questionable if we should decode that as a 0 or did the signal
just over-swing into the negative area.

Could it be a problem with the RFX2400 daughter boards? Any
suggestions on how to deal with this problem are appreciated.


On 5/3/06, Thomas Schmid <address@hidden> wrote:
Hi Matt,

Can you give me some pointers on what I would have to tweak in the
GMSK code? I read up on MSK but most demodulators I found use
integration of the I and Q phases and then proper slicing. I can see
how that works if I have synchronization, but I don't know how to
achieve that.

Here some more details on what I try to do: I try to decode a message
sent out by a chipcon CC2420 in the 2.4GHz ISM band. The over all
symbol rate is 2Mchip/s, thus I sample at 4MHz.


On 5/2/06, Matt Ettus <address@hidden> wrote:
> Thomas Schmid wrote:
> > Hello,
> >
> > Did anyone write blocks for MSK de-/modulation? I couldn't find
> > anything in the gnuradio-core directory and I wanted to ask before I
> > start implementing it in order to avoid duplicated work.
> The code for GMSK will work with some minor tweaks for plain MSK.
> Matt

Attachment: CC2420_MSK_signal.png
Description: PNG image

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