Hi, 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. Thomas On 5/3/06, Thomas Schmid <[EMAIL PROTECTED]> 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. Thomas On 5/2/06, Matt Ettus <[EMAIL PROTECTED]> 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 >
CC2420_MSK_signal.png
Description: PNG image
_______________________________________________ Discuss-gnuradio mailing list [email protected] http://lists.gnu.org/mailman/listinfo/discuss-gnuradio
