> I know very little about RFID, but I can make these observations on
> how I would probably perform a naive demodulation of this signal.
> 1. Tune the USRP to your carrier frequency and bring your signal
> down to baseband (centered around DC).
> 2. Choose an appropriate number of samples per symbol or bit (I'd
> arbitrarily choose 6 .. or 8).
> 3. Filter the incoming signal to some appropriate spectral mask.
> 4. Find the magnitude (or even power since it saves a sqrt) of the
> complex vector.
> 5. Develop some algorithm which accounts for frequency offset/tracking.
> 6. Perform hard decisions from the soft decisions you make from the
> algorithm developed in step (5).
Marco,
Are you trying to decode the reader transmission, or the tag reply? You may
be able to use a matched filter as a first step to clean up the signal before
demodulating. I'm not really sure what 13.56 MHz RFID signals look like, but that
helps a lot at 915 MHz.
Regardless, I think step 3 is the key to get you started. You don't want to just
throw out the I or Q channel. You probably want to use the complex_to_mag block,
and then the signal will look like what you expect.
michael