Re: [Discuss-gnuradio] Re: Complex samples format with ASK modulation

[Brian Padalino]

On Tue, Nov 25, 2008 at 9:46 AM, Marco Bottino <address@hidden> wrote:
> What do you mean with tune the target frequency? Isn't the carrier always at
> 13.56MHz?
>
> I asked that because in my opinion I should not need any kind of phase
> information because of the ASK modulation used that is an amplitude 
> modulation.
> So I don't know if I have simply to throw away the Q samples or not...
>
> I'm sorry for my ignorance but which is the information carried by the phase?
> With "demodulation" what do you exactly mean? USRP isn't able to sample
> directly at the RFID's frequency?
>
> Finally, reading the ISO standards it seems that is always used the ASK
> modulation... isn't it?

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).

If you didn't follow any of this, I might suggest you take a look at
the Suggested Reading page on the Wiki:

    http://gnuradio.org/trac/wiki/SuggestedReading

Good luck with your demodulator.

Brian

PS - Keep in mind that if you want want to open source your RFID
application, you can apply for a project on the Comprehensive GNU
Radio Archive Network:

    https://www.cgran.org/

It will provide you with a way to share your code with the community,
receive help from those interested, and get free version control all
very easily.