Re: Complex samples format with ASK


#1

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


#2

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.

  1. 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

To try to answer Marco’s original question about I/Q directly: the phase
info available from I/Q samples is not directly useful for RFID ASK
signals.
However, the calculated magnitude provides slightly better SNR than
would
samples of magnitude alone.

I’ve done a lot of work with 13 MHz RFID. Most common form of modulation
is
Amplitude Shift Keying, a form of AM, with the data stream Manchester
encoded.
http://en.wikipedia.org/wiki/Manchester_coding
A good example of channel filtering and AM demodulation using
complex_to_mag
is usrp_am_mw_rcv.py. That example is setup for AM broadcast, so you’ll
want
to widen the channel filter BW out to accomodate the baud rate you’re
using,
and, you can eliminate the audio filtering and sink code. The
demodulated
output is the ‘envelope’ of the signal, with an offset the depends on
the
percentage of modulation achieved. The RFID reader in Marco’s case
achieves
100% modulation, so the offset will be 50% of peak level, like this
(fixed
font spacing req’d):
___ ___ ___
| | | | | |
0 | |___| |___| |

RFID tags typically achieve much lower modulation percentages, depending
on
coupling with the reader, like this:

   ___     ___    ___
  |   |___|   |__|   |

0 | |

In either case, you can threshold the data at some value less than the
average peak to do a crude conversion from envelope to bits. In turn,
you
can examine the bitstream to locate bit transitions. The bit stream can
then
be decoded. There are better ways, e.g., using matched filters, to
identify
Manchester codes directly from the envelope data, but this should get
you
started.
Paul M.


#3

Hello,

Selon Paul M. removed_email_address@domain.invalid:

You have to choose reasonable values for the decimations and filter
parameters. The attached file runs without generating compiler errors, but
it’s up to you to take it from here.
Paul

If, as you say, it runs, what does “RuntimeError: complex_to_mag(4):
insufficient connected output ports (1 needed, 0 connected)” means?
Maybe that the output is not used (even if I don’t understand why), so
for that
it stops?

thank you

Marco