Complex samples format with ASK modulation


#1

hello everybody,

I am using the USRP with LFRX daughterboard to catch an RFID signal ASK
modulated (frequency 13.56MHz, decimation factor 16).
I really need an help because I don’t understand very well the meaning
of the
outputs of my USRP: if the information related to the ASK is only the
amplitude, so why USRP outputs I/Q samples?
Which is the meaning of the Q samples? Which is the information carried
by the
phase (that should not be important in ASK modulation)?
If I want to plot in the time domain the samples got, which kind of
operation is
necessary (given that I would compare my result with the
usrp_oscope.py)?

Any kind of help will be really well appreciated.
Thank you,

Marco


#2

Hello Marco

I not familiar with RFID ASK10% or ASK100%.

(1)
I use usrp -> sinkfft. I can see the wave and tune the target frequency
to 13.56MHz

However, I don’t understand the question “why USRP outputs I/Q samples?”
I have to reading more knowledge about communication theory.

(2) You want to plot in the time domain
When I got RFID signal, I will do demodulation and get the information
carried by the phase. I still trying.

CJay

Marco Bottino wrote:

hello everybody,

I am using the USRP with LFRX daughterboard to catch an RFID signal ASK
modulated (frequency 13.56MHz, decimation factor 16).
I really need an help because I don’t understand very well the meaning
of the
outputs of my USRP: if the information related to the ASK is only the
amplitude, so why USRP outputs I/Q samples?
Which is the meaning of the Q samples? Which is the information carried
by the
phase (that should not be important in ASK modulation)?
If I want to plot in the time domain the samples got, which kind of
operation is
necessary (given that I would compare my result with the
usrp_oscope.py)?

Any kind of help will be really well appreciated.
Thank you,

Marco


#3

Hello CJay,

thank you for answering me!

Selon “C.cc Jay” removed_email_address@domain.invalid:

Hello Marco

I not familiar with RFID ASK10% or ASK100%.

(1)
I use usrp -> sinkfft. I can see the wave and tune the target frequency
to 13.56MHz

What do you mean with tune the target frequency? Isn’t the carrier
always at
13.56MHz?

However, I don’t understand the question “why USRP outputs I/Q samples?”
I have to reading more knowledge about communication theory.

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…

(2) You want to plot in the time domain
When I got RFID signal, I will do demodulation and get the information
carried by the phase. I still trying.

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?

Thank you,

Marco


#4

Hello
(1) I mean “usrp.tune”, use the instruction i can set my target
frequency at 13.56MHz.

Now, I know you mean Usrp output I/Q samples. And you want to use one of
them (I or Q sample).
My answer is NO, because I/Q sample you can see it like SIN and COS
signal. So need both I/Q to demodulate signal. (If I am wrong, please
tell me.)

The detail in book is very diffcult to me. I don’t know how to explain.
Maybe someone can help me to explain it.

(2)
Sorry for my misunderstand before.
Use fft_sink, you can see usrp get the target frequency signal.
for example, my target is 13.56MHz. When my RFID device near USRP, I
will see a wavefrom raise in 13.56MHz.

(3) In ISO/IEC 14443A
PCD to PICC using ASK100%, PICC to PCD using different modulation (OOK).
I just got draft spec. I need more study to understand the spec.

(4) My question to RFID spec. Do you know, what is Pause_A?

CJay

Marco Bottino wrote:

Hello CJay,

(1)
What do you mean with tune the target frequency? Isn’t the carrier
always at
13.56MHz?

However, I don’t understand the question “why USRP outputs I/Q samples?”
I have to reading more knowledge about communication theory.

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…

(2) You want to plot in the time domain
When I got RFID signal, I will do demodulation and get the information
carried by the phase. I still trying.

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?

Thank you,

Marco


#5

On Tue, Nov 25, 2008 at 9:46 AM, Marco Bottino
removed_email_address@domain.invalid wrote:

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.