Forum: GNU Radio Please help me understand wave.jpg [attached] representing the plot of the usrp_rx_cfile.py output

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Da4bb53be2e97b4449cf2c49a959a78f?d=identicon&s=25 Bishal Thapa (Guest)
on 2009-03-06 11:10
(Received via mailing list)
Attachment: wave.jpg (40 KB)
Dear all,
  Please help me understand this output from usrp_rx_cfile.py. I have
attached the plot as the jpeg at the end:

1. I send a file with 1500 bytes of spaces (Space = 0x20 = 0010 0000 in
binary)
2. I use usrp_rx_cfile.py to capture the samples, and focus in on
roughly
24600 samples (because 1500 bdpsk modulated bytes = 1500*8*2 + preamble
+
CRC..)
3. multiplied by 2 because its 2 samples per symbol (one bit per
symbol).
Therefore, I ignore every other sample. So down to 12300 samples
roughly.
4. I turned off whitening and dewhitening. Hence, I should now see dbpsk
samples that represent 0010 0000 0010 0000.....
5. I do see the pattern when I look at the angle formed by those complex
symbols in MATLAB [using  plot(angle(read_complex_binary('s.dat')))]
6. But I also see some kind of unexplainable shifting once I take the
absolute value of the angles of the complex symbols [shown in the jpg].

Can somebody please tell me if there is some kind of shifting
implemented in
the dbpsk modulation which causes the plot of the angle of these complex
samples
captured by usrp_rx_cfile.py to look like this. Please note that I turn
off
whitening/dewhitening so the pattern do resemble 0010 0000 0010 0000
somewhat.

-Thank you for your time,
-B.
A576b201df57014077494da786319110?d=identicon&s=25 Tom Rondeau (Guest)
on 2009-03-06 17:09
(Received via mailing list)
On Fri, Mar 6, 2009 at 5:07 AM, Bishal Thapa <bdarealthug@gmail.com>
wrote:
> Therefore, I ignore every other sample. So down to 12300 samples roughly.
> captured by usrp_rx_cfile.py to look like this. Please note that I turn off
> whitening/dewhitening so the pattern do resemble 0010 0000 0010 0000
> somewhat.
>
> -Thank you for your time,
> -B.

Bishal,

In the future, please label your axes.

If I'm looking at what I think I'm looking at, then the answer is very
simple. You have a frequency mismatch between your transmitter and
receiver. And remember, you ALWAYS have a frequency mismatch unless
you are driven by some universal standard, and even then, well, we
won't get into that.

The digital demodulators we have implemented go through a lot of
trouble to track the frequency, phase, and timing at the receivers. If
you hare just using usrp_rx_cfile, there is no frequency correction
going on. The different frequencies used in both USRPs will cause an
envelop like you are seeing of fdel = |f_tx - f_rx|.

Tom
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