Le
sam. 5 déc. 2015 à 20:42, Haaris <address@hidden> a
écrit :
Hello
Almost a month ago i started working on a digital
communications project
which involves GNUradio.
And I am severely struggling to get past some errors or
mismatches I am
encountering in GNUradio.
I am desperately in need of some expert help.
I made a DQPSK modulator and demodulator using GRC. (
screenshots
provided )
Gave a vector source with bits 0,1,0,1 and repeat on, on the
input of
psk modulator.
I also used an error rate block to calculate bit error rate.
Now almost everything that is appearing on the scopes is
completely
wrong.
The bit error rate is 0.5, provided that i have added no
noise.(which is
max considering that we will recover 50 percent bits correctly
just by
chance).
The scope at the psk modulator output shows four constellation
points
even though i am transmitting only one symbol i.e (0,1).
What am i doing wrong?
Can someone please be kind enough to go through the
screenshots and tell
me the mistake(s).
Any help will be greatly appreciated.
Thanks in advance.
Hi Haaris,
The documentation of the PSK Mod says : "The input is a byte
stream (unsigned char), treated as a series of packed symbols.
Symbols are grouped from MSB to LSB."
You should add an "Unpacked to Packed block" with 2 bits per
chunk and MSB endianness before.
Likewise, you should add a "Pack K bits" block with K=2 after
the PSK Demod.
Also, your assumption that you should have one point in the
constellation sink is wrong.
You're using DQPSK so the (0, 1) symbol will add 90 degrees
to the phase, and you will cycle through the 4 points of your
constellation.
And last, keep in mind that each block has a delay, and you
can't compare the input and output bits directly.
Try to use a "Scope plot" with 2 inputs, and add a delay
block before the input bits to synchronise the two.
Timothée.
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