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[Discuss-gnuradio] QPSK demodulation problem

From: Ahmet Hasim Gokceoglu
Subject: [Discuss-gnuradio] QPSK demodulation problem
Date: Mon, 28 Apr 2008 19:56:44 +0300


I am trying to implement a QPSK receiver with GNU radio using the
board RFX2400 where I take the input from signal generator. The symbol
rate  of signal generator is 1Msym/sec sending a constant pattern of
"0010" and i have set the decimation in USRP to 16 to have 4 samples
per symbol. I suppose that I have done the calculation correct since
the ADC rate is 64 Msa/sec.

Since i am not very familiar with the parameters of frequency and
timing synchronization blocks i have just used the default ones:
costas_alpha=0.15 mu=0.05 omega_re_limit=0.5

When I look at the demodulated bit stream by reading the file
"rx_unpack.dat" (which should supposedly be showing the demodulated
bits)  what I get is arbitrary 0's and 1's which has nothing to do
with the pattern "0010" that I am sending. Does anybody have any idea
where might be the problem?

Secondly I am recording the data before and after the frequency/time
synchronization block so that I can plot the constellation points
before and after the synchronization. I suppose that the recorded data
is in the format [I Q] so for example when I read [32 64 128 196] then
32 is the decimal equivalent of the first 8 bits of I and 64 is the
decimal equivalent of last 8 bits. SÄ°milarly 128 is the first 8bits of
Q and 196 is the last 8.  Can anyone validate this ? and how about the
sign, how is it determined?

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