QPSK demodulation problem

hi,

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?

Ahmet Hasim G. wrote:

hi,

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.

Ahmet,
I’m not sure I understand how you are generating your input signal. What
signal generator? Is this an external sig gen, or are you somehow
generating the signal with GNU Radio?

Also, are you using the dqpsk.py for the DQPSK mod/demod functions? I
wasn’t sure if you were building your own QPSK receiver.

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

These defaults were set because they worked with the implementation or
the receiver. They should be fine.

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?

Again, it will depend on how you are generating the input symbols. If
you are using the packet modulator code we have in
/digital/benchmark_tx.py, the packets transmitted are first whitened.
The file rx_unpack.dat is captured before the signal is dewhitened. You
can turn whitening off by looking at
gnuradio-core/src/python/gnuradio/packet_utils.py in the “unmake_packet”
function for how.

If you are not using the benchmark_tx to generator your symbols and are
sending over the “0010” bit stream, what you are doing is sending over a
very specific pattern (0 degree shift followed by a 90 degree shift)
that will bias the recovery loops. Try sending a longer more random
sequence.

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?

If you’re using file_sink(gr.sizeof_float, “file.dat”), the output is
32-bit float for the I and 32-bit float for the Q. It’s standard IEEE
754 or whatever style.

You can use the Python/Matplotlib tools we have in gr-utils/src/python
for plotting these files. They are installed into your system now
(/usr/local/bin unless you changed the prefix) and called gr_plot_*. You
can look at the constellation with gr_plot_const.py

Tom

Do we have a QPSK (not a DQPSK)Demodulator available and operational in
gnuradio???

On Mon, Apr 28, 2008 at 8:20 PM, Tom R. [email protected] wrote:

Ahmet,
I’m not sure I understand how you are generating your input signal. What
signal generator? Is this an external sig gen, or are you somehow generating
the signal with GNU Radio?

I am using an external signal generator by which I generate the bit
stream and modulate it by QPSK.

Also, are you using the dqpsk.py for the DQPSK mod/demod functions? I
wasn’t sure if you were building your own QPSK receiver.

yeah I am using dqpsk.py for demod

are using the packet modulator code we have in /digital/benchmark_tx.py, the

I was also thinking about the same thing and will try with a longer
more random sequence

If you’re using file_sink(gr.sizeof_float, “file.dat”), the output is
32-bit float for the I and 32-bit float for the Q. It’s standard IEEE 754 or
whatever style.

I have read from http://www.nd.edu/~jnl/sdr/docs/tutorials/4.pdf
that the data coming from USRP is in 16 bit I and 16 bit Q format so
why is it changed to 32 bits ?

You can use the Python/Matplotlib tools we have in gr-utils/src/python for
plotting these files. They are installed into your system now
(/usr/local/bin unless you changed the prefix) and called gr_plot_*. You can
look at the constellation with gr_plot_const.py

Tom

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