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Re: [Discuss-gnuradio] QPSK phase noise

From: Paco Garcia
Subject: Re: [Discuss-gnuradio] QPSK phase noise
Date: Wed, 8 Apr 2009 06:26:36 -0700 (PDT)

Johnathan, you were right with the distortion issue at the TX, I am now using
these parameters:

./benchma-m dqpsk -f 1.25G -m dqpsk --tx-amplitude=500 -r 1M -M 99999999
./benchmark_rx.py -r 1M -m dqpsk --log --costas-alpha=0.05 -S 4 --verbose -r
1M --gain-mu=0.01 --excess-bw=0.35 -f 1.25G --rx-gain=45

But still, for a 40dB SNR (
http://img149.imageshack.us/img149/4583/screenshot4h.png ) I think we should
get a much better constellation than this one (
http://img149.imageshack.us/img149/7968/screenshot3t.png )

I am using the latest gnuradio trunk and the
I have disabled all the data loggins but the rx_receiver.dat from the 
/usr/local/lib/python2.5/site-packages/gnuradio/blks2impl/dqpsk.py to reduce
the computational requirements.

I tried the 0.5M as eric suggested but the results are even worse (
http://img18.imageshack.us/img18/2306/screenshot5hhm.png )

The 2M rate constellation is the same as 1M.

If I go with BPSK, I get this:
./benchmark_tx.py -m dbpsk -f 1.25G -m dbpsk --tx-amplitude=500 -r 200k -M
99999999 --excess-bw=0.35
./benchmark_rx.py -m dbpsk --verbose -S 4 -r 200k --excess-bw=0.35 -f 1.25G
--rx-gain=30 > debug

The image below shows the costas loop error: 
( http://img156.imageshack.us/img156/1521/screenshot11.png )
(zoom frequency > http://img514.imageshack.us/img514/2173/screenshot12.png )
blue: phase
red: phase_error
cian: Q
purple: I
green: freq

So once the costas locks, there is still some error which is causing the
blur constellation. But I think that for an almost 40dB SNR signal (
http://img212.imageshack.us/img212/551/screenshot13.png ) we should get much
better results.

The M&M block influences a lot, if I change the samples per symbol to 2 I
get this: ( http://img201.imageshack.us/img201/9436/screenshot14.png )

I have also tried via air interface getting similar results. My question is
whether this is normal for the mpsk gnuradio block receiver implementation
or I am doing something wrong.

What are your thoughts about this?

Thank you for your time.

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