[Discuss-gnuradio] Strange constellation appears on pi/4 dqpsk and dqpsk thru scope_sink.

[加藤義也]

Hello Friends.

Please someone advice me out for my situation.
I faced strange constellation at pi/4 dqpsk / dqpsk.

I am now trying to implement pi/4 dqpsk modulator/demodulator on GnuRadio3.2.2 with using USRP2(RFX2400).
And mostly is working now.

In these days, I tried to have BER ratio, by PN9 XORed my original bit pattern.(for spreading frequency)
and ratio shows very poor. If I compete with other systems.
which ratio is like 6 to 10dB more SNR is needed at same BER.

I've tried to have constellation of my implementation with using scope_sink.
Now, I found the implementation shows strange constellation as I attached file
http://www.telemann.co.jp/gnuradio-image/pi_4dqpsk-scope-sink.png

The demodulator is using origin as gr_make_mpsk_receiver_cc().

I sought where this strange constellation is come from.
Of course, I doubt this is because of my implementation at the first.

What I tried following things.
  -Used rented Agilent VecterSignalAnalyzer 89600/89640 to get constellation.
   for whether, At least Tx does work fine both pi/4 dqpsk and dqpsk.and shows beautiful constellation.
   as 
   http://www.telemann.co.jp/gnuradio-image/pi_4dqpsk-89600.png
   http://www.telemann.co.jp/gnuradio-image/dqpsk-89600.png

  -Used already impremented dqpsk as a modulator. and it's constellation shows
   similar strange plot. which is as 
   http://www.telemann.co.jp/gnuradio-image/dqpsk-scope-sink.png
  -Not PN9 XORed dqpsk shows sharpen constellation as shown in "dqpsk(notpn9)-scope-sink.png"
  -I have read gnuradio-core/src/lib/general/gr_mpsk_receiver_cc.cc:gr_make_mpsk_receiver_cc()
   and in my guess "phase_error_detector_generic" and "decision_generic" brought this issues.

Is my guess get pointed? or irrelevant?
If it is, I would like to think more how to make
 "phase_error_detector_pi_4qpsk" and "decision_pi_4qpsk".


I used following parameter for gr_make_mpsk_receiver_cc().
costas alpha = 0.15
gain mu = None
def mu = 0.5
omega relative limit = 0.005
fmin = -0.025
fmax= 0.025

and configuration of the test is as
http://www.telemann.co.jp/gnuradio-image/configuration.png
Thank you very much in advance.

Yoshiya Kato

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