Problems in Costas loop for QPSK

GNU Radio
1

Dear all,

I use mpsk_receiver_cc() to synchronze DQPSK symbols at a receiver.

Since I get too many packet losses when running the benchmark(in the
digital
folder) with DQPSK, I have closely looked at the codes.

I finally found that the output of costas_loop is not well synchronized
from
time to time. If I use higher rate like 500k, it happens more often.

Here’s the constellation of the output of the Costas loop (
http://bit.ly/ij4ytv) when it is not well synchronized. Sometimes the
constellation becomes worse than this.

I used the default parameters of dqpsk.py except fmin and fmax.
(When I set fmin/fmax=-0.1/0.1 instead of -0.25/0.25, I get better
constellation.)

Is there anyone who has encountered the same problem?
Or, is there any other code that I can use to synchronize QPSK symbols
at
the receiver?

PS. I’am using USRP1 / gnuradio 3.3.0 / Ubuntu 10.04

Thanks in advance.

2

i had problem at costas too, for example i used mpsk stage5 example(i
modified it a little bit):

file source-> packet encoder-> psk mod-> throttle-> multiple const->
channel model-> polyphase clock sync-> cma equalizer-> costas loop->
usrp tx

usrp rx-> psk demod-> packet decoder-> file sink

it is not giving file as a result of file sink(empty).

file disappers after multiple constant block.

when i send data as bit level(without encoder and decoder) it gives at
every level broken data FOR EXAMPLE:

  ...polyphase clock sync-> usrp tx   usrp rx-> psk demod-> packet

decoder-> file sink

  ...cma equalizer-> usrp tx    usrp rx-> psk demod-> packet

decoder-> file sink

  ...costas loop-> usrp tx     usrp rx-> psk demod-> packet

decoder-> file sink

so, any help for this too?