[Discuss-gnuradio] Comparison test results: new "2" series dqpsk blocks

[ikjtel]

I've run a comparison test of
 - the "old" GR dqpsk block
 - the "new" one (dqpsk2)
 - the "op25" cqpsk (Gardner/Costas) block

The resulting constellation diagrams are viewable at 
http://www.lightlink.com/mhp/qpsk2/

The results are: Gardner is the winner, the old GR blocks take second, the new 
ones are last
(intriguingly as shown in the symbol plot, things seem to start off nicely but 
then go awry)...

The problem could be caused by dumb user error - D.U.E. (TM) or something 
fixable by simple
parameter tweakage?

The input file used in all three cases is a complex capture taken from a system 
using so-called
"LSM/CQPSK".  Basically, that appears to mean something like: a) simulcast 
distortion is added;
b) PI/4 DQPSK is used; c) pulse shaping is stretched so as to fit the bandwidth 
of the signal into
a 12.5 KHz channel; d) a "non-standard" waveform is used which results in the 
signal passing
through the origin (0,0) point, introducing a rather severe AM component.

There are two further wrinkles in this particular case which should be noted:
1) the complex file was captured using a "very poor man's USRP", *not* a real 
USRP
   [ see http://www.lightlink.com/mhp/iq/ ].  However the file formats are 
fully compatible.
   Further, a DBSRX capture exhibits all the same characteristics.
2) the receiving location is outside the designed coverage area of the system

The results of all of this, plus the fact that the capture was made prior to 
verifying that the
hardware even was functional for LSM, makes for a very "difficult" specimen 
(perfect for
torture-testing RX codes)...

Best Regards

Max
--------------------------------------------------------------------------
#!/usr/bin/env python

import sys
import os
import math
from gnuradio import gr, gru, audio, eng_notation, blks2
from gnuradio.eng_option import eng_option
from optparse import OptionParser

from math import pi

class my_top_block(gr.top_block):
    def __init__(self):
        gr.top_block.__init__(self)
        parser = OptionParser(option_class=eng_option)

        parser.add_option("-d", "--debug", action="store_true", default=False, 
help="allow time at init to attach gdb")
        parser.add_option("-i", "--input-file", type="string", 
default="in.dat", help="specify the input file")
        parser.add_option("-o", "--output-file", type="string", 
default="out.dat", help="specify the output file")
        parser.add_option("-s", "--sample-rate", type="int", default=19200, 
help="input sample rate")
        (options, args) = parser.parse_args()

        sample_rate = options.sample_rate
        symbol_rate = 4800
        samples_per_symbol = sample_rate // symbol_rate

        IN = gr.file_source(gr.sizeof_gr_complex, options.input_file)

        print "sps %d" % samples_per_symbol

        DEMOD = blks2.dqpsk2_demod(samples_per_symbol = samples_per_symbol,
                                    excess_bw = 0.35,
                                    verbose = True,
                                    log = True,
                                    sync_out = False)

        OUT = gr.file_sink(gr.sizeof_char, options.output_file)

        self.connect(IN, DEMOD, OUT)

        if options.debug:
            print 'Ready for GDB to attach (pid = %d)' % (os.getpid(),)
            raw_input("Press 'Enter' to continue...")

if __name__ == "__main__":
    try:
        my_top_block().run()
    except KeyboardInterrupt:
        tb.stop()