# Question on DQPSK Modulation - issue with output **URL:** \ **Category:** GNU Radio\ **Created:** [June 3, 2012, 1:21am UTC](https://www.ruby-forum.com/t/question-on-dqpsk-modulation-issue-with-output/220230 "2012-06-03T01:21:25Z")\ **Posts on this page:** 3\ **Page:** 1
**Author:** ![aris](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/a/258eb7/32.png) [@aris](https://www.ruby-forum.com/u/aris)\ **Post date:** [June 3, 2012, 1:21am UTC](https://www.ruby-forum.com/t/question-on-dqpsk-modulation-issue-with-output/220230/1 "2012-06-03T01:21:25Z")
Dear all, I am currently trying to implement a pi/4 dqpsk modulator and demodulator. I have tried two approaches, one uses the CQPSK.py (included in osmocom’s TETRA project) and the other uses digital.qpsk.dqpsk\_mod. Both of these approaches have failed to return the same string of bits after the process of modulations-\>demodulation. The input comes from a .dat file and is a string of bits (tried different sizes, from 2 bits to 32 bits), then the output of modulation is written to a sink.dat as gr\_complex. This file is then fed to a demodulator which saves the output to a final.dat. I have attached important bits of the code which uses qpsk.dqpsk\_mod/demod. I would greatly appreciate any advice. Also, could anyone please confirm that what I can achieve a –PI/4–DQPSK using the dqpsk\_mod? Best Regards, ## Vahid Modulator: . . . sample\_rate = options.sample\_rate symbol\_rate = 18000 sps = 2 ``` IN = gr.file_source(gr.sizeof_gr_complex, options.input_file, repeat ``` = False) #IN = (1,0,0,0,1,1,0,0) src = gr.vector\_source\_b (IN) MOD = digital.qpsk.dqpsk\_mod( samples\_per\_symbol = sps, excess\_bw=0.35, log=options.log, gray\_coded=True, ``` verbose=options.verbose) self.connect(src,MOD) OUT = gr.file_sink(gr.sizeof_gr_complex, options.output_file) self.sink1 = gr.vector_sink_f() self.sink2 = gr.vector_sink_f() real = gr.complex_to_real() imag = gr.complex_to_imag() self.connect(MOD,real) self.connect(MOD,imag) self.connect(real,self.sink1) self.connect(imag,self.sink2) self.connect(MOD,OUT) def print_data(self): print "data in sink1 is: ",self.sink1.data() print "data in sink2 is: ",self.sink2.data() ``` ## . . . Demodulator: sample\_rate = options.sample\_rate symbol\_rate = 18000 sps = 2 # output rate will be 36,000 ntaps = 11 \* sps new\_sample\_rate = symbol\_rate \* sps ``` channel_taps = gr.firdes.low_pass(1.0, sample_rate, ``` options.low\_pass, options.low\_pass \* 0.1, gr.firdes.WIN\_HANN) ``` FILTER = gr.freq_xlating_fir_filter_ccf(1, channel_taps, ``` options.calibration, sample\_rate) IN = gr.file\_source(gr.sizeof\_gr\_complex, options.input\_file) #IN = (1,1,0,1,1,1,0,0) #src = gr.vector\_source\_b (IN) DEMOD = digital.qpsk.dqpsk\_demod( samples\_per\_symbol = sps, excess\_bw=0.35, gray\_coded=True, log=options.log, ``` verbose=options.verbose) OUT = gr.file_sink(gr.sizeof_char, options.output_file) self.sink1 = gr.vector_sink_b() r = float(sample_rate) / float(new_sample_rate) INTERPOLATOR = gr.fractional_interpolator_cc(0, r) self.connect(IN, FILTER, INTERPOLATOR, DEMOD, OUT) self.connect(DEMOD,self.sink1) def print_data(self): print "data in sink1 is: ",self.sink1.data() ``` – View this message in context: [http://old.nabble.com/Question-on-DQPSK-Modulation---issue-with-output-tp33950307p33950307.html](http://old.nabble.com/Question-on-DQPSK-Modulation---issue-with-output-tp33950307p33950307.html) Sent from the GnuRadio mailing list archive at [Nabble.com](http://Nabble.com). ---
**Author:** ![Vahid\_B](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/v/c68b51/32.png) [@Vahid\_B](https://www.ruby-forum.com/u/Vahid_B)\ **Post date:** [June 4, 2012, 1:37am UTC](https://www.ruby-forum.com/t/question-on-dqpsk-modulation-issue-with-output/220230/2 "2012-06-04T01:37:16Z")
Hi Vahid, First I’d try using a much longer string of bits and see if that help. Second I’d find an example in gnuradio (gr-digital/python/qa\_constellation.py should have a working dqpsk test) and then work from there. Thirdly if you want pi/4-dqpsk you won’t be able to use the standard dqpsk\_mod and dqpsk\_demod, but will need to define a two-dimensional constellation and use the generic\_mod and generic\_demod blocks. The two-dimensional constellation would be defined something like: normal\_qpsk\_points = [blah, blah, blah, blah] rotated\_qpsk\_points = [blah, blah, blah, blah] two\_d\_points = [] for p in normal\_qpsk\_points: for q in rotated\_qpsk\_points: two\_d\_points += [p, q] pi4dqpsk\_constellation = digital.constellation\_calcdist(two\_d\_points, [], 4, 2) Cheers, Ben ---
**Author:** ![Vahid\_B](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/v/c68b51/32.png) [@Vahid\_B](https://www.ruby-forum.com/u/Vahid_B)\ **Post date:** [June 4, 2012, 3:00am UTC](https://www.ruby-forum.com/t/question-on-dqpsk-modulation-issue-with-output/220230/3 "2012-06-04T03:00:13Z")
On Sun, Jun 3, 2012 at 4:31 PM, Ben R. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote: > constellation and use the generic\_mod and generic\_demod blocks. The > two-dimensional constellation would be defined something like: > > normal\_qpsk\_points = [blah, blah, blah, blah] > rotated\_qpsk\_points = [blah, blah, blah, blah] > two\_d\_points = > for p in normal\_qpsk\_points: > for q in rotated\_qpsk\_points: > two\_d\_points += [p, q] > pi4dqpsk\_constellation = digital.constellation\_calcdist(two\_d\_points, , 4, 2) Just checked the code and it looks like multi-dimensional constellations aren’t supported yet by generic\_mod and generic\_demod. Another way might be to edit digital.constellation\_receiver\_cb so that it shifts the phase back and forth by pi/4.