# Re: Distorted QPSK Constellation **URL:** \ **Category:** GNU Radio\ **Created:** [January 14, 2015, 11:36pm UTC](https://www.ruby-forum.com/t/re-distorted-qpsk-constellation/233819 "2015-01-14T23:36:43Z")\ **Posts on this page:** 4\ **Page:** 1
**Author:** ![addis\_a](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/a/c4cdca/32.png) [@addis\_a](https://www.ruby-forum.com/u/addis_a)\ **Post date:** [January 14, 2015, 11:36pm UTC](https://www.ruby-forum.com/t/re-distorted-qpsk-constellation/233819/1 "2015-01-14T23:36:43Z")
I believe that even with the MF you’ll have ISI because you have 4 samples/symbol in this example. Only if you down sample at 1 sample/symbol (at the right epoch) will you get rid of the ISI and get a clean QPSK. best Achilleas ---
**Author:** ![Achilleas\_A](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/a/74df32/32.png) [@Achilleas\_A](https://www.ruby-forum.com/u/Achilleas_A)\ **Post date:** [January 15, 2015, 7:42pm UTC](https://www.ruby-forum.com/t/re-distorted-qpsk-constellation/233819/2 "2015-01-15T19:42:49Z")
Hi to all, Thanks very much to all of you for your valuable suggestions.In accordance with your suggestions, I experimented with my designed DQPSK system and got few problems which I am listing down.All the referenced figures are attached. - I reduced the symbol per sample to 4 (previously it was 8) and added a RRC filter before constellation plot (figure RRC\_const) but constellation remains the same L - I successfully demodulated the distorted constellation output by using Cosine signal source as input (figure DQPSK\_demod\_S\_2). As someone previously suggested reducing the samples per symbol, I set it to 2 and ran the simulation. The demodulated output was out of phase w.r.t input (figure S\_2\_P\_1).As I varied the frequency, the phase difference also varied as shown in figures S\_2\_P\_2 and S\_2\_P\_3. When I set this parameter to 4 (figure DQPSK\_demod\_S\_4). The output got corrected (In-Phase) for all frequencies (figure Plot\_DQPSK\_S\_4). with frequency??Why this problem got corrected when Samples per symbol changed to 4? - In order to send my desired bit stream, I made a file in Libre office with bits 11110000 and saved it with txt extension. (i.e. data.txt). Before applying to the system, I tried to check the output of file source on scope. But when I ran the program (figure File\_source), the output was zero. After several trials, I put the bit streams in inverted commas “11110000” (figure file\_dat ) some data appeared at the output (figure output\_dat). But this is not the data which I wrote in the file. I am very sorry if my questions appear to be childish but I am not able to figure them out. Eagerly waiting for your expert opinions. Regards Salman D. ---
**Author:** ![Achilleas\_A](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/a/74df32/32.png) [@Achilleas\_A](https://www.ruby-forum.com/u/Achilleas_A)\ **Post date:** [January 15, 2015, 8:14pm UTC](https://www.ruby-forum.com/t/re-distorted-qpsk-constellation/233819/3 "2015-01-15T20:14:52Z")
On Thu, Jan 15, 2015 at 1:41 PM, Salman D. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote: > Hi to all, > > Thanks very much to all of you for your valuable suggestions.In accordance > with your suggestions, I experimented with my designed DQPSK system and got > few problems which I am listing down.All the referenced figures are > attached. > > I reduced the symbol per sample to 4 (previously it was 8) and added a RRC > filter before constellation plot (figure RRC\_const) but constellation > remains the same L You gave 0 taps to your RRC filter? Why would you do that? I’m not sure what that would do. At one exact phase you’ll get the points when your system is setup correctly. The other phases will get ISI. Brian ---
**Author:** ![Achilleas\_A](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/a/74df32/32.png) [@Achilleas\_A](https://www.ruby-forum.com/u/Achilleas_A)\ **Post date:** [January 15, 2015, 8:30pm UTC](https://www.ruby-forum.com/t/re-distorted-qpsk-constellation/233819/4 "2015-01-15T20:30:51Z")
Hi, For reading your data from file, you need to have a binary data file. Libre office file has a header and after that your text "11110000 ", in which “1” or "0’ is a char not a bit! GNURadio bit generation sources assume “0” is a byte of bits “00000000” and “1” is a byte of bits “10000000”. So you need to store your data in this binary format into a file. Great, Mostafa On Thu, Jan 15, 2015 at 10:11 PM, Salman D. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote: > ``` > Why the signal is out of phase and phase shift is varying with > > ``` > > * * * > > Discuss-gnuradio mailing list > [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) > [Discuss-gnuradio Info Page](https://lists.gnu.org/mailman/listinfo/discuss-gnuradio) – * * * Department of Electrical Engineering Aboureyhan Building MMWCL LAB Amirkabir University Of Technology Tehran IRAN Tel: +98 (919) 158-7730 LAB: [http://ele.aut.ac.ir/~mmwcl/?page\_id=411](http://ele.aut.ac.ir/~mmwcl/?page_id=411) Homepage: [http://ele.aut.ac.ir/~alizadeh/](http://ele.aut.ac.ir/~alizadeh/) * * *