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Re: [Discuss-gnuradio] Distorted QPSK Constellation

From: Mostafa Alizadeh
Subject: Re: [Discuss-gnuradio] Distorted QPSK Constellation
Date: Thu, 15 Jan 2015 22:59:32 +0330


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.


On Thu, Jan 15, 2015 at 10:11 PM, Salman Dinani <address@hidden> 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
  • 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).

           Why the signal is out of phase and phase shift is varying 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.

           Is there a special format for sending data via File Source??

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.


Salman Dinani



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