Re: Distorted QPSK Constellation

GNU Radio
1

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

2

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.

Regards

Salman D.

3

On Thu, Jan 15, 2015 at 1:41 PM, Salman D. [email protected]
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

4

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. [email protected]
wrote:

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

Discuss-gnuradio mailing list
[email protected]
Discuss-gnuradio Info Page

–

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
Homepage: http://ele.aut.ac.ir/~alizadeh/