Scatter Plot of OQPSK Below is my MATLAB code for OQPSK communication:

    data_in = [ 2 1 3 1 2 3 1 1 0 2 0 2 3 2 3 1 1 2 0 3 1 3  3 0 2 1 ];
    data_modualted=oqpskmod(data_in);
    filtmsg = filter(H,1,data_modualted); % Introduce channel distortion.
    eq1 = lineareq(nweigth,normlms(StepSize)); % Create an equalizer object. eq1.SigConst=oqpskmod(0:3);
    [symbolest,filtmsg_eq] = equalize(eq1,filtmsg,data_modualted(1:trainlen)); % Equalize.
    %%Scatter plot
    h = scatterplot(filtmsg,4,0,'g.');
    hold on ;
    scatterplot(symbolest,4,0,'kx',h);
    scatterplot(eq1.SigConst,1,0,'r*',h);
    legend('Filtered signal','Equalized signal','Ideal signalconstellation')


I am unable to understand which values to use to generate the phase
constellation diagram for my code?  How can i generate the correct
phase constellation diagram?



The first figure is constellation diagram of The filtered signal and the second is the constellation diagram of The Equalized signal
You'll need to plot your signal's imaginary and real (I and Q) part at the exact symbol time – in other words, you'll need timing recovery to get the constellation from a time signal.

Timing recovery is among the more complicated things you'll need to implement for a receiver, depending on the actual kind of transmission you're dealing with. 

Assuming you weren't aware of that, you might want to  look into these terms (I think the order makes sense, didactically):


pulse shaping filter
eye diagram
Nyquist ISI criteria
timing recovery


square timing recovery
Gardner timing recovery

phase error correction


Costas loop