QPSK Modulation and Demodulation

Here's QPSK modulation and demodulation code I got:

%XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
%XXXX QPSK Modulation and Demodulation without consideration of noise XXXXX
%XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
clc;
clear all;
close all;
data=[0  1 0 1 1 1 0 0 1 1]; % information
%Number_of_bit=1024;
%data=randint(Number_of_bit,1);
figure(1)
stem(data, 'linewidth',3), grid on;
title('  Information before Transmiting ');
axis([ 0 11 0 1.5]);
data_NZR=2*data-1; % Data Represented at NZR form for QPSK modulation
s_p_data=reshape(data_NZR,2,length(data)/2);  % S/P convertion of data
br=10.^6; %Let us transmission bit rate  1000000
f=br; % minimum carrier frequency
T=1/br; % bit duration
t=T/99:T/99:T; % Time vector for one bit information
% XXXXXXXXXXXXXXXXXXXXXXX QPSK modulatio  XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
y=[];
y_in=[];
y_qd=[];
for(i=1:length(data)/2)
    y1=s_p_data(1,i)*cos(2*pi*f*t); % inphase component
    y2=s_p_data(2,i)*sin(2*pi*f*t) ;% Quadrature component
    y_in=[y_in y1]; % inphase signal vector
    y_qd=[y_qd y2]; %quadrature signal vector
    y=[y y1+y2]; % modulated signal vector
end
Tx_sig=y; % transmitting signal after modulation
tt=T/99:T/99:(T*length(data))/2;
figure(2)
subplot(3,1,1);
plot(tt,y_in,'linewidth',3), grid on;
title(' wave form for inphase component in QPSK modulation ');
xlabel('time(sec)');
ylabel(' amplitude(volt0');
subplot(3,1,2);
plot(tt,y_qd,'linewidth',3), grid on;
title(' wave form for Quadrature component in QPSK modulation ');
xlabel('time(sec)');
ylabel(' amplitude(volt0');
subplot(3,1,3);
plot(tt,Tx_sig,'r','linewidth',3), grid on;
title('QPSK modulated signal (sum of inphase and Quadrature phase signal)');
xlabel('time(sec)');
ylabel(' amplitude(volt0');
% XXXXXXXXXXXXXXXXXXXXXXXXXXXX QPSK demodulation XXXXXXXXXXXXXXXXXXXXXXXXXX
Rx_data=[];
Rx_sig=Tx_sig; % Received signal
for(i=1:1:length(data)/2)
    %%XXXXXX inphase coherent dector XXXXXXX
    Z_in=Rx_sig((i-1)*length(t)+1:i*length(t)).*cos(2*pi*f*t); 
    % above line indicat multiplication of received & inphase carred signal
    
    Z_in_intg=(trapz(t,Z_in))*(2/T);% integration using trapizodial rull
    if(Z_in_intg>0) % Decession Maker
        Rx_in_data=1;
    else
        Rx_in_data=0; 
    end
    
    %%XXXXXX Quadrature coherent dector XXXXXX
    Z_qd=Rx_sig((i-1)*length(t)+1:i*length(t)).*sin(2*pi*f*t);
    %above line indicat multiplication ofreceived & Quadphase carred signal
    
    Z_qd_intg=(trapz(t,Z_qd))*(2/T);%integration using trapizodial rull
    if (Z_qd_intg>0)% Decession Maker
        Rx_qd_data=1;
    else
        Rx_qd_data=0; 
    end
    
    
    Rx_data=[Rx_data  Rx_in_data  Rx_qd_data]; % Received Data vector
end
figure(3)
stem(Rx_data,'linewidth',3) 
title('Information after Receiveing ');
axis([ 0 11 0 1.5]), grid on;
% XXXXXXXXXXXXXXXXXXXXXXXXX    end of program    XXXXXXXXXXXXXXXXXXXXXXXXXX

I've changed the code

Tx_sig=y;

into:

Tx_sig=y;
Tx_sig_transmit = awgn(Tx_sig,10,'measured');

and in demodulation side into:

Rx_sig=Tx_sig_transmit;

But the demodulation result remains the same as the signal before modulation (should be different because AWGN has been added)

what should be changed so that the result can match the theory (signal before modulation differs from demodulation because it passes through the awgn channel)? And sorry for my English

Answer 1

[Manimegalai Munisamy]

The received signal should be added with awgn signal

Rx_sig=y+Tx_sig_transmit