%QPSK Modulation
clear all;
close all;

%Generate Quadrature Carrier Signal
Tb=1; t=0:(Tb/100): Tb; fc=1;
c1=sqrt(2/Tb)*cos(2*pi*fc*t);
c2=sqrt(2/Tb)*cos(2*pi*fc*t);

%Generate Message Signal
N=8; m=rand(1,N);
t1=0; t2=Tb
for i=1:2:(N-1)
    t=[t1:(Tb/100):t2]
  if m(i)>0.5
     m(i)=1;
     m_s=ones(1,length(t));
  else
     m(i)=0;
     m_s=-1*ones(1,length(t));
  end

%ODD Bits Modulated Signal
odd_sig(i,:)=c1.*m_s;
  if m(i+1)>0.5
     m(i+1)=1;
     m_s=ones(1,length(t));
 else
     m(i+1)=0;
     m_s=-1*ones(1,length(t));
  end

%EVEN Bits Modulated Signal
even_sig(i,:)=c2.*m_s;

%QPSK signal
qpsk=odd_sig+even_sig;

%Plot the QPSK Modulated Signal
subplot(3,2,4);plot(t,qpsk(i,:));
title('QPSK Signal'); xlabel('t---->'); ylabel('s(t)'); grid on; hold on;
t1=t1+(Tb+0.1); t2=t2+(Tb+0.1);
end

%Plot the Binary Data Bits and Carrier Signal
subplot(3,2,1); stem(m);
title('Binary Data Bits'); xlabel('n---->'); ylabel('b(n)'); grid on;
subplot(3,2,2); plot(t,c1);
title('Carrier Signal-1'); xlabel('t---->'); ylabel('c1(t)'); grid on;
subplot(3,2,3); plot(t,c2);
title('Carrier Signal-2'); xlabel('t---->'); ylabel('c2(t)'); grid on;

%QPSK Demodulation
t1=0; t2=Tb
for i=1: N-1
   t=[t1:(Tb/100):t2]
   %Correlator
   x1=sum(c1.*qpsk(i,:));
   x2=sum(c2.*qpsk(i,:));
   %Decision device
   if (x1>0&&x2>0)
      demod(i)=1;
      demod(i+1)=1;
   elseif (x1>0&&x2<0)
      demod(i)=1;
      demod(i+1)=0;
   elseif (x1<0&&x2<0)
      demod(i)=0;
      demod(i+1)=0;
   elseif (x1<0&&x2>0)
      demod(i)=0;
      demod(i+1)=1;
   end
   t1=t1+(Tb+.01); t2=t2+(Tb+.01);
   end
subplot(3,2,5); stem (demod);
title('QPSK Demodulated Bits'); xlabel('n---->'); ylabel('b(n)'); grid on;