function qpsk_output=qpsk_code(code_input,f)
%Modulation QPSK
%qpsk_output is output of modulation QPSK
%code_input is a binay vector
%f is the carrier frequency
%Example: 
%qpsk_output=qpsk_code([0 0 0 1 1 0 1 1],2)
%Author is lipeng
%date 2008 11 15

clc;
code_input=[1 0 1 1 0 0 1 1];
f=2;
nargin=2;

if nargin > 2
    error('Too many input arguments');
elseif nargin==1
    f=1;
end
if f<1;
    error('Frequency must be bigger than 1');
elseif f~=ceil(f)
    error('Frequency must be a integer!');
end
l=length(code_input);
if rem(l,2)~=0
    code_input=[code_input 0];
    l=l+1;
end
t=0:2*pi/(100*f-1):2*pi;
cp=[];
sp=[];
mod=[];
mod1=[];
bit=[];
for n=1:2:length(code_input);
    if code_input(n)==0 && code_input(n+1)==0;
        die=sqrt(2)/2*ones(1,100*f);
        die1=sqrt(2)/2*ones(1,100*f);
        se=[zeros(1,50*f) zeros(1,50*f)];
    elseif code_input(n)==0 && code_input(n+1)==1;
        die=sqrt(2)/2*ones(1,100*f);
        die1=-sqrt(2)/2*ones(1,100*f);
        se=[zeros(1,50*f) ones(1,50*f)];
    elseif code_input(n)==1 && code_input(n+1)==0;
        die=-sqrt(2)/2*ones(1,100*f);
        die1=sqrt(2)/2*ones(1,100*f);
        se=[ones(1,50*f) zeros(1,50*f)];
    elseif code_input(n)==1 && code_input(n+1)==1;
        die=-sqrt(2)/2*ones(1,100*f);
        die1=-sqrt(2)/2*ones(1,100*f);
        se=[ones(1,50*f) ones(1,50*f)];
    end
    c=cos(f*t);
    s=sin(f*t);
    cp=[cp die];    %Amplitude cosino
    sp=[sp die1];   %Amplitude sino
    mod=[mod c];    %cosino carrier (Q)
    mod1=[mod1 s];  %sino carrier   (I)
    bit=[bit se];
end
qpsk_output=cp.*mod+sp.*mod1;
subplot(8,1,5);
plot(bit,'LineWidth',1.5);
grid on;
title('Binary Signal');
axis([0 50*length(code_input)*f -1.5 1.5]);

subplot(8,1,6);
plot(qpsk_output,'LineWidth',1.5);
grid on;
title('QPSK modulation');
axis([0 50*length(code_input)*f -1.5 1.5]); 

clc;