function code_output=qpsk_decode(qpsk_input,f)
%Demodulation QPSK
%code_output is output of demodulation QPSK
%qpsk_input is a modulation signal
%f is the carrier frequency
%Example: 
%code_output=qpsk_decode(qpsk_input,2)
%Author is lipeng
%date 2008 11 16
clc;
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

t=0:2*pi/(100*f-1):2*pi;
l=length(qpsk_input)/length(t);

sp=[];
mods=[];
modc=[];
code_output=[];
for n=1:l;
    sp=qpsk_input((n-1)*length(t)+1:n*length(t));
    mods=sin(t).*sp;
    modc=cos(t).*sp;
    means=sum(mods);
    meanc=sum(modc);
    if means>0 && meanc>0
        code_output=[code_output zeros(1,50*f) ones(1,50*f)];
    elseif means>0 && meanc<0
        code_output=[code_output ones(1,50*f) ones(1,50*f)];
    elseif means<0 && meanc<0
        code_output=[code_output ones(1,50*f) zeros(1,50*f)];
    else means<0 && meanc>0
        code_output=[code_output zeros(1,50*f) zeros(1,50*f)];
    end
end
subplot(8,1,7);
plot(qpsk_input,'LineWidth',1.5);
grid on;
title('QPSK Signal')
axis([0 50*l*2*f -1.5 1.5]);

subplot(8,1,8);
plot(code_output,'LineWidth',1.5);
grid on;
title('QPSK demodulation');
axis([0 50*2*l*f -1.5 1.5]); 
clc;