qpsk.m

clear;
clc;
b = input('Enter the Bit stream \n ');
%b = [0 1 0 1 1 1 0];

n = length(b);
t = 0:.01:n;
x = 1:1:(n+2)*100;
for i = 1:n
    if (b(i) == 0)
        b_p(i) = -1;
    else
        b_p(i) = 1;
    end
    for j = i:.1:i+1
        bw(x(i*100:(i+1)*100)) = b_p(i);
        if (mod(i,2) == 0)
            bow(x(i*100:(i+1)*100)) = b_p(i);
            bow(x((i+1)*100:(i+2)*100)) = b_p(i);
        else
            bew(x(i*100:(i+1)*100)) = b_p(i);
            bew(x((i+1)*100:(i+2)*100)) = b_p(i);
        end
        if (mod(n,2)~= 0)
            bow(x(n*100:(n+1)*100)) = -1;
            bow(x((n+1)*100:(n+2)*100)) = -1;
        end

    end    
end
%be = b_p(1:2:end);
%bo = b_p(2:2:end);

bw = bw(100:end);
bew = bew(100:(n+1)*100);
bow = bow(200:(n+2)*100);

cost = cos(2*pi*t);
sint = sin(2*pi*t);
st = bew.*cost+bow.*sint;
 

subplot(4,1,1)
plot(t,bw)
grid on ; axis([0 n -2 +2])
subplot(4,1,2)
plot(t,bow)
grid on ; axis([0 n -2 +2])
subplot(4,1,3)
plot(t,bew)
grid on ; axis([0 n -2 +2])
subplot(4,1,4)
plot(t,st)
grid on ; axis([0 n -2 +2])