function [S] = QPSK(bits)
nb = length(bits); % number of bits
ns = ceil(nb/2); % number of symbols
b0 = zeros(1,ns*2); % zero padding
b0(1:nb) = bits;
j = sqrt(-1);
for s = 1 : ns

ba = b0(((s-1)*2)+1);
bb = b0(((s-1)*2)+2);
k = bb + ba*2;
p = ((pi/4)*(2*k-1))-pi;

Sc(s) = cos(p);
Ss(s) = sin(p);
S(s) = Sc(s) + j*Ss(s);

end