%----------------------------------------------------------------
%  Spectra of different Modulations
%  : QPSK, MSK, QPSK with raised cosine spectral shape
%----------------------------------------------------------------
samprt = 64;
Nq_bw  = samprt;  % 8x oversampled
blksz  = 4*1024;
ii     = [1:blksz];
fTs    = (ii-blksz/2-1) * Nq_bw/blksz;

%--- QPSK ------------------
sQPSK = sinc(fTs).^2;

%--- MSK spectrum ----------
t     = [0:1/Nq_bw:1];
Pulse = sin(pi*t);
sMSK  = (fftshift(abs(fft(Pulse,blksz)))).^2;
sMSK  = sMSK / max(sMSK);

%--- Raised cosine spectrum ----
rolloff = 0.5; 
z      = 1E-8*ones(size(fTs));

ind    = find(abs(fTs) <= ((1+rolloff)*0.5));
z(ind) = 0.5*(1-sin(pi*(abs(fTs(ind))-0.5)/rolloff));

ind    = find(abs(fTs) <= ((1-rolloff)*0.5));
z(ind) = ones(size(ind));

sRRC   = z;

%--- Plot results --------------
plot(fTs, 10*log10(sQPSK));
hold on
plot(fTs, 10*log10(sMSK),'r');
plot(fTs, 10*log10(sRRC+1E-8),'g');
grid on, axis([-4 4 -60 10])
hold off
xlabel ('FTs')
ylabel('Spectrum (dB)')