GMSK In the process of implementing the GMSK modulator and demodulator, I ran into a problem in the work of the demodulator. Maybe you will have thoughts as to why the demodulator is not working. Thank you!

Maybe you can find a critical error

%% %modulation
Nfft = 512;
Fc = 1600;
Br = 800;
sps = 4;
BT = 0.3;
NT = 1;
Fadc = sps * Br;
Ts = 1/Fadc;
Tb = sps * Ts;
bit = [1,0,1,0,1,0,1,0,1,1]; 
N = length(bit);
Ng = N;
bits = 2*bit - 1;
H = gaussdesign(BT,NT,sps);
Rashir = repmat(bits,sps,1);
Rashir = Rashir(:);
bt = conv(H,Rashir,'full');
bt = bt/max(abs(bt));
integral = cumsum(bt);
%integral = filter(1,[1,-1],bt);
phi = integral *0.5*pi/Tb;
I = cos(phi);
Q = sin(phi);
S = I - 1i .* Q;
t = ((0:length(I)-1)*Ts).';
St = I .* cos(2*pi*t*Fc) - 1i .* Q .* sin(2*pi*t*Fc);
%%  %demodulation
f1 = Fc + 1/(4*Tb);
Fadc = 2*Fadc;
 I_reciver = real(St).*cos(2*pi*t*Fc);
 Q_reciver = -imag(St).*sin(2*pi*t*Fc);

[b,a] = butter(3,2*f1/(Fadc));
fvtool (b, a, 'Fs' , Fadc)
y1 = filter(b,a,I_reciver);
y2 = filter(b,a,Q_reciver); 
z1 = y2.*[zeros(sps,1); y1(1:length(y1)-sps)]; 
z2 = y1.*[zeros(sps,1); y2(1:length(y1)-sps)];
Z2 = z1 - z2;
a_cap1 = (Z2(2*sps:sps:end-sps)>0);