This loop doesn't involve the loop variable k - you're doing the same thing N times:
for k=1:N
if (Bit_wat1 ==1)
y = Tx .*exp(1i.*(2*pi*fc*t+wat)) ;
else
y = Tx .*exp(1i.*(2*pi*fc*t-wat)) ;
end
end
Also, these loops
b_inf=zeros(1,N);
b_w=zeros(1,N);
for w= 1:N
if real(y(w)) >= 0
b_inf(w) = 1;
else
b_inf(w) = 0;
end
end
for x= 1:N
if imag(y(x)) >= 0
b_w(x) = 1;
else
b_w(x) = -1;
end
end
can be replaced with:
b_inf = double( real(y) >= 0 );
b_w = -1 + 2*( imag(y) >= 0 );
