phase estimation from I and Q
I think you want the 2-argument version of atan2d,

atan2d(I, Q),

instead of atan2d(Q/I).

With the one argument form, there's no way to distinguish between the 2nd and 4th quadrants, so you probably got an answer of -30 degrees or something.

Caveat: I did not test this in Matlab, only in SciLab.

Edit Here is my SciLab code:

n1=1024;
n=[0:n1-1];
f=50;
sf=4*f;
PHI=150; 
x=(1*sin(2*%pi*(f*1e6)*((1/sf)*1e-6)*n+PHI*%pi/180));
xx=matrix(x,4,n1/4);
xrr=[xx(1,1:n1/4) ;-xx(3,1:n1/4)];
xii=[xx(2,1:n1/4) ;-xx(4,1:n1/4)];
I=matrix(xrr,1,n1/2);
Q=matrix(xii,1,n1/2);

phi = atan(I, Q) * 180 / %pi;


You see this is essentially identical to yours, except 


SciLab has matrix instead of reshape.
SciLab doesn't have atan2d.
SciLab uses %pi instead of pi.


With this code I get 150 in all elements of the phi array.