interpolation

Hi

I want to interpolate a complex signal given at points x1,x2,x3....xn
to get signal at points x1+a,x2+a....xn+a.

1) To interpolate complex signal, I am interpolating the magnitude and
phase separately and later combining them. Is this an appropriate
method?

 2) I am trying to get FIR filter coefficients using Spline method. The
equation that i am using is:

   y(i+a)= ((a^3)/6)*(x(i+2))+ ((1 + 3*a +
3*(a^2)-3*(a^3))/6)*(x(i+1))+ ((1 + 3*a +
3*(a^2)-3*(a^3))/6)*(x(i+1))+((4-6*(a^2) + 3*(a^3))/6)*(x(i)) +((1-3*a
+ 3*(a^2)-(a^3))/6)*(x(i-1));

But the values I am getting are far away from the desired one.

Can anybody give me suggestions?

Thanks