How to get the correct asin value

I have a simple question! How can I get the correct value of asin function (Inverse sin)? Consider this:

asin(sin(-3*pi/2))

The answer is pi/2. However, I need the exact value -3*pi/2 to be the result!

I'd like to thank Azzi and Walter for their answers. However. I would clarify some more aspects of the challenge that I'm trying to solve. Suppose that We have Two Signals:

Yi=X.*h_i+N_i; 
Yj=conj(X).*h_j+N_j

Now, I'm trying to extract X out of Yi and Yj. I assume that N_i and N_j are White complex Gaussian noise which are uncorrelated with the data. I can't simply add them and subtract them because this would double the noise. I can extract the magnitude of x.^2 by simply:

sqrt(Yi.*Yj)
However. I'm trying to do the following for the phase:
q=Yi./Yj;
w=1./q;
ph=(asin((q-w)/2j))/2;

Which is giving me the wrong answer for X, because of the 2pi difference! Any suggestions.

Answer 1

[Walter Roberson]

Are you requiring that asin(sin(A)) be A for all (real-valued) A ? If so then you cannot do that. sin() is a periodic function, so for any A there exists multiple B not equal to A such that sin(A) = sin(B), and asin() is not going to be able to tell the difference.

[Mohammad]

Thank you for your clarifications. I totally agree with you. However, I was looking for a trick to solve it! I added some more details to the question!

[Walter Roberson]

Try with atan2() and appropriate arguments, instead of asin()

[Huang Tzu-Hua]

Great solution! You save me!

Answer 2

[Azzi Abdelmalek]

You can't get -3*pi/2, you have to add k*2*pi, in your case

asin(sin(-3*pi/2))-2*pi

[Mohammad]

Dear Azzi, Thank you for your answer. However, this doesn't help my case. Suppose that we interested in calculating the output of a, where:

asin(sin(a))

Where a can be pi/2 or -3*pi/2.

[Azzi Abdelmalek]

By definition, asin(x) takes its values from -pi/2 to pi/2. In your case the result will be pi/2.

[Azzi Abdelmalek]

The equation

a=sin(y)

has an infinity of solutions

y=asin(a)+2*k*pi

If you have specific conditions that allows to find the appropriate values of k, you have to add them.

[Mohammad]

Thank you again. However, I have no means of specifying k! I added some more details to the question.