channel estimation for cdma I am looking for a method to make a multi-user channel estimation with CDMA.

Here is my question:

Is there a way to estimate multi-user channels with their spreading codes? If yes, what is the method?
Sure. The idea behind CDMA is that different users codes are orthogonal.

So, since your codes probably are pretty white, meaning that their autocorrelation function is essentially a dirac delta, you can simply encode an pulse for every user (or by every user), and use the standard correlation receiver to despread and thus recover each individual channel impulse response.