The most basic and necessary preprocessing is to center , i.e. subtract its mean vector so as to make a zero-mean variable. This implies that is zero-mean as well, as can be seen by taking expectations on both sides of Eq. (4).
This preprocessing is made solely to simplify the ICA algorithms: It does not mean that the mean could not be estimated. After estimating the mixing matrix with centered data, we can complete the estimation by adding the mean vector of back to the centered estimates of . The mean vector of is given by , where is the mean that was subtracted in the preprocessing.