  AIMS: One of the most challenging and important problem of digital signal
processing in Cosmology is the separation of foreground contamination from
cosmic microwave background (CMB). This problem becomes even more difficult in
situations, as the CMB polarization observations, where the amount of available
"a priori" information is limited. In this case, it is necessary to resort to
the "blind separation" methods. One important member of this class is
represented by the "Independent Components Analysis" (ICA). In its original
formulation, this method has various interesting characteristics, but also some
limits. One of the most serious is the difficulty to take into account any
information available in advance. In particular, ICA is not able to exploit the
fact that emission of CMB is the same at all the frequencies of observations.
Here, we show how to deal with this question. The connection of the proposed
methodology with the "Internal Linear Composition" (ILC) technique is also
illustrated.
  METHODS: A modification of the classic ICA approach is presented and its
characteristics are analyzed both analytically and by means of numerical
experiments.
  RESULTS: The modified version of ICA appears to provide more stable results
and of better quality.
