Channel Correlatoin Function & Noise Varaince

In OFDM Channel Estimation, we firstly get a raw channel estimate from the
pilots and then perform interpolation for the remaining subcarriers.  A
popular cited method is the 1D Wiener method which basically looks for a
set of MMSE weights.  The formula requires the correlation of the channel
in the each subcarrier and the noise variance.  Most text will assume that
this is known.  Being new to this, I have following questions:

1) If the channel of each subcarrier is already known, why bother
interpolate? 
2) It looks like there must be a separate process to get the channel for
each subcarrier and the noise variance.  Is there an easy method of getting
the channel correlation and noise variance for each subcarrier?  Any
rference will be appreciated.

Thanks.