1. A method for processing communication signals, the method comprising: quantizing a change in channel state information in a multiple-input multiple-output (MIMO) pre-coding system onto a codebook, which comprises one or more unitary matrices, using a cost function, wherein said cost function f(A) is defined by the following relationship: f ⁡ ( A ) = ( 1 N ⁢ ∑ j = 1 N ⁢  a jj  2 ) where A is a matrix of size N by N and a ij is element (i,j) of matrix A; and generating said codebook based on at least said channel state information.