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 at least a first and a second codebook, each of 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 j ⁢ ⁢ j  2 ) where A is a matrix of size N by N and a ij is element (i,j) of matrix A; and generating said at least first and second codebook based on at least said channel state information.