An apparatus for deriving a submatrix {tilde over (G)}−1 is described. The apparatus for deriving a submatrix {tilde over (G)}−1 is configured to select an N-elements-column and an N-elements-row of an N×N-Matrix G or G−1. The apparatus for deriving a submatrix {tilde over (G)}−1 is configured to rearrange the selected column to the rightest column and the selected row to the lowest row of G or G−1 so as to generate a N×N-matrix Gp or Gp−1. The apparatus for deriving a submatrix {tilde over (G)}−1 is configured to calculate a submatrix {tilde over (G)}−1 by G ~ - 1 = A - AbcA d - 1 + c T ⁢ Ab , wherein the parameters (N−1)×(N−1)-submatrix A, b, d, c are obtained from the Gp or the Gp−1; wherein G p = [ G ~ b c T d ] , ⁢ G p - 1 = [ A b ~ c ~ T d ~ ] .