Exemplary embodiments of the present disclosure propose multiple codebook searching methods for a receiver in a communication system, wherein the communication system uses a codebook having L pre-coding matrices. First, L matrices are calculated according to the L pre-coding matrices, and the L matrices are inversed mean square error matrices of the L pre-encoding matrices. Then, diagonal cofactors or determinants of at least S matrices of the L matrices are calculated, and a pre-coding matrix used by the receiver is found according to the maximum cofactors of the diagonal of the at least S matrices or ratios between the maximum cofactors of the diagonals and the determinants of the at least S matrices.