1. A method to constrain power allocation in a multiple input multiple output (MIMO) system, used in a communication device comprising a programmable coordinate rotation digital computer (CORDIC) circuit comprising: executing, by the programmable CORDIC circuit, a singular value decomposition (SVD) operation upon a channel matrix and obtaining a diagonal matrix, wherein the diagonal matrix is composed of a plurality of local matrixes; and transforming, by the programmable CORDIC circuit, diagonal elements of the diagonal matrix into diagonal elements of a specific diagonal vector from an upper-left corner local matrix and a lower-right corner local matrix, in parallel, simultaneously, and according to a sequence, by using a generalized triangular decomposition, to reduce a dimension of the channel matrix which reduces power consumption of the CORDIC circuit, wherein the transforming of the diagonal elements of the diagonal matrix into diagonal elements of the specific diagonal vector further comprises: transforming remaining diagonal local matrixes into the upper triangular matrix from the upper-left corner local matrix according to a upper-left to lower-right sequence; and transforming remaining diagonal local matrixes into the upper triangular matrix from the lower-right corner local matrix according to a lower-right to upper-left sequence.