1. A method of compressing data for transmission in a first direction across a communications channel, the data representing channel conditions for the communications channel in a direction opposite to the first direction, comprising; arranging the data as a matrix comprising a number of orthonormal vectors derived from a channel matrix; determining a singular value decomposition of a subset of the orthonormal matrix to generate matrices respectively of left and right singular vectors, the number of vectors in the subset being equal to the order of the vectors; and right multiplying the remainder orthonormal vectors not included in the singular value decomposition by a matrix product of the matrix of right singular vectors and the matrix of left singular vectors to generate a matrix of compressed data.