Embodiments of the invention provide a Space-Time coding device for encoding a digital data sequence according to a predefined space-time coding rate, said data sequence comprising a set of modulated symbols to be transmitted over at least two propagation modes, during a number of time slots, through an optical transmission channel in a single-core multimode optical fiber transmission system, wherein the device comprises: a codebook generator (53) configured to determine a set of codeword matrices defining a space-time code represented by a space dimension and a temporal dimension, each codeword matrix comprising complex values and a number of columns equal to said code temporal dimension, and a number of rows equal to said code space dimension, the codebook generator (53) being further configured to associate a difference codeword matrix and a distance metric to each pair of codeword matrices among said set of codeword matrices, each component of a difference codeword matrix associated with a given pair of codeword matrices being equal to the difference between the corresponding components of the codeword matrices of said pair, the distance metric associated with a given pair of codeword matrices being determined by computing the Euclidean norm of the difference codeword matrix associated with said pair, wherein the codebook generator (53) is configured to select the components of the codeword matrices depending on the number of pairs of codeword matrices that are associated with a unitary difference codeword matrix and with a distance metric equal to a given Euclidean distance; and an encoding unit (55) configured to encode said set of modulated symbols into a codeword matrix selected from said set of codeword matrices.