A method and device are provided for selecting a transmission mode, for a first telecommunications entity having a plurality of different modes for transmitting a communications signal to a second telecommunications entity. The various transmission modes provide the same data rate D. The method includes: for a given transmission mode, determining the value of a first metric α that measures degradation relating to a given distance d stemming from the transmission medium of the communications signal for a given environment compared with a reference model for the transmission medium, as a result of a multi-path effect and/or of an attenuation effect of the transmission medium; and comparing values of the metric α for different modes in order to select at least one transmission mode.