A method is provided which improves reliability of channel estimation in a digital communication system by reducing the ambiguity in the recognition of received symbols evaluated for the channel estimation. A first plurality of bits is mapped to a modulation state according to a given Gray mapping of binary numbers to modulation states and transmitted. The plurality of bits is re-transmitted at least once, with a sub-set of bits contained in the plurality of bits inverted, and mapped to further modulation states according to the same Gray mapping. The bits to be inverted are determined in a way that the number of different vector sum results obtainable, for all combinations of bit values within the first plurality of bits, by adding vectors representing complex values of the first and further modulation states in a complex plane, is lower than the number of different modulation states within the Gray mapping.