Transmit diversity system in which constellation rearrangement is used. Both diversity signals contain the same data and both are 16-QAM, but the location of the data bits within the constellation is different. This averages the effect of the different levels of reliability of the different constellation points. A method for modifying a quadruple of data bits in a data transmission system using Quadrature. Amplitude Modulation with 16 different modulation states, 16-QAM, using an Antipodal Inverted Constellation AICO) mapping, swapping bits selecting contiguous symbol regions with bits selecting non-contiguous symbol regions. Depending on the particular pre-defined mapping and on the particular permutation of bits, inversion of selected bits may be performed in addition. The quadruples thus obtained are mapped to modulation symbols according to a pre-defined AICO mapping. In the corresponding method for receiving the symbols, likelihood values are swapped and modified in a way which is complementary to the permutation and inversion of bits, before they are combined with likelihood values from other symbols representing the same bits. Method ensures that there is a one to one correspondence between Hamming distance and Euclidian distance for the combined received data. In other words it ensures that, once combined, all 4 bit data words which differ by the same number of bits (Hamming distance) are the same distance apart in the constellation (Euclidian distance).