Group trellis codes over multidimensional unbalanced PSK and their optimality

New geometrically uniform trellis codes over multidimensional unbalanced 4-, 8-, and 16-PSK constellations, obtained with the group code approach, are presented. They improve by up to 1.25 dB the asymptotic performance of the best known trellis codes over multidimensional balanced PSK. To check their "optimality", an upper bound on the free distance of group trellis codes is derived. Some of the new codes achieve the largest obtainable free distance, and several others are close to the bound. Exploiting the symmetry properties of the codes, curves of tight upper bounds to the error probability are also presented.