A data communication apparatus, for transmitting one or more blocks of data within a frame including a two-dimensional grid of transmission symbol positions is configured to select a group of transmission symbol positions, which is a subset of the two-dimensional grid of transmission symbol positions, for a transmission of a data portion. The data communication apparatus is configured to select one or more reference symbol positions associated to the selected group of transmission symbol positions, out of a plurality of possibilities, based on an information describing a desired relative position of the reference symbol position with respect to the selected group of transmission symbol positions. Other apparatuses also use a flexible selection of transmission symbol positions and a flexible selection of reference symbol positions. A system, methods and computer programs are also described.