The paper deals with space-time-coded transmissions in the presence of channel estimation errors. A simple transmission model is described which gives insight into the basic relations between space-time block coding schemes and the general concept of spreading. On the basis of this model, diagonal algebraic space-time (DAST) block codes are taken as an example. For maximum ratio combing and equal gain combining it is shown, that in case of channel estimation errors suboptimum algorithms at the receiving side might outperform optimum ones. The mathematical equivalence between DAST and multicarrier code division multiplexing as a by-product of the modelling is also discussed