In this paper, we propose a technique to decouple the problems of maximizing spatial diversity gain, temporal diversity gain, and coding gain, involved in the design of space-time codes in Rayleigh fading channels. For this purpose, we will use a set of concatenated codes with orthogonal transmit diversity system as their inner code. In the case of slowly fading channels, no temporal diversity is available, and the proposed technique decouples the problems of maximizing spatial diversity gain and coding gain. The design criterion for the outer codewill be shown to be based on the maximization of the free Euclidean distance. For fast fading channels, by using the new idea of constellation expansion (in dimension or size), we decouple the code design problem into two simpler problems, namely maximizing the spatial diversity gain, and maximizing the temporal diversity and coding gain. In this case, the code design criteria for the outer encoder will be shown to reduce to the maximization of Hamming and product distances in the expanded constellation. The proposed techniques are illustrated by designing multilevel and multiple trellis coded modulation schemes for orthogonal transmit diversity systems. These codes are shown to have better performance compared to some existing space-time trellis codes with the same complexities.