In this paper, we propose a technique to decouple the problems of spatial and temporal diversity gain maximization involved in the design of space-time codes in fast Rayleigh fading channels by using orthogonal transmit diversity systems. We will also introduce the idea of constellation expansion (in dimension or size) to design coded modulation schemes with maximum temporal diversity gain for orthogonal transmit systems. The Chernoff upper bound for the error probability is used to show that the code design criterion reduces to the maximization of Hamming and product distances in the expanded constellation. The proposed technique is demonstrated by designing multilevel and multiple trellis coded modulation schemes for orthogonal transmit diversity systems. These codes are shown to have better performance compared to the coding schemes designed for single transmit and receive antennas.