We propose a systematic method for the design of space-time codes for AWGN slowly and fast Rayleigh fading channels. This can be accomplished by adopting a concatenated space-time code structure, where an orthogonal transmit diversity system constitutes the inner encoder. We will show that this will cause in decoupling of the problems of spatial and temporal diversity gains maximization, involved in the design of space-time codes. This decoupling significantly simplifies the code design procedure and presents a systematic code construction technique. In the case of slowly fading channels, where no temporal diversity gain is available, the concatenated structure of the space-time code, will help to decouple the problems of spatial diversity and coding gains maximization. However in a fast fading channel, the proposed system will decouple the problems of spatial and temporal diversity gains maximization. At the end, some issues involved in designing codes for downlink broadcast channels will be discussed.