Modified design criteria for space-time codes over quasi-static fading channels are introduced; they capture the joint effects of error exponents and multiplicities in the code's distance spectrum. Distance spectrum based optimization produces a new set of constituent codes for parallel-concatenated space-time turbo coded modulation, resulting in a robust space-time coding scheme with improved performance over both quasi-static and fast fading channels. A recursive systematic form with a primitive equivalent feed-backward polynomial is assumed for constituent codes to assure good convergence of iterative decoding.