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Performance analysis of open-loop transmit diversity schemes for orthogonal frequency-division-multiplexing (OFDM) systems is addressed by investigating the outage capacity limits in frequency-selective fading channels. It is shown that the variance of the mutual information provides details on the amount of available diversity, and it suggests guidelines on transmitter design. The optimal delays, in the outage capacity maximization sense, are found for linear and cyclic delay diversity (LDD and CDD) schemes. It is proved that the outage rates for subcarrier diversity (SD) systems are the same as for the optimized CDD case. The effect of spatial correlation on system performance for a uniform linear transmit array is discussed and numerically evaluated.