Outage Behavior for Transmit Diversity OFDM-Based Systems

he following transmit diversity schemes for OFDM-based systems are analyzed: Linear Delay Diversity (LDD), Cyclic Delay Diversity (CDD), Subcarrier Diversity (SD), and Time-Switched Transmit Diversity (TSTD). The overall system performance is investigated by computing the theoretic outage rates achievable over the composite radio channels induced by the different transmission strategies. In particular, the analysis points out that a convenient exploitation of the available diversity branches leads to lower outage probabilities and, consequently, to higher outage rates. Incidentally, the ergodic capacity limit for the considered diversity schemes remains equal to the single transmit antenna case. On the other hand, transmit diversity reduces the statistical dispersion of the mutual information about its mean. As a result, the optimal selection of the delay for both LDD and CDD is addressed. Nevertheless, in many cases of interest the failure probability is the same with either SD, LDD or CDD. It is also mentioned that SD and TSTD are equivalent if the propagation medium is sufficiently slow time-varying, while SD outperforms TSTD over highly time-selective radio channels.