  In this work, the delay limited capacity (DLC) of orthogonal frequency
division multiplexing (OFDM) systems is investigated. The analysis is organized
into two parts. In the first part, the impact of system parameters on the OFDM
DLC is analyzed in a general setting. The main results are that under weak
assumptions the maximum achievable single user DLC is almost independent of the
distribution of the path attenuations in the low signal-to-noise (SNR) region
but depends strongly on the delay spread. In the high SNR region the roles are
exchanged. Here, the impact of delay spread is negligible while the impact of
the distribution becomes dominant. The relevant asymptotic quantities are
derived without employing simplifying assumptions on the OFDM correlation
structure. Moreover, for both cases it is shown that the DLC is maximized if
the total channel energy is uniformly spread, i.e. the power delay profile is
uniform. It is worth pointing out that since universal bounds are obtained the
results can also be used for other classes of parallel channels with block
fading characteristic. The second part extends the setting to the broadcast
channel and studies the corresponding OFDM DLC BC region. An algorithm for
computing the OFDM BC DLC region is presented. To derive simple but smart
resource allocation strategies, the principle of rate water-filling employing
order statistics is introduced. This yields analytical lower bounds on the OFDM
DLC region based on orthogonal frequency division multiple access (OFDMA) and
ordinal channel state information (CSI). Finally, the schemes are compared to
an algorithm using full CSI.
