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Delay matters in future wireless communication. An appropriate limit for rates achievable under delay constraints is the delay limited capacity (DLC). In this work, the DLC of OFDM systems is investigated. Despite its complicated correlation structure the OFDM DLC is fully characterized for low and high SNR. It is shown that (under weak assumptions) the OFDM DLC is almost independent of the fading distribution in the low SNR region but strongly depends on the delay spread thereby achieving a capacity gain over AWGN capacity. In the high SNR region the roles are exchanged. Here, the impact of delay spread is negligible while the impact of the fading distribution becomes dominant. The relevant quantities and their asymptotic behaviour are derived without employing simplifying assumptions on the OFDM correlation structure. Using a general convergence framework the analysis further shows that if the delay spread becomes large even the predominant impact of the fading distribution vanishes and DLC capacity loss compared to AWGN capacity approaches 0.58[nats/s/Hz]. The convergence speed, the loss due to non-uniform power delay profile, and the relation to ergodic capacity is also analyzed and underlined with simulations and application examples. The main conclusion here is that OFDM fully takes advantage of the degrees of freedom of the underlying fading channel in terms of delay spread and, regardless of the fading distribution, delay sensitive capacity measures such as the DLC converge to the ergodic capacity. Finally, since universal bounds are obtained which apply to any fading distribution the results can also be used for other classes of parallel channels extending the range of applicability.
Date of Publication: July 2009