The Delay-Limited Capacity Region of OFDM Broadcast Channels

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. ∗ The material in this paper was in part presented at IEEE Information Theory Workshop, Punta del Este, March 2006, IEEE SPAWC, Cannes, July 2006 and 44th Allerton Conference, Monticello, 2006 † The authors are with the Fraunhofer German-Sino Mobile Communications Lab, Heinrich-Hertz-Institut, Einstein-Ufer 37, D-10587 Berlin, Germany. Tel/Fax: +493031002-872/-863; Email: {wunder,michel}@hhi.fhg.de