On the OFDM multiuser downlink capacity

We characterize the OFDM multiuser downlink capacity region under a sum power constraint and perfect channel knowledge at the base station. This serves as a fundamental limit for all transmission strategies (Tse, D.N.C. and Hanly, S.V., IEEE Trans. Inf. Theory, vol.44, no.7, p.2796-815, 1998). We derive the corresponding power control strategy and provide necessary and sufficient conditions for both the single user and the frequency division multiple access (FDMA) optimality range. It is shown that, unlike the sum capacity case, FDMA is not always optimal and power control is complex (Ohno, S. et al., Proc. Int. Conf. on Commun., vol.3, p.1656-60, 2002). On the other hand, the solution can be simplified in the single user and FDMA region by applying a modified weighted water-filling over the selected subcarriers.