Multi-carrier multiple access is sum-rate optimal for block transmissions over circulant ISI channels

We establish that practical multiple access based on finite size information blocks transmitted with prescribed power and with loaded multicarrier modulation, is optimal with respect to maximizing the sum-rate of circulant inter-symbol interference (ISI) channels, that are assumed available at the transmitter. Circulant ISI channels are ensured either with cyclic prefixed block transmissions and an overlap-save reception, or, with zero-padded block transmissions and an overlap-add reception. Analysis asserts that sum-rate optimal multicarrier users could share one or more subcarriers depending on the underlying channels. Optimal loading is performed by specializing an existing iterative low-complexity algorithm to circulant ISI channels.