Impact of multiuser diversity and channel variability on adaptive OFDM

A downlink radio interface for cellular packet data systems with wide area coverage and high spectral efficiency is evaluated. A slotted OFDM radio interface is used, in which time-frequency bins are allocated adaptively to different mobile users within a downlink beam, or sector, based on their channel quality. Frequency division duplex (FDD) is assumed, which requires channel prediction in the terminals and feedback of this information to a packet scheduler. The adaptive modulation scheme is optimized by a novel approach which maximizes the throughput, including also the ARQ part of the transmission. A theoretical evaluation of the resulting multiuser diversity under some idealized assumptions shows that the spectral efficiency increases significantly with the number of active users. The simulations indicate that the loss of performance due to channel variability within the bins for vehicular users in frequency-selective fading environments is rather small.