Optimal OFDM downlink scheduling for UMTS HSDPA evolution

Orthogonal frequency division multiplexing (OFDM) systems offer significantly more flexibility in resource allocation due to frequency-selective channel reporting. In this paper we show that maximizing the buffer-length-weighted-sum-rate in each time slot is a convex problem and achieves the maximal stability region which is equal to the ergodic capacity region of the underlying OFDM broadcast channel. Unfortunately, the optimal solution causes collision of multiple users on the subcarriers. Thus in addition to the comparatively complex optimal scheduling policy we present also a frequency division multiple access (FDMA) solution. It is shown that the FDMA solution comes quite close to the optimal performance and is also optimal in low SNR regime. We investigate the stability region for the optimal and suboptimal scheduling strategy and show the effect of limited feedback capacity. Generally, the stability region may serve as a fundamental limit for all other strategies.