Optimization of Hierarchical-QAM based Downlink Multiple Access with Statistical CSI

This paper considers a downlink multiple access scenario with OFDM transmissions in frequency selective channels. The users are assigned with different bit streams from hierarchical quadrature amplitude modulation (QAM) rather than assigned with different subcarriers as in orthogonal frequency division multiple access (OFDMA). Based on statistical channel state information in the form of the average signal-to-noise ratios of the users, we derive a closed-form solution on the hierarchical constellation design. The numerical results demonstrate the performance advantage of the proposed system over OFDMA where power control is also applied based on the same channel feedback.