Hierarchical modulation for the downlink of MIMO multi-user channels

This paper introduces a signal transmission technique for the downlink of multiple-input multiple-output (MIMO) multi-user wireless systems. By applying non-uniformly spaced modulation constellations, i.e., hierarchical modulation constellations, we show that multiple services with different QoS requirements or the same service of randomly located multiple users in a cell can be supported simultaneously. In a MIMO cellular system as specified by 3GPP/3GPP2 work group, we present both the link-level and system-level simulation results. The link-level simulations shows the great potentials of the proposed scheme to provide unequal error protections. The system-level simulations consider two application scenarios: single user with multiple services and single service to multiple users, and show that the proposed scheme requires less average transmission power at the base station than the traditional TDMA system with uniformly spaced QAM modulation. It is also shown that the peak transmission power will be reduced. This scheme can be used to increase the size of the future cellular cell or the number of users in each cell.