We consider a downlink multicast and unicast superposition transmission in muti-layer Multiple-Input Multiple-Output (MIMO) Orthogonal Frequency Division Multiple Access (OFDMA) systems when only the statistical channel state information is available at the transmitter (CSIT). Multiple users can be scheduled by using the time/frequency resources in OFDMA, while for each scheduled user MIMO spatial multiplexing is used to transmit multiple information layers, i.e., single user (SU)-MIMO. The users only need to feedback to the base-station the rank-indicator and the long-term average channel signal-to-noise ratio, to indicate a suitable number of transmission layers, a suitable modulation and coding scheme and allow the base-station to perform user scheduling. This approach is especially relevant for the delivery of common (e.g., popular live event) and independent (e.g., user personalized) content to a high number of users in deployments in the lower frequency bands operating in Frequency-Division-Duplex (FDD) mode, e.g., sub-1 GHz. We show that the optimal resource allocation that maximizes the ergodic sum-rate involves greedy user selection per OFDM subchannel and superposition transmission of one multicast signal across all subchannels and single unicast signal per subchannel. Degree-of-freedom (DoF) analysis shows that while the lack of instantaneous CSI limits DoF of unicast messages to the minimum number of transmit antennas and receiver antennas, the multicast message obtains full DoF that increases linearly with the number of users. We present resource allocation algorithms consisting of user selection and power allocation between multicast and unicast signals in each OFDM subchannel. System level simulations in 5G rural macro-cell scenarios show overall network throughput gains in realistic network environments by superposition transmission of multicast and unicast signals.

翻译：本文研究在发射端仅已知统计信道状态信息（CSIT）的多层多输入多输出（MIMO）正交频分多址（OFDMA）下行系统中，多播与单播叠加传输问题。通过OFDMA的时频资源可调度多个用户，而对每个被调度用户采用MIMO空间复用技术传输多层信息（即单用户MIMO）。用户仅需向基站反馈秩指示符和长期平均信道信噪比，用以指示合适的传输层数、调制编码方案，并辅助基站进行用户调度。该方法特别适用于低频段（如低于1 GHz）频分双工（FDD）模式下，向大量用户传输公共内容（如热门直播事件）及独立内容（如个性化用户信息）。研究表明，最大化遍历和速率的最优资源分配策略包括：每OFDM子信道的贪婪用户选择，以及跨所有子信道传输一个多播信号与每子信道传输一个单播信号的叠加传输方案。自由度（DoF）分析表明，虽然瞬时CSI的缺失将单播消息的自由度限制为发射与接收天线数的最小值，但多播消息可获得随用户数量线性增长的全自由度。本文提出包含用户选择与每OFDM子信道内多播/单播信号功率分配的资源分配算法。在5G农村宏蜂窝场景下的系统级仿真验证了，通过多播与单播信号的叠加传输可在实际网络环境中获得整体网络吞吐量增益。