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：我们研究了在下行链路多层多输入多输出（MIMO）正交频分多址（OFDMA）系统中，当发射端仅知统计信道状态信息（CSIT）时的多播与单播叠加传输问题。通过OFDMA的时间/频率资源可调度多用户，而对每个被调度用户，采用MIMO空间复用传输多个信息层，即单用户MIMO（SU-MIMO）。用户只需向基站反馈秩指示符和长期平均信道信噪比，以指示合适的传输层数、调制编码方案，并支持基站进行用户调度。该方法尤其适用于在低频段（如sub-1 GHz）频分双工（FDD）部署场景中，向大量用户传输公共内容（例如热门直播事件）和独立内容（例如个性化信息）。我们证明，最大化遍历和速率的优化资源分配涉及：每个OFDM子信道的贪婪用户选择、跨所有子信道传输一个多播信号的叠加传输，以及每个子信道传输单一单播信号。自由度（DoF）分析表明：虽然瞬时CSI缺失将单播消息的DoF限制为发射天线和接收天线数的最小值，但多播消息可获得与用户数呈线性增长的完整DoF。我们提出了资源分配算法，包含每个OFDM子信道中的用户选择以及多播与单播信号间的功率分配。5G农村宏小区场景的系统级仿真显示，通过多播与单播信号的叠加传输，在实际网络环境中可实现整体网络吞吐量增益。