The aim of this paper is to develop hybrid non-orthogonal multiple access (NOMA) assisted downlink transmission. First, for the single-input single-output (SISO) scenario, i.e., each node is equipped with a single antenna, a novel hybrid NOMA scheme is introduced, where NOMA is implemented as an add-on of a legacy time division multiple access (TDMA) network. Because of the simplicity of the SISO scenario, analytical results can be developed to reveal important properties of downlink hybrid NOMA. For example, in the case that the users' channel gains are ordered and the durations of their time slots are the same, downlink hybrid NOMA is shown to always outperform TDMA, which is different from the existing conclusion for uplink hybrid NOMA. Second, the proposed downlink SISO hybrid NOMA scheme is extended to the multiple-input single-output (MISO) scenario, i.e., the base station has multiple antennas. For the MISO scenario, near-field communication is considered to illustrate how NOMA can be used as an add-on in legacy networks based on space division multiple access and TDMA. Simulation results verify the developed analytical results and demonstrate the superior performance of downlink hybrid NOMA compared to conventional orthogonal multiple access.

翻译：本文旨在开发混合非正交多址接入辅助的下行链路传输技术。首先，针对单输入单输出场景（即每个节点配备单天线），提出了一种新型混合NOMA方案，其中NOMA作为传统时分多址网络的附加技术实现。由于SISO场景的简洁性，可推导出分析结果以揭示下行链路混合NOMA的重要特性。例如，当用户信道增益排序且时隙时长相同时，下行混合NOMA性能始终优于TDMA，这与现有上行混合NOMA的结论不同。其次，将所提出的下行SISO混合NOMA方案扩展到多输入单输出场景（即基站配备多根天线）。针对MISO场景，考虑了近场通信以说明NOMA如何在基于空分多址与TDMA的传统网络中作为附加技术应用。仿真结果验证了理论分析的正确性，并展示了下行混合NOMA相较于传统正交多址接入的优越性能。