This paper investigates an analytical model for low-earth orbit (LEO) multi-satellite downlink non-orthogonal multiple access (NOMA) networks. The satellites transmit data to multiple NOMA user terminals (UTs), each employing successive interference cancellation (SIC) for decoding. Two ordering schemes are adopted for NOMA-enabled LEO satellite networks, i.e., mean signal power (MSP)-based ordering and instantaneous signal-to-inter-satellite-interference-plus-noise ratio (ISINR)-based ordering. For each ordering scheme, we derive the analytical expression for the coverage probability of each typical UT. Moreover, we discuss how coverage is influenced by SIC, main-lobe gain, and tradeoffs between the number of satellites and their altitudes. Additionally, two user fairness-based power allocation (PA) schemes are considered, and PA coefficients with the optimal number of UTs that maximize their sum spectral efficiency (SE) are studied. Simulation results show that there exists a maximum effective signal-to-inter-satellite-interference-plus-noise ratio (SINR) threshold for each PA scheme that ensures the operation of NOMA in LEO satellite networks, and NOMA provides performance gains only when the target SINR is below a certain threshold. Compared with orthogonal multiple access (OMA), NOMA increases UTs' sum SE by as much as 35%. Furthermore, for most SINR thresholds, the sum SE increases with the number of UTs to the highest value, whilst the maximum sum SE is obtained when there are two UTs.

翻译：本文研究了一种面向低地球轨道多卫星下行链路的非正交多址网络分析模型。卫星向多个采用连续干扰消除技术进行解码的非正交多址用户终端传输数据。针对支持非正交多址的低轨卫星网络，本文采用两种排序方案：基于平均信号功率的排序与基于瞬时卫星间干扰加噪声比的排序。针对每种排序方案，我们推导了典型用户终端覆盖概率的解析表达式。此外，我们探讨了连续干扰消除、主瓣增益以及卫星数量与轨道高度之间的权衡关系如何影响网络覆盖性能。本文同时考虑两种基于用户公平性的功率分配方案，并研究了在最大化总频谱效率条件下最优用户终端数量对应的功率分配系数。仿真结果表明：每种功率分配方案均存在一个确保非正交多址在低轨卫星网络中正常运行的最大有效卫星间干扰加噪声比阈值，且仅当目标信干噪比低于特定阈值时，非正交多址才能提供性能增益。相较于正交多址技术，非正交多址可将用户终端总频谱效率提升高达35%。此外，在多数信干噪比阈值条件下，总频谱效率随用户终端数量增加而达到峰值，且当存在两个用户终端时获得最大总频谱效率。