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 coverage probabilities of UTs under different channel conditions. 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 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 the benefit of NOMA only exists 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：本文研究了一种低地球轨道（LEO）多卫星下行链路非正交多址接入（NOMA）网络的分析模型。卫星向多个NOMA用户终端（UT）传输数据，每个终端采用连续干扰消除（SIC）进行解码。针对支持NOMA的LEO卫星网络，采用了两种排序方案：基于平均信号功率（MSP）的排序和基于瞬时信号与星间干扰加噪声比（ISINR）的排序。针对每种排序方案，我们推导了不同信道条件下用户终端的覆盖概率。此外，我们讨论了SIC、主瓣增益以及卫星数量与高度之间的权衡如何影响覆盖性能。同时，考虑了两种基于用户公平性的功率分配（PA）方案，并研究了在最大化用户总频谱效率（SE）时，最优用户终端数量下的PA系数。仿真结果表明，每种PA方案都存在一个最大信号与星间干扰加噪声比（SINR）阈值，以确保NOMA在LEO卫星网络中的运行，且NOMA的优势仅当目标SINR低于特定阈值时才存在。与正交多址接入（OMA）相比，NOMA可将用户终端的总频谱效率提升高达35%。此外，对于大多数SINR阈值，总频谱效率随用户终端数量增加而达到最高值，而最大总频谱效率在存在两个用户终端时获得。